Underbalanced Drilling Operations Research Articles

Underbalanced Drilling Dynamics: Two-Phase Flow Modeling and Experiments

Summary A dynamic model based on the drift-flux formulation is presented for treating transient phenomena in underbalanced drilling (UBD) operations. A set of mechanistic steady-state procedures for dealing with the definition of flow patterns, pressure drops, gas volumetric fractions, and in-situ velocities completes the model. The iteration between those mechanistic laws and the conservation equations is discussed. The numerical solution uses a composite explicit scheme, which reduces false oscillations and numerical diffusion. In addition, details about the boundary treatment are also presented. Model predictions are validated through comparison to fullscale experimental data in two distinct transient situations: first, a typical liquid unloading, and second, the injection of a high-velocity single pulse and a sequence of two pulses of a gas/liquid mixture. The comparisons address model limitations and improve the understanding of the physics involved.

Wellbore-Stability Simulation for Underbalanced Drilling in Highly Depleted Reservoirs

This article is a synopsis of paper SPE 65512, "Wellbore Stability Simulations for Underbalanced Drilling Operations in Highly Depleted Reservoirs," by J.G. Parra, E. Celis, SPE, and S. De Gennaro, SPE, PDVSA Intevep, originally presented at the 2000 SPE International Conference on Horizontal Well Technology,Calgary, 6-8 November.

Application of Multiphase Flow Methods to Horizontal Underbalanced Drilling

Abstract Multiphase flow can be present in all aspects of underbalanced drilling. This paper outlines the ways in which multiphase flow pressure loss calculations can be used by drilling engineers in the design and optimization of underbalanced drilling operations. Then, detailed field measurements for several horizontal wells drilled underbalanced with coiled tubing are used to evaluate the application of existing pressure loss calculation methods to this unique application. Introduction Underbalanced drilling (UBD) is rapidly gaining popularity in the oil and gas industry. The Alberta Energy and Utilities Board ID94–3(1) defines underbalanced drilling as follows: "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. The hydrostatic head of the drilling fluid may be naturally less than the formation pressure or it can be induced. The induced state may be created by adding natural gas, nitrogen, or air to the liquid phase of the drilling fluid. Whether induced or natural, this may result in an influx of formation fluid which must be circulated from the well and controlled at surface." Benefits of Underbalanced Drilling Maintaining the pressure in the wellbore below the reservoir pressure allows reservoir fluids to enter the wellbore while UBD operations proceed, thus preventing flow of drilling fluids (and associated solids) into the formation, thereby minimizing or even eliminating formation damage. This is of particular importance in the drilling of horizontal wells as the formation is exposed to the drilling fluids for an extended period of time. Although formation damage reduction is the most widely recognized benefit of underbalanced drilling, several additional benefits are outlined below: Increased Penetration Rates Underbalanced drilling can achieve higher rates of penetration due to reduced "chip holdown" and decreased hydrostatic pressure at the bit face. Reduction in "chip holdown" refers to easier removal of drilled solids from the vicinity of the drill bit due to the flow of drilling and reservoir fluids, thus allowing the bit to drill into fresh rock continuously. The decreased hydrostatic pressure at the bit face reduces stress in the rock being drilled, allowing it to fail more easily. The experience of drilling engineers(2) familiar with UBD is that the rate of penetration can be increased by between three and ten times that of conventional drilling. Minimal Lost Circulation Underbalanced drilling gives better control in situations where fractured, low pressure, or high permeability formations may lead to the loss of drilling fluids and the associated problems that can cause. Evaluation While Drilling Data acquired in real time during underbalanced drilling operations allow for both the short term on site optimization of the UBD operation, and the longer term assessment of the well's potential. On site, the data acquired can be used to optimize drilling parameters such as the well's horizontal length, vertical depth, and orientation. Other data that can be obtained, useful both on site and long term, include fluid properties, productivity, and geological interpretations of the formation.

Performance of Positive Displacement Downhole Motors Under Two-phase Flow

Abstract Positive displacement motors have been used extensively in underbalanced drilling operations. Motor performance characteristics are essential to ensure drilling success and efficiency. Pressure drop across the motor must be understood to accurately simulate underbalanced conditions bottom-hole. In this study, five positive displacement motors were tested under a variety of mixing ratios of nitrogen and water with different back pressures. Comparing with the baseline test at maximum liquid flow rate, the replacement of water with nitrogen decreased the motor performance by as much as 95% in terms of maximum torque output and maximum mechanical power output. The pressure drop across the motor was lower with commingled fluid, and the greater the nitrogen ratio, the lower the pressure drop across the motor. It was also found that back pressure decreased the motor performance. This paper also discusses the testing procedures, the results, and how commingled fluid affects underbalanced drilling operations. Introduction Underbalanced drilling is a process where the aerated or nitrified fluid is pumped through the drilling string to the bottom-hole. The pressure at the bottom-hole should be less than the formation pressure and allow wellbore fluids to flow during the drilling period. The circulated fluids power the downhole motor to rotate the drill bit to cut the formations. The cuttings are then picked up by the jetting action of the nozzles on the bit and transported by the drilling fluid to the surface through the annulus between the drilling string and casing. Currently, there are two main types of downhole motors: positive displacement (PDM) and turbines. In general, turbine motors have a high rotational speed, whereas PDMs have a low speed and high torque output. The application of turbine motors has been limited to less than 1% of the total footage drilled in the United States. PDMs are the most widely used in the world at the present time(l). The PDM is a hydraulically-driven downhole motor that is based on the Moineau principle. It consists of a bypass valve, power section, transmission assembly, and bearing assembly (Figure 1). In the power section of a PDM, a helicoidal rotor with one or more lobes is placed eccentrically inside a stator having one more cavity than the rotor. This difference between the rotor/stator lobe configuration creates cavities. Under pressure, the drilling fluid will drive the rotor in an eccentric rotation, which is then translated into concentric rotation through the transmission assembly and transferred to the drill bit. Motor performance characteristics are essential to ensure drilling success and efficiency. Pressure drop across the motor must be understood to accurately simulate underbalanced conditions at the bottom-hole. In general, motor suppliers provide motor performance data at the baseline condition. A baseline test consists of testing a motor at designated flow rates with water and motor outlet open to atmosphere. When conventional drilling fluids are used, output torque and bit speed are determined based on differential pressure and flow rate. Due to the compressible nature of gas, it is difficult to predict the gas flow and RPM/torque relationship based on baseline tests.

Special Core Analysis Designed to Minimize Formation Damage Associated With Vertical/Horizontal Drilling Applications

Abstract Laboratory testing of core material to optimize drilling fluid composition and procedures has been used for many years to minimize invasive formation damage of a mechanical, chemical or biological nature which can occur during the drilling of horizontal or vertical wells. This paper discusses the limitations of past practices such as the use of non-representative core or fluids, non-preserved or non-restored state core, ambient conditions of temperature and overburden pressure, direct injection of muds/filtrates into samples and unrealistically high drawdown gradients for cleanup. The paper describes the current technology used to eliminate many of these concerns and also to extend drilling fluid evaluation technology to extremely heterogeneous carbonate and sandstone formations, fractured formations and specific test equipment and procedures used to evaluate the effectiveness and utility of underbalanced drilling programs. Introduction Formation damage occurring during the drilling of horizontal or vertical wells can be a significant cause of ultimate reduced productivity in both oil and gas bearing formations. In some cases a combination of petrographic and special core analysis techniques is used to evaluate the potential effectiveness of proposed drilling fluids and procedures, prior to incurring the actual cost and risk of implementation. These tests are conducted to obtain a better assessment of the risk associated with the use of proposed drilling fluids and to optimize the fluid and procedures which will be utilized in a given horizontal or vertical well operation to maximize ultimate productivity of oil or gas. Considerable advances have been made in recent years in both the execution and interpretation of the results of laboratory coreflow tests to obtain representative results for effective field design of well drilling programs. This paper reviews test procedures used in the past, describes potential problems with these test procedures, and outlines current laboratory technology with respect to specialized laboratory testing to evaluate drilling induced damage for vertical and horizontal well applications. Common Mechanisms of Formation Damage During Overbalanced and Underbalanced Drilling Operations A number of authors have provided a detailed discussion of potential formation damage mechanisms which may occur during overbalanced and underbalanced drilling operations. A summary of this work is provided in the literature(l-31). These mechanisms include: Mechanically Induced Formation Damage Physical migration of in situ fines and mobile particulates. The introduction of extraneous solids of either an artificial nature (i.e., weighting agents, fluid loss agents, or artificial bridging agents) or naturally occurring drill solids generated by the milling action of the drill bit on the formation. Relative permeability effects associated with the entrainment of extraneous aqueous or hydrocarbon phases within the porous medium. Formation damage effects associated with the use of extreme underbalance or overbalance pressures and associated fines migration or spontaneous imbibition phenomena. Direct mechanical glazing phenomena associated with bitformation interactions. This particular damage mechanism is usually associated with gas drill operations where high bitrock temperatures commonly occur.

Underbalanced Drilling And Completion Operations to Minimize Formation Damage-Reservoir Screening Criteria For Optimum Application

Abstract Underbalanced drilling (UBD) is becoming increasingly utilized worldwide as an alternative technique to conventional overbalanced drilling to reduce invasive near wellbore formation damage problems in oil and gas producing formations. It is also increasingly used as a method for low damage completion of injection or disposal wells. Properly designed and executed underbalanced drilling operations can eliminate or significantly reduce formation damage concerns with respect to such phenomena as mud or drill solids invasion, lost circulation, fluid entrainment and trapping effects, and potential adverse reactions of invaded drilling or completion fluids with the reservoir matrix or in situ reservoir fluids. Underbalanced drilling and completion operations are not necessarily a panacea for all formation damage problems and, if an underbalanced drilling operation is poorly designed and executed, there is the potential for greater damage than if a properly implemented overbalanced operation had been used. This paper reviews reservoir characteristics and properties which should be considered to ascertain if a reservoir is a prime candidate for a UBD operation. This would include such items as reservoir lithology, matrix composition, micro or macrofracturing and heterogenous porosity features, such as large interconnected vug systems, permeability variations, heterogeneity, reservoir pressure, reservoir fluid types and other factors. A discussion of optimum operating parameters with respect to the design and execution of an underbalanced drilling operation for a particular reservoir application is provided. The selection of the appropriate type of equipment to be used (jointed pipe vs. coiled tubing), mode of underbalance condition generation (through string injection, parasite string or microannular injection), base fluid selection (oil, water or gas), gasification medium selection (nitrogen, oxygen content reduced air, air, flue gas, natural gas) and surface equipment considerations for a given UBD operation, are reviewed. Introduction Underbalanced drilling (UBD) is utilized with increasing frequency as a method to increase oil and gas well profitability by reducing invasive formation damage to maximize production rates, increase ROP to reduce drilling time and costs, and to obviate various technical problems associated with typical overbalanced drilling operations such as differential sticking and lost circulation. There have been many cases in which underbalanced drilling has been spectacularly successful in providing added value to the operators utilizing the technology. These examples have been highly publicized and have resulted to some degree in the rapid increase in popularity of the technology. Unfortunately, there have also been many instances where the results from underbalanced drilling operations have been less successful. In many cases this is not due to a deficiency in the basic technology of underbalanced drilling, but rather is related to the faulty execution or inappropriate application of the UBD technology. Not every reservoir is an ideal candidate for an underbalanced drilling operation and, in some cases, distinct disadvantages may exist in trying to execute an UBD operation in comparison to a simpler more conventional overbalanced application. This paper reviews some of the key criteria and design considerations that should be examined when evaluating a well and provides a discussion of screening criteria to be applied to ascertain if a given formation is, in fact, a viable UBD candidate.

Safety Considerations For Underbalanced Drilling of Horizontal Wells Using Air Or Oxygen-containing Gas

Abstract The underbalanced drilling of horizontal wells involves the injection of gas either with the drilling mud or via a separate string to that through which the drilling mud is injected. Nitrogen is generally used to prevent fire and explosion hazards during drilling; however, this adds considerably to the cost. Costs can be reduced by the replacement of nitrogen injection with normal air, de-oxygenated air or vitiated air (air/nitrogen mixture), but this increases the potential for flammable, or explosive, mixtures to be present in the wellbore and surface piping during underbalanced drilling. Safe operational ranges of oxygen-containing drilling mud mixtures can be determined if the flammability of the mixtures is known. Several studies were carried out to determine the flammability of these mixtures as a function of pressure to minimize the potential for downhole and surface explosion, and to set out the guidelines for safe underbalanced drilling operations of horizontal wells. A specially-designed apparatus was calibrated utilizing methane/air mixtures, then the ignition and the flammability characteristics of various mixtures of air, drilling mud and live oil were established over a range of pressures from atmospheric to 2,100 kPa at 21 ° C. Additional tests were carried out utilizing drilling mud, live oil, and either a nitrogen/air mixture (vitiated air) or de-oxygenated air. Flammability limits were determined for each series of experiments, and the results used to design the safety features for the underbalanced drilling of several horizontal wells. The field trials proved successful, and significant cost savings were realized. Introduction A growing demand for maximizing heavy oil production in a cost-effective manner has led to the development of a variety of novel technologies. One such technology involves the drilling of wells employing underbalanced or near-balanced drilling procedures in order to minimize the possibility of formation damage due to drilling fluid invasion. There is a potential for flammable, or explosive, mixtures to be present in the wellbore and surface piping during this type of drilling operation. In underbalanced drilling, normal air or an oxygen-containing gas (usually vitiated air, which is air mixed with nitrogen, or de-oxygenated air, which is air with some of the oxygen removed) is injected and/or circulated with the drilling mud to reduce the potential for formation damage due to whole mud, fluid filtrate and solids invasion into the hydrocarbon-bearing formation. The study presented in this paper was performed to determine the safe conditions for underbalanced drilling of Husky Wainwright Well 15B-31-44-4 W4 using compressed air and liquid nitrogen. This well, located in the Camrose reservoir, which is a fractured vugular carbonate at 690 m TVD, was proposed to be drilled 550 m laterally(1). The main objectives of this investigation were:To establish the flammability of mixtures of air, live heavy oil and drilling mud as a function of pressure, andTo determine the optimum composition of vitiated air (nitrogen- air mixture), based on the flammability data, in order to minimize the potential for ignition or explosion during the underbalanced drilling operation.

Optimization of Underbalanced Drilling Operations to Improve Well Productivity

Abstract In excess of 100 horizontal wells have been drilled underbalanced in the Weyburn Unit in S.E. Saskatchewan. Initial underbalanced drilling (UBD) operations intermittently placed horizontal wellbores in an overbalanced condition due to large variations in bottomhole pressure while drilling. Pressure fluctuations acting at the sandface were found to have resulted in reduced productivity in many of these wells by forcing drilled fines into productive pore throats. In an effort to reduce formation damage, UBD procedures were modified to better maintain an underbalanced condition and decrease the magnitude of bottom-hole pressure surges occurring during drilling operations. Initial steps taken to optimize Weyburn UBD operations included the use of electromagnetic survey tools, the acquisition of bottomhole pressure data and adding on-site UBD technical support. This led to the implementation of basic operational practices such as minimizing survey and connection times, displacement of the drill string past the first float to nitrogen prior to making connections, shutting in the annulus during connections, and adjustment of circulation parameters to account for formation fluid influx, penetration rates, and wellbore length. Wells drilled using the adjusted UBD practices resulted in approximately 40% more production, on average, than those wells drilled using the initial UBD practices. Introduction During conventional drilling operations the hydrostatic pressure exerted by the column of drilling fluid is, by design, greater than the formation pressure of the zone being penetrated. This overbalanced pressure differential promotes both drilling mud filtrate and particle invasion into the target formation and can reduce well productivity. Fortunately, stimulation techniques can reach beyond damaged zones in most cased wellbore applications to provide access to undamaged reservoir. Horizontal well bores, however, are generally more susceptible to drilling induced damage than vertical wellbores due to their increased exposure time to drilling fluids and increased circulating pressure losses as horizontal length increases(1). Many horizontal well applications also involve underpressured or pressure depleted reservoirs, and are frequently completed open hole. Damage removal in these wells is often costly. Underbalanced drilling (UBD) has emerged as an effective technique to reduce weIIbore damage, particularly when drilling horizontally into underpressured formations. UBD refers to the drilling process involving downhole circulation systems designed with a hydrostatic pressure lower than the pressure of the reservoir section being penetrated. Properly designed, the process results in a continuous and controlled influx of formation fluids into the wellbore while drilling. Although both the magnitude of wellbore damage and the impact that damage has on well productivity are difficult to quantify in most horizontal wells, laboratory evaluations(2) have measured the effect that overbalanced, balanced, and underbalanced conditions can have on formation damage. These tests have demonstrated that underbalanced pressure conditions can significantly reduce the potential for formation damage both y mud solids blocking pore throats and by filtrate invasion. Field tests(3) have similarly identified benefits of UBD.

Underbalanced Drilling And Formation Damage-Is It a Total Solution?

Abstract Underbalanced drilling has been utilized as a technique to minimize damage- due to whole mud, fluid filtrate and solids invasion into high permeability" and fractured formations and formations which exhibit a high degree of sensitivity to invaded fluids. While it is generally generally accept that the process of underbalanced drilling reduces the potential for formation damage, the possibility of invasive formation damage ‘still’ exists in certain reservoirs where the initial saturation (either hydrocarbon or water) is substantially less than the irreducible saturation of the phase which is being utilized in the drilling or completion process. This situation could conmonly occur when drilling in an underbalanced mode in gas reservoirs that are in a desiccated state and exhibit abnormally low initial water saturations, or in oil and gas reservoirs in a portion of the reservoir significantly, above the oil-water or water-gas transition zone. Although dynamic fluid flow is occurning from the formation during the drilling process, the circulating drilling fluids are in continual contact with the formation face. At the low initial liquid saturations which exist in these reservoirs, there is an ability for strong countercurrent spontaneous imbibition effects which can, in some situations counteract pressures that are far greater than the apparent underbalance pressure which is present during the drilling or completion process. If the formation contains potentially sensitive materials .sucl;1 as deflocculatable fines or swelling clays, exhibits the potential for the formation of in situ emulsions or the possibility of permeability -reductions due to the establishment of near wellbore aqueous phase traps, these phenomena could occur and cause potential reductions in productivity of the well. The majority of underbalanced drilling fluid systems are not designed with ultra-low fluid loss in mind as extensive mud losses to the formation are not contemplated. However, the fact that a stable filter cake is not developed during underbalanced drilling, due to continual inflow from the formation face, could increase the severity of losses to the formation if underbalanced conditions are not maintained 100% of the time during drilling and completion. Overbalanced 'pulses may occur due to mud pulsed logging programs, bit trips or mechanical problems which may occur during the drilling process of most horizontal wells. This results in the majority of underbalanced horizontal wells being overbalanced for some fraction of the time during the drilling and completion process. A discussion of mechanical damage due to near surface glazing effects is also presented as a possible mechanism of damage which can occur during some underbalanced drilling operations. The paper elaborates on potential damage scenarios based on the results of specific laboratory tests which have been conducted and reviews a variety of new techniques 'which have been designed to evaluate and pre-screen fluids and procedures prior to costly and possibly ineffective implementation of an underbalanced drilling operation in the field.