Drilling Fluid Flow Research Articles

Analysis of the Influences of Drilling Fluid Flow Velocity inside and outside the Drilling String on Drilling String Lateral Vibration Frequency

Drilling string lateral vibration is a severe damage,which is influenced by many complex factors. This paper makes a theoretical analysis of the drilling string lateral vibration frequency caused by drilling fluid flow velocity inside and outside the drilling string, which supplies some evidence for calculating rotation speed of rotary table and reduces the times of drill string resonance. The influence of drilling fluid flow velocity inside the drilling string on lateral vibration frequency is analyzed by Euler method. Besides, through the use of mechanical analysis method and the introduction of additional mass coefficient. A differential equation of drilling string lateral vibration with the consideration of drilling fluid flow velocity inside and outside the drilling string is built up, and a mathematical model of drilling string lateral vibration under the influence of drilling fluid flow velocity is obtained. Field applications indicate that this model is in agreement with the field condition.

A laboratory investigation of the factors controlling the filtration loss when drilling with colloidal gas aphron (CGA) fluids

Abstract The colloidal gas aphron (CGA) drilling fluids are designed to minimize formation damage by blocking pores of the rock with microbubbles and reducing the filtration loss. In order to gain a better understanding of the factors controlling the pore blocking mechanisms of microbubbles, a set of core flooding experiments was conducted by using various CGA drilling fluid formulations. The differential pressure drop along the sand-pack was measured. Effects of CGA fluid composition, flow rates, type of reservoir saturating fluids, permeability and wettability on the resistance to CGA drilling fluid flow through porous media (i.e., pressure drop due to CGA fluid flow) have been investigated. An increasing resistance to flow of CGA drilling fluids through porous media was observed as more CGA fluid was injected. Results confirmed that CGA microbubble build-up across the pore structure could establish an effective seal for controlling filtration loss.

Special aspects of ice drilling and results of 5G hole drilling at Vostok station, Antarctica

AbstractThis paper documents the drilling of the 5G deep hole at the Russian Vostok station, Antarctica. The hole construction is described and the specifications of the drill and surface drilling equipment are given. The peculiarities of drilling at various depths are considered. Based on the extensive experimental data collected at Vostok station, the processes occurring at the hole bottom are investigated: ice breaking and cutting, bottom cleaning and chip transport, and accumulation of the chips in the screen. The main factors affecting ice-drilling efficiency are the coarseness of ice crystals and the ice temperature. When ice crystal size exceeds 10 mm the cutting efficiency sharply decreases, and when ice temperature exceeds –5°C the chip transport from the hole bottom and the chip density in the screen are reduced. The drill advance then becomes irregular, slows down and may even be terminated in some cases. Optimal values of the basic drilling parameters (e.g. cutter head rotation rate and drilling fluid flow rate in the circulation system) have been deduced empirically. The designs of the cutter head and circulation system including screens have been significantly modified, allowing the drilling process to proceed at a normal rate even in the vicinity of the subglacial lake surface.

Analysis of the Influences of Drilling Fluid Flow Velocity inside and outside the Drilling String on Drilling String Lateral Vibration Frequency

Drilling string lateral vibration is a severe damage,which is influenced by many complex factors. This paper makes a theoretical analysis of the drilling string lateral vibration frequency caused by drilling fluid flow velocity inside and outside the drilling string, which supplies some evidence for calculating rotation speed of rotary table and reduces the times of drill string resonance. The influence of drilling fluid flow velocity inside the drilling string on lateral vibration frequency is analyzed by Euler method. Besides,through the use of mechanical analysis method and the introduction of additional mass coefficient. A differential equation of drilling string lateral vibration with the consideration of drilling fluid flow velocity inside and outside the drilling string is built up, and a mathematical model of drilling string lateral vibration under the influence of drilling fluid flow velocity is obtained. Field applications indicate that this model is in agreement with the field condition. Key words: mass coefficient;differential equation; drilling fluid flow velocity; lateral vibration frequency;

Multi‐physics design of a novel turbine permanent magnet generator used for downhole high‐pressure high‐temperature environment

Conventional turbine generator with magnetic coupling is becoming difficult to meet large electric power demand because of complex structure, low-efficiency and lower-power density in the downhole environment. In this study, a novel stator-sealed downhole turbine permanent magnet generator is presented. Conventional magnetic coupling is replaced by the separate seals of stator and magnet, allowing drilling fluid flow from the air gap of the generator through the sliding bearings. Then the effective lubrication of bearings and cooling of generator are achieved as well as high power density and pressure balance of the generator. The Halbach array and fractional slot winding are both utilised to further improve the power density, reduce the loss, and keep a lower voltage regulation. A multi-physics design procedure is proposed in this study involving the electromagnetic, thermal, fluid and stress field for downhole harsh environment. In the design process, the coupling electromagnetic–thermal analysis based on two-way multiple iterations and the coupling thermal–fluid analysis based on conjugate heat transfer is incorporated to guarantee high accuracy. The experimental data are compared with simulation results to verify the correctness of proposed method and the feasibility of this novel turbine generator for downhole application.

Reelwell Drilling Method Makes Use of Dual-Conduit Drillstring

Young Technology Showcase The Reelwell drilling method (RDM) is a drilling solution using a unique flow arrangement. RDM employs drillpipe where an inner string is installed to form a dual-conduit drillstring. This allows the fluid and cuttings from the bottom of the well to return to the surface through the inside of the drillstring. RDM enables improved hole cleaning, reduced possibility for washouts, and improved downhole pressure control. RDM has unique features for managed-pressure drilling (MPD) and extended-reach drilling (ERD). A special feature of RDM is that it allows MPD and underbalanced operations (UBO) to be performed with a heavy fluid in the well annulus by means of the Reel-well multigradient system. This implies that MPD and UBO can be performed with two well barriers and, thus, improved safety. RDM started its development in 2004 and has been through several full-scale tests. In 2009, the system was used for the first time for directional drilling with MPD in a land well in Norway. In 2010, RDM was used for the first time in a live shale gas well in Canada. These operations have validated RDM and its features and are important steps proving its potential. Arrangements and Rig Description Fig. 1 presents a schematic of the system. The system is based on a dual-channel drillstring, where one flow path is used to convey the drilling fluid into the well down to the bit and the other channel is used for the return fluid from the bottom of the well. This arrangement implies a modification of the drilling fluid flow arrangement. The following special tools and arrangements are used. Dual Drillstring (DDS). The DDS can be manufactured in various sizes; the current size used consists of 6⅝-in. drillpipe adapted with 3.5-in. inner pipe that has been fitted with stab-in innerstring connectors. The DDS is handled on the rig floor as conventional drillpipe. Topdrive Adapter (TDA). A dual-conduit swivel connects the rig’s topdrive unit to the dual drillstring and enables rotation of the drillstring by the top drive. The TDA is connected to the surface flow control unit through an additional separate stand pipe and mud hose. Flow Control Unit (FCU). The FCU is a control valve arrangement equipped with pressure and flow sensors and permits accurate control of the in/out flow in the RDM system. The control unit connects to all the flow paths of the system, as shown in Fig. 1. Dual-Float Valve (DFV). The DFV is a device located at the bottom of the DDS. The DFV includes a flow cross-over from the well annulus into the return channel of the DDS. The DFV incorporates valves that isolate the drillstring from the well during connections. In the fail-safe default position, the DFV closes both channels of the DDS. A conventional BHA is made up below the DFV.

A Novel Tool To Improve the Rate of Penetration–Hydraulic-Pulsed Cavitating-Jet Generator

Summary On the basis of the analysis of the jet-modulating mechanism, a novel tool—a hydraulic-pulsed cavitating-jet generator (HPCJG)— is designed to further improve the rate of penetration (ROP). The HPCJG combines the advantages of a pulsed jet and a cavitating jet. When the drilling fluid flows through this tool, the fluid will be modulated to the pulsed and cavitating jet by application of the impellers and by entering into the self-resonant chamber. During this process, a pulsed cavitating jet is formed at the outlet of the bit nozzle. Because of jet pulsation, cavitating erosion, and the local negative-pressure effect, the cleaning efficiency of the well-bottom cuttings and the ROP will be improved. Laboratory and oilfield tests have indicated that the pulsed-pressure amplitude and the pressure drop of the generator increase with the increase of the flow rate. However, the frequency of the pressure pulsation is invariable in this process. This generator has been applied to more than 100 wells in eight oil fields throughout China. The results show that the maximum density of the test drilling fluid is 1.93 g/cm3, the maximum test-well depth is 6162 m, and the operation time of the generator is more than 230 hours (the maximum operation time recorded was more than 520 hours). Furthermore, the average ROP was increased by approximately 16.7% to 104.4%. In conclusion, the generator has the characteristics of a simple structure, has a long lifetime, is able to adapt well with existing drilling equipment, and can provide a safe and efficient drilling technique for deep wells.

HYDRAULIC STUDY OF DRILLING FLUID FLOW IN CIRCULAR AND ANNULAR TUBES

This study investigates the drilling fluid flow behavior of two water-based drilling fluids in circular and annular tubes. The study has four main objectives: 1) to evaluate correlations between the Power Law and the Casson rheological models, 2) to characterize the flow behavior, 3) to evaluate five hydraulic-diameter equations, and 4) to evaluate the correlations of five turbulent flow-friction factors. The experimental fluid flow loop consisted of one positive displacement pump of 25 HP connected to a 500-liter tank agitated by a 3-HP mixer. The fluids passed through six meters long tubes, arranged in three horizontal rows with independent inlets and outlets. The circular tubes had a 1 inch diameter and were configured as two concentric annular tubes. Annular Tube I had an outer diameter of 1 ¼ inch and an inner diameter of ½ inch. Annular Tube II had an outer diameter of 2 inches and an inner diameter of ¾ inch. The results show that, for the fluids in exam, correlations proposed in the literature were inaccurate as far as predicting hydraulic diameter, estimating pressure drop, and defining the flow regime. In general, the performance of those correlations depended on the fluid properties and on the system’s geometry. Finally, literature parameters for some of the correlations were estimated for the two drilling fluids studied. These estimations improved the predictive capacity of calculating the friction factor for real drilling fluids applications for both circular and annular tubes.

Experimental Research on the Particles Reflux in the Particle Impact Drilling System

Particle Impact Drilling technology (PID) is a new drilling technology, which is designed especially to solve the oil and gas exploration under hard terrane. In PID system, the steel particles were added in the drilling fluid to impact rock. The particles would be recycled and put to use again, thus it is of great significance to adjust proper drilling fluid flow rate for steel particle’s reflux. The flow rate of drilling fluids carrying particles is influenced by the fluid viscosity, the annular gap between drill pipe and wellbore, the particle volume fraction and particle size, etc. This paper mainly studied the influence of the annular gap and the flow rate, while the other factors keep constant. Both experimental method and dimension theory were employed in the research. Furthermore, empirical formula was proposed to describe the mechanism.

New Approach To Extend Horizontal-Drilling Reach

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 137821, ’New Approach To Improve the Horizontal-Drilling Reach,’ by O.M. Vestavik, SPE, H. Syse, O. Hole, SPE, and J. Aleksandersen, Reelwell, prepared for the 2010 CSUG/ SPE Canadian Unconventional Resources & International Petroleum Conference, Calgary, 19-21 October. The paper has not been peer reviewed. A drilling method was developed to enhance extended-reach managed-pressure drilling. A conventional drillstring with a special inner string forms a dual concentric drillstring (DCD). The inner string is used to remove drill cuttings from the bottom of the well. A separate hydraulic-fluid system is used to push the drilling assembly forward using fluid pressure from the surface. Development began in 2004, and the system has been through several full-scale operations in Norway. In 2009, the DCD method was used to directionally drill the 81/2-in. section of an onshore well. Introduction As targets for new petroleum resources become increasingly challenging, there is a need for new technology to keep exploration and production costs at an acceptable level. Development of the DCD method was supported by the Research Council of Norway, Statoil, and Shell. DCD method The DCD method, shown in Fig. 1, is based on conventional drilling, with the following components added to the system. The dual drillstring (DDS) facilitates closed-loop circulation of the drilling fluid. The cuttings are trans-ported to the surface by the drilling fluid through the center pipe, leaving the wellbore annulus free of cuttings. The top-drive adapter (TDA) is a dual-conduit swivel that facilitates rotation of the drillstring and accommodates the discharge and return conduits and a slip ring for power and data transmission. The dual-float valve (DFV) enables downhole pressure isolation of the well and facilitates controlled-pressure drilling and nonpressured pipe connections. The sliding piston is used to pump the bottomhole assembly (BHA) into the horizontal section of the well. The piston provides hydraulic weight on bit (WOB) and provides the ability to push the limits of extended-reach-drilling. The flow-control unit is a control valve for the active drilling fluid to ensure constant downhole pressure during drilling and pipe connections. The unit is equipped with pressure and flow sensors on both the drilling-fluid inlet and the return lines. A computer monitors and controls the status of the well. An annulus-control unit is used to keep a constant pressure in the annulus behind the piston. A main difference between this method and conventional drilling is the circulation path of the drilling fluid. In conventional drilling, the fluid returns through the wellbore annulus. The DCD method pumps the drilling fluid into the DDS annulus through the TDA. The drilling fluid flows down the DDS annulus to the DFV, which terminates the DDS at the top of a conventional BHA.

Modelling partially-emptied compaction bands induced by borehole drilling

Drilling into rock blocks subjected to critical far-field stresses can result in localized failure around the created borehole, manifested by the formation of two diametrically opposed breakouts. In weakly bonded quartz-rich porous sandstones breakouts take the form of narrow tabular (slot-like) openings extending along a plane perpendicular to the maximum applied-stress direction. SEM images strongly suggest that these breakouts are compaction bands (CB’s) that have been emptied to different degrees. Debonded grains, some whole, others cracked or crushed, within the compaction band are totally or partially removed by the circulating drilling fluid, creating the breakout. A new analytical model is developed here, which computes the stress field around partially-emptied CBs and predicts the expected lengths of these defects and their dependence on borehole radius and the extent of the CB emptying by the drilling fluid. It is found that boreholes with larger radius generate longer partially-emptied CBs. The results also demonstrate that when the ratio of the filled to the emptied part of a CB increases (addressing a smaller drilling fluid flow rate), the resulting total length of partially-emptied CB decreases. Both of these findings are in good agreement with results of drilling experiments conducted by Sheets and Haimson (2004).

Developing a Managed Pressure Drilling Strategy for Casing Drilling Operations

Casing drilling can be an effective method of reducing drilling costs and minimising drilling problems but its uptake around the world has been slow with only a few wells drilled so far with casing. Complex geological features like the high overburden on top of shallow unconsolidated reservoirs characteristic of offshore West Africa can benefit from casing drilling when effectively combined with Managed Pressure Drilling technique. For the industry to develop a managed pressure drilling capability that will allow today’s generation of complex wells to be drilled safely with casing, it is necessary to develop models that include the effect of eccentricity , rotation and fluid rheology at bottom hole conditions on flow and pressure regimes, and to embed these models within an easy to use, intuitive well design package for pre planning and as a real time tool to monitor and provide forward simulations based on real time rig and downhole data. The paper presents new results of the theoretical predictions of the wellbore pressure regimes incurred when different types of drilling fluid flows in concentric and eccentric horizontal annuli. The concentric and eccentric casing drilling results are compared with parallel predictions from conventional drillstring results from developed analytical solutions integrated into the VisWELL(DeskTop Simulator) , which is used in simulating well operations.

Hydraulic Pulsed Cavitating Jet-Assisted Drilling

How to improve drilling rate in deep wells has been a hot subject. Based on modulating pulse jet and cavitating jet, a new drilling tool is designed which couples advantages of both pulse jet and cavitating jet. When drilling fluid flows through the tool during the drilling process, fluid is modulated to pulse and cavitate. Thus, pulse cavitating jet is formed at the outlet of the bit nozzle. Because of jet pulsation, cavitating erosion and local negative pressure affect bottomhole rock. Cleaning and breaking is enhanced and penetration speed is improved. Oil field tests in five wells show good applicability of the tool to bit types, formation, drilling densities, flow rates, and dynamic hydraulic drilling motors, etc. As a result, penetration rates are improved ranging from 10.1 to 31.5%; the maximum working time is about 235.5 hr in downhole. Pulse and cavitating jet coupling will afford an effective means to improve drilling rate for deep wells.

Numerical Modelling of Cuttings Transport in Horizontal Wells Using Conventional Drilling Fluids

Abstract A one-dimensional transient mechanistic model of cuttings transport with conventional (incompressible) drilling fluids in horizontal wells has been developed. The model is solved numerically to predict cuttings bed height as a function of drilling fluid flow rate and rheological characteristics (n, K), drilling rates, wellbore geometry and drill pipe eccentricity. The results of the sensitivity analysis showing the effects of various drilling operational parameters on the efficiency of solids transport are presented. The model developed in this study can be used to develop computer programs for practical design purposes to determine optimum drilling fluid rheology (n, K) and flow rates required for drilling horizontal wells. Introduction Poor hole cleaning can lead to pipe sticking, higher drag and torque, slower rate of penetration, formation fractures and wellbore steering problems(1). It has been estimated that one third of stuck pipe problems are due to inadequate hole cleaning(2). Optimization of drilling hydraulics design for trouble free drilling, therefore, requires a good understanding of the mechanics of cuttings transport. Experimental studies have shown that hole cleaning is affected by many parameters, such as well geometry (diameter, inclination, eccentricity), cuttings characteristics (size, porosity of bed), drilling fluid properties (rheology, density, drag coefficient) and drilling operational parameters (drilling rate, drilling fluid circulation rate)(3–7). Based on the results from experimental studies, empirical models were developed(8–13) and rules of thumb for field practices were also suggested(14–16). Cuttings transport has also been investigated extensively by using analytical models. Two-layer(1,17–22) and three-layer(23,24) mechanistic models of cuttings transport were developed. Most of the existing models do not consider the slip velocity between solids and drilling fluids. In some cases, the authors simply neglected the slip velocity and treated the flow of the liquid-solids mixture as homogeneous flow(20,21,23,24), while others simply assumed the slip velocity is equal to the solids terminal settling velocity(19). Steady-state transport of cuttings is the major assumption inherent in all of the previous cuttings transport models. Therefore, the transient nature of the cuttings transport cannot be studied adequately by using these models. As the drilling goes on, cuttings are continuously deposited along the horizontal wellbore. There is a critical limit for cuttings bed height above which drilling operation exhibits a high risk of operational integrity, such as risk of lost circulation due to high ECD, increased probability of pipe sticking, high drag and torque. The common practice is to stop drilling occasionally and clean the borehole by using viscous pills, pipe rotation and reciprocation, and drilling fluid circulation. It is very critical to be able to predict after how many hours of operation drilling should be stopped and remedial wellbore cleaning actions be resumed. A transient cuttings transport model is, therefore, needed for better design of drilling hydraulics; in particular for drilling long horizontal sections of the wellbore and extended reach (ERD) wells. In this study, a 1D transient, mechanistic model of cuttings transport with conventional (incompressible) drilling fluids in horizontal wells has been developed. The prediction of cuttings bed as a function of various drilling operational parameters is presented. A detailed description of t

USING NMPC BASED ON A LOW-ORDER MODEL FOR CONTROLLING PRESSURE DURING OIL WELL DRILLING

During petroleum well drilling operations, the pressure gradient in the well has to be maintained within the pressure restrictions of the formation. The well pressure can be controlled by restricting the drilling fluid flow through a choke valve at the top of the well. This paper proposes a closed-loop control algorithm using a finite horizon nonlinear model predictive control scheme based on a low order well model for this problem. The algorithm is tested on a rigorous high order model.

Annular flow analysis by tracers in drilling operations

A study of drilling fluid flow in the annular space and drill pipe by residence time distribution analysis using a tracer is presented. This is a new method which can be used to assess the rate of removal of cuttings during the drilling process. Two field trials were conducted to evaluate the technical feasibility and potential application. The results are analyzed using the flow behavior, carrying capacity of the drilling fluid, and hole cleaning conditions. Analysis of tracer response indicates the presence of anomalous flow in both trials, characterized by two fluid volume fractions traveling with different velocities. The magnitude of these volume fractions is related directly to the carrying capacity of the drilling fluid (hence, the hole cleaning conditions) which is explained. The dispersion coefficient, as well as other distribution functions, is suggested to measure the overall behavior of fluid in a drill-hole. This method is compared with empirical correlations employed in the industrial practice and potential advantages are demonstrated using tracers.

Borehole instability in high-porosity Berea sandstone and factors affecting dimensions and shape of fracture-like breakouts

Stress-induced breakouts in vertical boreholes are failure zones caused by excessive compressive stress concentration at the borehole wall along the springline of the least horizontal far-field stress. Wellbores are sometimes drilled into aquifers or oil reservoirs that are weak, poorly consolidated, and highly porous sandstone formations, which are often conducive to breakout formation. Breakouts are an expression of borehole instability and a potential source of sand production. On the other hand, the breakout phenomenon can be used advantageously in obtaining an estimate of the in situ stress condition. The average orientation of breakouts, as identified by borehole geophysical logging, is a reliable indicator of in situ stress directions. It has also been suggested that breakout dimensions could potentially be used as indicators of in situ stress magnitudes. The research reported here has concentrated on the unique type of breakouts observed for the first time in high-porosity Berea sandstone. Drilling experiments in rock blocks subjected to critical far-field true triaxial stress regimes, simulating in situ conditions, induced breakouts that were unlike the ‘dog-ear’ ones previously observed in granites, limestones, and low-porosity sandstones. The newly observed breakouts were thin, tabular, and very long, resembling fractures that counterintuitively extended perpendicular to the maximum principal stress. We found that a narrow zone ahead of a fracture-like breakout tip underwent apparent localized grain debonding and compaction. In the field, such zones have been termed ‘compaction bands’, and are a source of concern because in oil fields and aquifers they constitute curtains of low permeability that can impede the normal flow of oil or water. In order to determine whether a correlation exists between fracture-like breakouts and in situ stress, we conducted several series of tests in which the minimum horizontal and vertical stresses were held constant and the maximum horizontal stress ( σ H) was increased from test to test. These tests showed strong dependence of the breakout length on far-field stress, signaling that potentially the ability to assess fracture-like breakout length in the field could be used to estimate in situ stress magnitudes in conjunction with other indicators. Another series of tests revealed that breakout length increased substantially when borehole diameter was enlarged. This result suggested that in the field, where wellbore size is considerably larger, fracture-like breakout could extend to sizable distances, creating a sand production hazard. Two series of tests, one to evaluate the effect of drill-bit penetration rate, and the other to verify the drilling-fluid flow rate effect on breakout formation and dimensions yielded inconsistent results and showed no unique trends. Remarkably, fracture-like breakouts maintained a consistent narrow width of about 5–10 grain diameters, irrespective of the test conditions. This characteristic supports the suggestion that fracture-like breakouts are emptied compaction bands.

A New Hole Cleaning Criteria for Drilling Operations of Oil Wells

A study of drilling fluid flow in the annular space and drill pipe through residence time distribution (RTD) analysis of a tracer injected in impulse form while drilling an oil well is presented in this article. Two field trials were carried out in order to evaluate the technical feasibility and potential practical application of the RTD theory and the dispersion model. From the results it is possible to explain physically the flow behavior and its relation with parameters such as carrying capacity of the drilling fluid and hole cleaning conditions. The RTD analysis of tracer response indicates the presence of anomalous flow in both trials, characterized by two fluid volume fractions traveling with different velocities. The magnitude of these volume fractions concerns directly with the carrying capacity of the drilling fluid, hence, the hole cleaning conditions as is explained along the work. The dispersion number (RDN) as well as other distribution functions are suggested as a measure of the overall behavior of the fluid in a hole. This criterion is compared with empirical correlations employed in the industrial practice.

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.

Solids-Conveyance Dynamics and Shaker Performance

Summary No comprehensive mathematical models are available that can predict solids conveyance and shaker fluid handling capacity for arbitrary solids in a specified drilling fluid. This paper addresses the solids loading problem and presents an analytical approach to the design of shale shakers, based on a dynamic model for solids conveyance off the screen and fluid flow through the screen. First, the solids loading is derived as a function of shaker geometry, solids-conveying speed, bit penetration rate, hole size, drilling fluid flow rate, particle size distribution of drilled solids, and screen opening size. Second, the solids-conveyance model is developed, based on the physics of the problem, accounting for the adhesive forces resulting from wet solids. The effect of various parameters, such as vibration pattern and normal acceleration on solids conveyance, is discussed. The solids-conveyance model can give the optimum set of shaker design parameters for efficient solids conveyance off the screen. For a given shaker, the solids conveying speed predicted by the model can also be used to obtain the flow capacity of the shaker in the presence of solids. Experimental and field results relevant to this study are also discussed.