Underbalanced Drilling Operations Research Articles

Pressure Loss at the Bit While Drilling with Foam

Foam is one of the most frequently used drilling fluids at underbalanced drilling operations. As foam flows, due to the pressure drop, a volumetric expansion is observed, which causes the foam quality to increase in the same direction with flow. Flow of foams through circular pipes and annular geometries are well studied. Interestingly, although one of the major sources of pressure drop is at the bit, there have been few studies of this subject for foams. Many drilling parameters including hole cleaning capacity, volumetric requirements of liquid and gas phase, and backpressure are out of control if the pressure drop at the bit is not accurately determined, even though pressure drops inside the pipes and wellbore are properly determined. This article introduces a more accurate model for estimating the pressure drop of foam flowing through the bit. The major difference between the proposed and the existing models is that the proposed model includes the effect of foam expansion and velocity change as a function of pressure. Pressure drop has been observed to increase significantly as the upstream pressure and foam average velocity increases when compared with the existing models. For the same flow conditions, pressure drop decreases as the foam quality increases, and as the upstream pressure increases, pressure drop also increases. The existing models cannot detect this event at all. In some cases, the pressure drop at the bit can be 10 times greater than the pressure drop predicted from existing models.

Hydraulic Parameters of Underbalanced and Managed-Pressure Drilling

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 114667, "Some - times-Neglected Hydraulic Para meters of Underbalanced and Man aged-Pressure Drilling," by Charles R. Stone, SPE, and Shifeng Tian, SPE, Signa Engineering Corporation, originally prepared for the 2008 SPE/IADC Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition, Abu Dhabi, UAE, 28-29 January. The paper has not been peer reviewed. Successful drilling, especially in very deep wells, may be driven by hydraulic limiting parameters. Two new technologies, underbalanced drilling (UBD) and managed-pressure drilling (MPD) have emerged as solutions to specific hydraulics issues during drilling. A hydraulic parameter can be defined as any factor—mechanical, structural, or fluid—that affects the exertion of hydrostatic head on the open hole. Hydraulic parameters, as a group, must be planned for and managed during all drilling operations to prevent unwanted or unsafe conditions. Introduction Hydraulic limits occur in both conventional wells and unconventional wells. The more critical wells experience limits that are unmanageable with conventional techniques; thus the emergence of UBD and MPD. MPD is defined by the International Association of Drilling Contractors as "an adaptive drilling process used to more precisely control the annular pressure profile throughout the wellbore." Simply put, drillers are concerned with the entire pressure profile in the open hole, including the annulus pressure at the casing shoe and the bottomhole pressure (BHP). MPD does not encourage formation influx. UBD operations involve drilling into any formation where the pressure exerted by the drilling fluid is less than the formation pressure. The technique reduces the hydrostatic pressure of the drilling-fluid column so that the net pressure in the wellbore is less than the formation pressure. Consequently, the formation pressure may cause permeable zones to flow if conditions at the surface allow flow. UBD can facilitate drilling of pressure-depleted formations and lessen formation damage for better productivity. UBD operations include formation influx in the operating plan except in the case of a hole being drilled for rate-of-penetration (ROP) applications in impermeable rock. In MPD, the driller seeks to stay slightly above or "at-balance" with the downhole pore pressure (PP), or as close to near balance as possible during the entire section of problem hole, during both drilling and connections. Precise control of downhole pressure allows the driller to drill within the window between PP and fracture gradient (FG) without setting casing prematurely.

Technology Focus: Drilling Technology (February 2009)

Technology Focus To reach reserves in increasingly complex wells, drilling challenges are numerous and very impressive, requiring highly skilled engineers who must deal with many disciplines, such as tubular and rock mechanics, hydraulics, data acquisition and processing, and modeling, among others. Nowadays, drilling is faced with several limits that intensify uncertainty and risks. Hydraulics and mechanics issues illustrate these drilling limits. From a hydraulic point of view, because of narrow drilling margin (e.g., very small difference between fracture and pore pressure) coupled with inaccurate knowledge of what is happening downhole, automated control and measurements while drilling are required to maintain equivalent circulating densities within a tight margin. Indeed, the uncertainty of temperature and pressure downhole may be greater than the margin itself. As a result, this managed- or controlled-pressure drilling technology enables reducing lost circulation, formation fracturing, and differential sticking, and it enables close monitoring of static, dynamic, and transient hydraulic behavior. From a mechanical point of view, because wells are deeper and temperatures and pressures are higher, drilling equipment is pushed to its mechanical and endurance limits. Even though it is recognized that new resistant and higher-strength material needs to be developed for the future, downhole measurements with a high-speed-telemetry drillstring will aid in determining accurate mechanical limits of existing technology. This information should lead to a better understanding of drillstring mechanics to prevent vibration, friction heating, wear, fatigue, and finally failure. To reduce uncertainty on downhole mechanical and hydraulic conditions, one must measure accurately, transmit quickly, and react accordingly to make the best drilling decisions. Therefore, data processing and modeling are important aspects of measurements or control while drilling. To manage, and add value to, this massive amount of information collected downhole, real-time and accurate mechanics and hydraulics models are required. Future generations of drillers will have to push these technical limits even further. Drilling Technology additional reading available at the SPE eLibrary: www.spe.org SPE 112669 • "Challenges of Directional Drilling Through Salt in Deepwater Gulf of Mexico" by R.R. Israel, Schlumberger, et al. SPE 114050 • "Modeling of an Underbalanced-Drilling Operation Utilizing Supercritical Carbon Dioxide" by Faisal Al-Adwani, Kuwait University, et al. SPE 112683 • "A New Mud-Pulse-Telemetry System for Enhanced MWD/LWD Applications" by C. Klotz, SPE, Baker Hughes, et al. Additional reading available at OnePetro: www.onepetro.org OTC 19550 • "Overcoming Weight-Transfer Challenges in Complex, Shallow, Extended-Reach Wells on Alaska's North Slope" by Randy Thomas, SPE, ConocoPhillips Alaska, et al. See JPT November 2008, page 70.

Legends of Drilling

During SPE's Annual Technical Conference and Exhibition in Denver, JPT honored ?ve pioneers in the drilling industry with its ?rst Legends of Drilling Award. The reception, held on 23 September, honored Leon Robinson, Martin E. Chenevert, Marvin Gearhart, William A. Rehm, and William C. Maurer, and was sponsored by Schlumberger. Although the pioneering achievements of the early days of drilling are well known and documented, little attention has been given to the achievements of the late 20th and early 21st centuries. The people this award honors have been instrumental in the creation and development of these more recent drilling accomplishments. Robinson has contributed greatly to the industry's technical literature, and has made many notable technological contributions in the areas of mud cleaners, explosive drilling, and drilling data telemetry. He has received 24 international patents, 34 US patents, and many service and achievement awards. Chenevert is considered a legend in the world of oil and gas drilling. In addition to 100 published works and three books, he has developed petroleum engineering software applications and holds nine US patents. He has earned international recognition for his work in the area of wellbore stability and drilling ?uids, has received numerous industry awards and recognitions, and has been active in SPE and many other associations. Gearhart has had a long track record for creating companies that specialize in high-performing technologies. During the 1970s, his company developed a major advance in openhole logging equipment and became a forerunner in measurement-while-drilling technology. His Gearhart Company is currently putting together an integrated line of drilling tools and systems designed for vertical and horizontal drilling in the Barnett Shale play. Rehm helped develop well control and pressure measurement from electric logs and wrote the ?rst manual on well control accepted by the US Minerals Management Service. He has contributed to some of the most signi?cant technological advancements in recent history, including the development of directional drilling, coiled tubing, underbalanced drilling, and high-pressure drilling operations. He is a recognized expert in underbalanced drilling and conducts schools and management seminars internationally and has written several books and manuals on the technology. Maurer's work has contributed to pioneering changes in the industry, including extensive research into novel drilling techniques, drilling mechanics, rock mechanics, drill bit design, downhole drilling motors, high-pressure jet drilling, horizontal drilling, and advanced drilling tools. His Maurer Engineering was instrumental in the development of measurement-while-drilling technology. During the 1980s, he organized an effort to develop tools to economically and reliably drill horizontal wells, and was instrumental in the development of the then-revolutionary PDC bit. It is my hope that reading about the remarkable achievements of these extraordinary people will inspire others to create and contribute to new technologies and solutions that will bene?t the oil and gas industry in the years to come.

Lubrication Process at the Wall in Foam Flow Application to Pressure Drop Estimation While Drilling UBD Wells

Abstract When drilling underbalanced, the pressure of the drilling fluid is maintained at a value below the formation pressure. In order to lower the well pressure, specific low density fluids are needed, i.e., gas, aerated mud or foam. Foam is particularly beneficial for drilling mainly due to its low density coupled with good carrying capability, but its use remains hazardous due to the incomplete knowledge of its bottomhole properties, and especially of its flowing properties. Pressure drop estimation is crucial for underbalanced drilling (UBD) operations in order to be able to keep the bottom hole pressure in the adequate range in real time. In this study, we analyze the pressure drop variation with the flow rate in a circular pipe for different foam qualities and formulations. Experimental investigations are realized in a pressure and temperature circular conduct flow. We show that lubrication at the wall plays a crucial role. Indeed, the intrinsic viscosity of the foam can be very high, leading to the development of a water layer at the wall responsible for the lubrication of the flow. A two-phase description of the system allows for the analytical estimation of the pressure drop. The size of the lubricated layer is then deduced and its range of existence is discussed. Main parameters of its formation are also discussed. We show that this lubrication effect can be significant in pressure drop estimation for underbalanced operations. Introduction Underbalanced drilling is recognized as one of the most effective solutions to prevent formation damage, differential sticking or fluid losses. When drilling underbalanced, the pressure of the drilling fluid is maintained at a value below the formation pressure, and for this purpose, low density fluids are used. Foam, defined as a mixture of a gas phase and a foaming solution, is one of the most versatile aerated fluids. It allows very low densities from 0.2 s.g., and is one of the most efficient fluids for lifting the cuttings and cleaning the wellbore. However, it is very important when drilling underbalanced to keep the bottom hole pressure in an adequate range in real time. Indeed, foam, as other underbalanced fluids, is not designed to create a mud cake at the wall. Therefore, a short overbalanced period can lead to the deep penetration of a filtrate into the reservoir, severely reducing the expected productivity. One of the key features of a successful underbalanced operation is control of the bottom hole pressure. Wellbore hydraulic simulation is essential as well, to design the operation and for monitoring the drilling phase or evaluating changes in the drilling program(1). While pressure estimation for aerated fluids is often realized through multiphase modelling (with estimation of the different phase's velocities), it is possible to consider flowing foam as a fluid with a given rheology that varies with the pressure, temperature and quantity of gas. Predicting the bottomhole pressure while drilling with foam implies knowledge of foam rheological properties that depend on the foam density; the foam density being, itself, a function of the frictional and hydrostatic pressure.

Drill Bit Pressure Drop During Foam Drilling Operations

Abstract Foam is one of the most frequently used drilling fluids for underbalanced drilling operations. This paper introduces a more accurate model than the existing models for estimating the pressure drop of foam flowing through the drill bit. The major difference between the proposed and existing models is that the proposed model includes the effect of foam expansion and velocity change as a function of pressure. It has been observed that pressure drop increases significantly as the upstream pressure and foam average velocity increases. For the same flow conditions, pressure drop decreases as the foam quality increases, and as the upstream pressure increases, pressure drop also increases. These events cannot be detected by existing models. In some cases, the pressure drop at the bit can be 10 times greater than the pressure drop predicted by existing models. Introduction Foam has been used as a drilling fluid in many drilling operations; especially in underbalanced drilling applications. In a number of cases, drilling with foam has shown to provide significant benefits, including increased productivity (by reducing formation damage), increased drilling rate, reduced operational difficulties associated with drilling in low pressure reservoirs (e.g. lost-circulation and differentially stuck pipe) and improved formation evaluation while drilling. Foams consist of a continuous liquid phase, forming a stable cellular structure that surrounds and entraps a gas phase. Special chemicals, called surfactants, are used to capture the gas phase; at least for a desired period of time. Foams are considered to be dry or wet, depending on the gas content. Wet foams have spherical bubbles with large amounts of liquid between the bubbles. Dry foam bubbles are polyhedral in shape, with definite contact between the bubbles. In between these two extremes, geometrical figures having both curved and flat faces can exist. Foams are thermodynamically unstable systems because they always contain more than a minimal amount of gas solution interface(1). This interface represents surface free energy, the amount of which can be estimated from knowledge of the surface tension and the interfacial area of the foam. Wherever a foam membrane breaks and the liquid coalesces, there is a decrease in surface free energy. Thus, the decomposition of foam into its constituent phase is a spontaneous process. Since the solution phase is always denser than the gaseous phase, there is a strong tendency for the liquid to separate or drain from the main body of foam unless it is circulated or agitated in some way. Foams can have extremely high viscosity. In all instances, their viscosity is greater than either the liquid or the gas that they contain(2). At the same time, their densities are much lower than the density of water. They are stable at high temperatures and pressures. So, by using foam as a drilling fluid, its high viscosity allows efficient cuttings transport and its low density allows underbalanced conditions to be established; thereby minimizing formation damage. Foams are also preferred when water influx is a problem because they can handle large amounts of water.

Development of Three-Layer Model and Numerical Simulation for Cuttings Transport in Horizontal Foam Drilling

Foam drilling has grown tremendously in the last few years. Foam is often used as a circulating fluid in underbalanced drilling operations (i.e. to keep the pressure of the drilling fluid less than the pressure of the formation fluid). Foam drilling has been shown to provide significant advantages, such as higher penetration rate, effective cuttings transport, less formation damage, etc. However, although, a few studies have been conducted on vertical wellbores, there is little detailed published information related to its use in horizontal wellbores. In this study, the cuttings behavior in horizontal foam drilling was numerically investigated. First of all, the experimental results of foam rheology and particle settling test were shown. Experimental results showed that the foam behaves like power law fluid and foam volumetric quality has a major impact on the settling phenomena. The next, a three layer model was developed to predict and interpret the cutting transport behavior in horizontal foam drilling. This model consists of a three-layer configuration, i.e. stationary bed of drilled cuttings at the bottom; a moving bed above it; and a heterogeneous suspension layer at the top. This model is based on conservation laws of mass and linear momentum as well as on constitutive relations that describe the interactions between each phase and the walls. The model was solved numerically to predict the cuttings bed height, pressure drop at different foam flow rates under several conditions. Furthermore, numerical simulations were carried out in order to investigate some factors affecting the cutting transport in horizontal foam drilling. The results of numerical simulation are presented.

Sour-Well Serviceability of Higher-Strength Coiled Tubing

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 107014, "Sour- Well Serviceability of Higher-Strength Coiled Tubing," by H.B. Luft, T. Padron, and E. Kee, BJ Services Co. Canada Ltd., and E. Szklarz, Shell Canada Ltd., prepared for the 2007 SPE/ICoTA Coiled Tubing and Well Intervention Conference and Exhibition, The Woodlands, Texas, 20–21 March. A joint-industry project (JIP) was initiated approximately 2 years ago to extend the research conducted into the serviceability of coiled tubing (CT) for underbalanced sour-well drilling and workover operations to higher-strength [90- to 110-ksi specified-minimum-yield-stress (SMYS)] grades. A significantly different and unique laboratory testing protocol is used in the present JIP research. The methodology calls for full-body tubing specimens that have been immersed in a sour solution of varying severity followed by testing in a bend-fatigue machine (BFM) to determine the low-cycle fatigue performance of high-strength CT materials degraded by exposure to a sour environment. Introduction Over many years, sour-well interventions have been performed with CT with relatively few failures. This success likely is the result of a variety of factors including time of exposure to sour conditions, adequate corrosion-protection programs, inherent resistance to hydrogen sulfide (H2S) damage by different CT materials, continuing research into the response of CT materials to sour conditions, and generally good CT-management procedures to minimize the risk of failure. A better understanding of the response of CT materials while exposed to sour downhole conditions has been the focus of research for several years. The testing used both coupon and full-body CT test specimens. The testing protocol included standard and custom-designed fixtures comprising slow-strain-rate testing (SSRT), axial low-cycle corrosion fatigue (LCCF), double-cantilever beam (DCB), Natl. Assn. of Corrosion Engineers (NACE) hydro-gen-induced cracking (HIC) and proof-ring tests, acoustic-emission (AE) measurements of crack-incubation times, constant-load tests (CLTs) on full-body specimens for sulfide stress cracking (SSC), BFM tests on CT sour-fatigue specimens, and a custom-designed hydrogen-diffusion vacuum cell for full-body CT specimens. Extensive metallographic examinations were performed to investigate failure mechanisms, fracture characteristics, and relationships to CT material properties and environmental severity. The initial JIP considered only CT-strength grades of 70 and 80 ksi and concluded with a recommendation that sour-service CT strings be limited to a maximum strength of 80-ksi yield. This was based on SSRT strain-to-fracture limitations for CT of higher strength using conventional A606/607 modified materials. Bend-fatigue lives of CT materials were estimated on the basis of LCCF measurements by assuming that the ratio of axial- to bend-fatigue life in sweet environments applied equally to sour conditions. The 80-grade sour CT limit served the JIP companies well but imposed an undesirable limitation on sour-well interventions for which higher-strength CT strings were required. The purpose of the full-length paper is to share the major interim findings of the second JIP, involving tests conducted over the last 1 1/2 years on higher-strength CT90, CT100, and CT110 grades. The majority of these data involves actual low-cycle fatigue tests on 7-ft-long CT specimens that were submerged in a sour solution for 4 days before fatigue loading to failure.

The effect of oxygen partial pressure on oxidation of an oil base liquid

Oil base liquids are used as the continuous phase in the majority of oil-based drilling fluid systems in conventional drilling and as the liquid phase in gasified liquids (aerated muds) in some underbalanced drilling operations. These fluids can also be used as the friction reducing agents in situations where a stuck drillstring is to be freed. Understanding the oxidation behavior of these liquids is paramount in the drilling industry, as it may lead to explosion and cause changes in the rheological properties. An experimental investigation was conducted to establish the effect of oxygen partial pressure on the oxidation of an oil base liquid that is used as the continuous phase in oil-based drilling fluid systems. It was aged for 7 days at an initial total pressure of 14 MPa and temperatures of 120, 135, and 150 °C in the presence of oxygen/nitrogen mixtures with oxygen mole fractions of 0.1019, 0.2189, 0.3971, and 0.7968. Characterization of the post-test gases and liquid phases before and after oxidation and measurements of the amounts of solids formed and viscosity of the clear and oxidized liquid were carried out. The results show that the extent of oxidation is reflected by the cumulative concentration of CO plus CO 2 as well as by the solids precipitated and viscosity. Pressure decline curves indicated two oxidation regions. The main consumption of oxygen occurs in the second region. The division between the two regions reflects an induction delay time. As the percentage of oxygen in the initial gas or oxygen partial pressure is increased, the induction delay time is decreased. Furthermore, the variations of the solid precipitated and viscosity ratio with oxygen mole fraction in the initial gas are in a direct relation to the amount of total CO and CO 2 produced. These three parameters are in turn related to the extent of oxidation as measured by the cumulative oxygen uptake.

Bottomhole Pressure Control During Drilling Operations in Gas-Dominant Wells

Summary To obtain an underbalanced pressure condition, nitrogen gas can be injected into the drillstring. Simultaneous injection of liquids and gases leads to a highly dynamic flow system. During pipe connections, pressure transients can cause the bottomhole pressure to rise above the pore pressure of the reservoir or fall below the reservoir collapse pressure. Migration of gas during pipe connection and inflow from the reservoir will also cause bottomhole pressure changes. This paper presents a methodology for controlling the bottomhole pressure during drilling operations in gas-dominant wells. The methodology incorporates a dynamic model of the well fluid flow and the well-reservoir interaction. Available control actions during the drilling process are the gas injection rate prior to the pipe connection and choke valve settings during the pipe connection. Measurement of the pump rates, pump pressures, choke pressure and the bottomhole pressure are also available to support the control actions. However, during pipe connections and in the event of transient signal failures, the bottomhole pressure measurements will be suppressed. The control methodology used is based on a nonlinear model predictive control system, which predicts the near-future behavior of the well, and uses these predictions to obtain the optimal choke settings. The model parameters are calibrated using measurements from the well to ensure that the model is suitable for the predictions. A field-based case with gas injection has been examined using this control methodology. The results indicate that model based control can be utilized in developing an automated and integrated pump rate and choke-control system for underbalanced drilling operations.

Basis of Design for Coiled-Tubing Underbalanced Through-Tubing Drilling in the Sajaa Field

Summary This paper describes the technical basis for the design and implementation of a coiled-tubing (CT) underbalanced drilling (UBD) campaign in the Sajaa field in Sharjah, UAE. The campaign calls for up to three multilaterals per well to be drilled underbalanced from the existing wells' through tubing. The paper describes the analysis, modeling, and result, and their implication in the selection of the operational approach. The key issues associated with the campaign, many of which were unique and challenging within the context of UBD, are discussed, with specific reference to the Sajaa-3 well, which was the first candidate. These include Temperature modeling and thermal considerations because of the high bottomhole temperature (BHT). Compositional pressure/volume/temperature (PVT) behavior of the produced fluid. Feasibility, underbalanced operability, and controllability of through-tubing UBD operations. Risk to underbalance from near-wellbore depletion of the reservoir caused by long-term production from the motherbore. Estimation and impact of the productivity of the motherbore and open laterals while drilling subsequent laterals. Risk from reservoir parameter uncertainties. Hole-cleaning risks. Designing for zero flare. Consideration of mechanical and hydraulic limitations to the drillable lengths. Equipment limitations. The paper details the analysis approach used, assumptions made, models used, and the results. It should be noted that the Sajaa produced fluid is a retrograde condensate and the inflow performance was nonlinear and, therefore, consideration of these in the basis of design (BOD) becomes critical. The goal of this work was not only to create the technical BOD for the well, Sajaa-3, but also to set the parametric limitations for a wide range of well types that were likely to be encountered in the continuation of the campaign. On the basis of this work, an approach to drilling and monitoring the well to minimize the risks was developed, and it formed the basis of the drilling operation. The ongoing campaign, now well past 30 wells, has been one of the most successful applications of CT UBD. The results of the drilling operation of Sajaa-3 and the subsequent wells and their implications to the design basis are also discussed.

The Effect of Oxidation on Viscosity of Oil-Based Drilling Fluids

Abstract True downhole rheological properties affect equivalent circulating density, hole cleaning, barite sag, surge/swab pressures during tripping, pump pressure, and bit hydraulics. Gelation and excessive viscosity are major concerns at high temperatures. When using oil-based drilling fluids lightened by an injection of de-oxygenated air (containing small amounts of oxygen, usually around five per cent) in underbalanced drilling (UBD) operations, there is a need to be able to predict the effect of oxidation on the viscosity of the oil-based muds as a function of temperature and pressure. This paper presents the results of an experimental investigation that was aimed at establishing the effect of oxidation on the viscosity of an oil-based drilling fluid. The drilling fluid was aged for 2.5 days and for 7 days in the presence of air at temperatures ranging from 100 to 150 °C and at pressures ranging from 14 to 44 MPa. The viscosity of the drilling fluid samples after aging is compared with the corresponding fresh samples (before aging). The results show that oxidation causes an increase in viscosity. The amount of the increase depends on the amount of oxygen reacted, which is a function of temperature, pressure, and time. The higher temperatures of the reactors lead to the higher increases in viscosity. Furthermore, at higher temperatures, solid (mostly coke) formation was observed. Introduction Underbalanced drilling (UBD), in which drilling fluid pressure is lower than reservoir pore pressure, has been studied by many researchers since the late 1980s. This method can improve the financial returns on drilling a well by reducing formation damage (especially in horizontal wells and depleted reservoirs), minimizing lost circulation (especially in fractured reservoirs and depleted reservoirs), increasing the penetration rate of drilling, increasing bit life, minimizing differential sticking, reducing stimulation, and improving formation evaluation(1). The underbalanced condition is created by using different drilling fluids such as dry air, nitrogen, natural gas, mist, stable foam, stiff foam, gasified liquids, glass bubbles, and liquids. In gasified liquids, a gas is injected into the liquid to reduce the density of the drilling fluid. In these mixtures, since the pressure difference between the formation and the wellbore is relatively low (1.72 - 3.45 MPa), these drilling fluids have fewer UBD related problems such as wellbore instability and formation fluid inflows(1). These specifications have made them the most appropriate drilling fluid in UBD. The unique performance of oils makes them preferable to water as the liquid phase in gasified liquids. Oils prevent clay swelling and they are resistant to contaminants. Their rheological properties are stable at high pressures and temperatures. Also, they are highly lubricious and non-corrosive. On the other hand, to prevent downhole fires, de-oxygenated air from membrane units with maximum five per cent oxygen is usually used as the gas phase in gasified liquids(2). The presence of oxygen causes the oil in the gasified liquid to be oxidized. In this paper, the effect of oxidation on the viscosity of the gasified liquids is investigated.

Oxidation Studies of Light Hydrocarbons for Underbalanced Drilling Safety

Abstract Economic development of many of the world's oil and gas reserves will require the application of improved drilling methods such as underbalanced drilling (UBD), a technique used to minimize formation damage in hydrocarbon reservoirs. In UBD operations, oxygen-reduced air is often injected along with an oil-based drilling fluid. This oxygen-reduced air may contain up to 5% oxygen which can react with oil and/or an oil-based drilling fluid during the drilling process to form potentially hazardous oxidized hydrocarbons. This could lead to the presence of potentially flammable or explosive mixtures in the surface piping and/or drill string, particularly the return annulus. Discussion of the oxidation behaviour of light oils and oil-based drilling fluids in the literature is limited, particularly regarding the effects of long-term exposure to oxygen. This paper characterizes the oxidation behaviour of two light oils and an oil-based drilling fluid using thermogravimetry (TG) and differential scanning calorimetry at 89 and 9,811 kPa (1,410 psig). The effects of core, pressure, and pre-exposure to oxygen on subsequent oxidation behaviour were studied. The results show that the addition of crushed core enhanced the oxidation reactions of the oil samples and delayed exothermic activity of the drilling fluid. Elevated pressure accelerated the oxidation reactions. Pre-exposure to oxygen was shown to increase the reactivity of the oils and drilling fluid during subsequent oxidation. The results indicate that under certain conditions, oxidation processes could begin at significantly reduced temperatures, possibly leading to ignition of the hydrocarbon mixtures. Introduction Underbalanced drilling is a technique used to minimize formation damage caused by drilling fluid invasion in hydrocarbon reservoirs. In order to achieve this technique, the hydrostatic head of liquid in the hole must be decreased, which can be accomplished by co-injecting nitrogen, natural gas, flue gas, vitiated air (air mixed with nitrogen), or oxygen-reduced air (air with some of the oxygen removed). The oxygen containing gases are the least expensive, but the possibility of flammable or explosive mixtures is an important safety consideration. Oxygen-reduced air is normally obtained using portable membrane separation systems but often still contains up to 5% oxygen by volume. During the drilling process, this oxygen can react with produced hydrocarbons or the oil-based drilling fluid to form peroxides and other oxidized hydrocarbons. These oxidized hydrocarbons could pose a safety hazard. While the oxygen concentrations under consideration are relatively small, the pressures and temperatures in the reservoir and along the drill string are significant. Added to this is the effect of prolonged exposure to oxygen. Drilling fluid is re-circulated throughout the drilling operation and may be used in multiple operations. It may also be exposed to oxygen while in the drilling mud tanks at the surface (during the drilling operation) and in storage tanks. Over time, the fluid may be gradually oxidized, posing an increased risk of combustion or even explosion. Due to the serious ramifications of undesired oxidation processes, it is necessary to establish fundamental information on the oxidation behaviour of the hydrocarbons involved in these processes.

Optimization of Big-Bore HP/HT Wells to Exploit a Low-Pressure Reservoir in Indonesia

Summary A big-bore, high-pressure/high-temperature (HP/HT) drilling and completion program of seven wells was undertaken under challenging conditions in a low-pressure carbonate reef in Indonesia. Significant enhancements made during the program substantially reduced well times and cost. These wells incorporated the largest tubingless gas completions in the world. Higher-than-anticipated initial flow rates were achieved through an innovative strategy to decrease formation damage. The wells were completed openhole without pumping kill-weight fluids at the end of underbalanced drilling (UBD) operations. These wells are the first to combine big-bore completions, drill-in liners, and UBD in individual wellbores.

Numerical Modelling of Cuttings Transport With Foam in Vertical Wells

Abstract In this study, a 1D unsteady-state two-phase mechanistic model of cuttings transport with foam in vertical wells has been developed. The model is solved numerically to predict the optimum foam flow rate and rheological properties to maximize the cuttings transport efficiency in vertical wells. Comparisons of model predictions with the field test data have shown that model predictions are in close agreement with the field test results. The numerical solution allows analyzing the effects of borehole geometry, drilling rate, foam rheological properties, gas and liquid flowing rates, and reservoir fluid influx on the cuttings transport efficiency when drilling vertical wells. Results of sensitivity analysis study are presented. Introduction The Alberta Energy and Utilities Board defines underbalanced drilling (UBD) as, "when the hydrostatic head of a drilling fluid is intentionally designed to be lower than the pressure of the formation being drilled(1)." The benefits of UBD include increased productivity by reducing formation damage, increased rate of drilling bit penetration, minimization or elimination of lost circulation, improved formation evaluation while drilling, reduction or elimination of differential pipe sticking, reduced stimulation requirements, and earlier production. Four drilling methods are referred to as UBD:gas drilling- air, nitrogen, and some hydrocarbon gases can be used as a circulating medium;flow drilling-fluid with density below the formation's hydrostatic gradient is used as circulating fluid;gas/ liquid drilling-a gas-liquid two-phase drilling fluid is used in the drilling operation; and,foam drilling-stable foams are used as cuttings transport media. Because of its variable density and good cuttings transport ability, foam is often used in UBD operations. The use of foam as a drilling fluid in many drilling operations and the results from various field cases have been well documented in the literature(2–5). Field application of the foamed drilling fluids is very often complicated because of the difficulties encountered in controlling fluid hydraulic properties. In general, compared to conventional (incompressible) drilling fluids, much less is known about the hydraulic and rheological properties of foamed drilling fluids, and even less is known about their cuttings transport capabilities. The complex flow mechanism involving in compressible drilling fluid circulation makes the determination of the optimum combination of liquid and gas injection rates very difficult. Other questions also exist, such as how to predict the bottom-hole pressure and how to combine different controllable variables in order to obtain optimum cuttings transport performance and bit hydraulics. In order to realistically simulate the hydraulics of foam drilling, factors including foam rheological properties, drag coefficient of cuttings in foam, formation fluid influx, drillpipe eccentricity, and drilling rate need to be considered in any modelling study. An example of such a modelling study is presented in this article. Literature Review The following section provides a review of literature on cuttings transport in general, the rheology of foam, and of cuttings transport with foam. Cuttings Transport Hole cleaning (cuttings transport) is one of the major factors affecting cost, time, and quality of directional, horizontal, extended reach, and multilateral oil/gas wells.

Numerical Modelling of Cuttings Transport With Foam in Horizontal Wells

Abstract In this study, a one-dimensional unsteady-state two-phase mechanistic model of cuttings transport with foam in horizontal wells has been developed. A new critical deposition velocity correlation for foam-cuttings flow is introduced. The model is solved numerically to predict cuttings bed height as a function of the drilling rate, the gas and the liquid injection rates, the rate of gas and liquid influx from the reservoir, and the borehole geometry. Results of the sensitivity analyses study are presented. Introduction The Alberta Energy and Utilities Board defines underbalanced drilling (UBD) as "when the hydrostatic head of a drilling fluid is intentionally designed to be lower than the pressure of the formation being drilled, the operation will be considered underbalanced drilling(1)." The benefits of UBD include increased productivity by reducing formation damage, increased rate of drilling bit penetration, minimization or elimination of lost circulation, improved formation evaluation while drilling, reduction or elimination of differential pipe sticking, reduced stimulation requirements, and earlier production. Foam is favourably used in UBD operations because of its variable density and good cuttings transport ability. Field application of the foamed drilling fluids is very often complicated because of the difficulties encountered in controlling their hydraulic properties. The complex flow mechanism involved in the circulation of foam makes a determination of the optimum combination of liquid and gas injection rates very difficult. Other questions also exist, such as how to predict the bottom-hole pressure and how to combine different controllable variables in order to obtain optimum cuttings transport performance and bit hydraulics. In order to simulate the hydraulics of the foam drilling realistically, factors including foam rheological properties, drag coefficient of cuttings in foam, formation fluid influx, drillpipe eccentricity, and drilling rate need to be considered in any modelling study. An example of such a modelling study is presented in this article. Okpobiri and Ikoku(2) developed a semi-empirical model for cuttings transport with air, mist, and foam in vertical wellbores. Harris et al.(3) suggested that foam-particle flow for hydraulic fracturing could be treated as a homogeneous flow. Medley and Liu(4) presented a one-dimensional steady-state foam flow model by modifying earlier models presented by Lord(5) and Sp?rker et al.(6) Drill cuttings and gas were considered as homogeneous internal phases of foam and the mixture properties were used to solve the steady-state mechanical energy balance equation. Owayed(7) developed a one-dimensional steady-state computational model for UBD by assuming a homogeneous flow of foam in a vertical well. The model included the effect of water influx into the wellbore during drilling. Valk? and Economides(8) established a method that combined the principle of volume equalization(9) with the method of constant-internal-phase(3) for foam-proppant flow. Guo et al.(10) presented an analytical model to estimate the bottom hole pressure when drilling with foam in directional wells. They recognized that the compressibility of foam could cause the cuttings concentration at a given depth to be different from the ones at the surface, rendering inappropriate calculations of the minimum required cuttings transport velocities.

A New Comprehensive, Mechanistic Model for Underbalanced Drilling Improves Wellbore Pressure Predictions

Summary A new comprehensive, mechanistic model that allows more precise predictions of wellbore pressure and two-phase flow parameters for underbalanced drilling (UBD) is proposed. The model incorporates the effects of fluid properties and pipe sizes and, thus, is largely free of the limitations of empirically based correlations. The model is validated against actual UBD field data and fullscale experiments in which the gas and liquid injection flow rates as well as drilling fluid properties were similar to those used in common UBD operations. Additionally, a comparison against two different commercial, empirically based UBD simulators shows better performance with the mechanistic model.

Oxidation Characteristics of Light Hydrocarbons for Underbalanced Drilling Applications

This investigation is directly relevant to various applications associated with the safety aspects of underbalanced drilling operations where de-oxygenated air may be co-injected with oil-based drilling fluid. However, de-oxygenated air often still contains up to 5% oxygen by volume. This residual oxygen can react with oil during the drilling process, thereby forming potentially hazardous oxidized hydrocarbons and compromising the safety of drilling operations. This article examines the conditions and processes by which oxidation reactions occur and may be helpful in reducing risk in drilling operations. This project characterizes the oxidation behavior of several oils and a typical oil-based drilling fluid at atmospheric and elevated pressures using thermogravimetry (TG) and pressurized differential scanning calorimetry (PDSC). Tests performed on mineral matrix (core) from the oil reservoirs showed no reactivity in both inert and oxidizing atmospheres. In an inert atmosphere, tests on all hydrocarbon samples showed only vaporization, no reactivity. In an oxidizing environment, the tests on hydrocarbons showed several oxidation regions. The presence of core had no effect on the behavior of the hydrocarbons tested in an inert atmosphere but accelerated the higher temperature oxidation reactions of the oil samples. The oil-based drilling fluid exhibited the opposite effect—the presence of core material retarded the oxidation reactions. This is perhaps due to the presence of an oxygen scavenger reacting with oxygen-containing clays present in the mineral matrix. In all tests performed on mixtures of hydrocarbon and core in oxidizing atmospheres, elevated pressures resulted in acceleration of the lower and higher temperature reaction regions.

Wellbore Stability Simulations for Underbalanced Drilling Operations in Highly Depleted Reservoirs

Summary The need to increase productivity and to reduce drilling formation damage favors the use of underbalanced drilling technology. The main idea is to drill with equivalent circulating densities (ECD) that are less than the formation pore pressure and to avoid contact between the drilling fluid and the formation. In highly depleted reservoirs, pore pressures can be very low. Therefore, extremely low-density fluids, such as foams, are used to achieve circulating densities lower than the pore pressure. In such cases, the induced modification of the in-situ stresses has to be supported mainly by the rock, with little contribution from the drilling fluid pressure. In that sense, the application of underbalanced drilling depends on the mechanical stability of the drilled formation, among other factors. In general, poorly consolidated, depleted formations are not suited for that technology. This paper presents the wellbore stability simulation performed to establish the feasibility of using underbalanced drilling in highly depleted reservoirs in western Venezuela. The in-situ stress field and the mechanical properties of the formation were obtained. Pore pressure as low as 800 psi at 5,500 ft (2.7 lb/gal equivalent fluid density) was measured. The finite difference method and an elastoplastic constitutive model was used to obtain the new stress, deformation, and pore pressure distribution. The undrained condition (immediately after the wellbore is drilled) as well as the drained condition were analyzed. The analysis showed that horizontal wells could be drilled in an underbalanced condition with low instability risk. Following the recommendations, four horizontal wells were drilled in underbalanced conditions. Values as low as 2.0 lb/gal ECD were used to drill the wells, and no wellbore instability problems were reported. Production tests showed an enormous increase in the well productivity index in comparison with conventional overbalanced drilling.

Foam Flow Modelling for UBD Applications

Introduction While naturally occurring foaming crude oils can present unwanted operating problems in production facilities, specially created foams have been used for a variety of applications in the petroleum industry for many years. These include, but are not limited to, foam drilling fluids, foam stimulation fluids, and foams used for controlling the mobility of various other fluids and solids. More recently, interest in the properties and behaviour of foams has increased significantly due to the use of foam drilling fluids in underbalanced drilling (UBD). In UBD, the bottomhole pressure is intentionally maintained lower than the reservoir pressure during the drilling operations. This underbalanced state is usually achieved by using a gas-liquid mixture in place of traditional (and much heavier) drilling mud. The combination of low density (i.e., low pressure loss due to hydrostatic head) and high viscosity (i.e., good capacity for cuttings transport) makes foam a strong candidate as a UBD drilling fluid. Because of the technical complexity of UBD, the success of such operations can depend on the accuracy of a detailed engineering analysis that is typically carried out both before and during the actual drilling. Accurate computer modelling of the hydraulics of flow in the drill string, the bottomhole assembly (BHA), and the return annulus is a critical component of this design and performance analysis process. When foam is used as the drilling fluid, this generally means that complex questions of foam rheology and flow behaviour must be answered using technology which is simple and can be applied with relatively limited measured data. A very important component of the modelling process, from a cuttings transport perspective, is the determination of the local foam velocity in the entire return annulus, due to changes in the cross-sectional area (e.g., between the BHA and the open hole, the open hole-drill tubing, presence of casings). Since this parameter can (and does) change during drilling because of reservoir fluid influx, the modelling must be an ongoing part of the monitoring. This paper examines the extent to which relatively simple methods, based on limited data, can be applied to give the accurate performance predictions required for the planning of complex underbalanced drilling operations. Some of the basic concepts of foam rheology are introduced, and some recent work based on detailed measurements gathered during a UBD pilot study is reviewed, along with several applicable technologies. Foam Rheology Foam is created by mixing gas, liquid, and a foaming agent (i.e., surfactant). The latter stabilizes the foam by reducing the gas-liquid surface tension. The liquid phase is frequently an aqueous solution of one or more polymer materials that are used to increase the effective viscosity of the resulting foam over that obtained with pure water. For modelling purposes, foam is generally treated as a single phase fluid using a homogeneous mixture density and an effective viscosity. The latter is computed using a yield-power law model (frequently referred to as a Herschel-Bulkley model), over some range of conditions.