The use of permeability tensors is required when modelling fluid flow in anisotropic and heterogeneous reservoirs presenting multiple zones of directional permeability, or those categorized as naturally fractured reservoirs. A general procedure for characterizing complex reservoirs utilizing their permeability tensor is being developed by integrating data and methods from different disciplines. Permeability tensors for geologically defined fracture patterns are derived, and finally these small-scale descriptors are incorporated into a reservoir simulation program capable of handling full tensor permeabilities. The application and convenience of the method presented in this paper is illustrated with a field example from a naturally fractured reservoir. Introduction For many years, substantial research has been conducted in the areas of geosciences and engineering in order to characterize naturally fractured reservoirs. Numerous approaches have been presented to properly overcome this difficult task. Geoscientists have focused their research towards understanding the process of fracturing (rock mechanics), and the subsequent description of fracture characteristics such as density and orientation. Engineers, on the other hand, have focused their attention to the description of the fluid flow in the fracture systems, and in the development of accurate models (reservoir simulators) to reproduce the history, and predict the hydrocarbon production, for these complex systems. One of the first matrix-fracture models was presented by Warren and Root(1), who presented an idealized sugar cube model with two classes of porosity, a primary porosity that is intergranular, and a secondary porosity that is induced by fractures. Even though the sugar cube model has been widely accepted as the forerunner of the modern interpretation of dual-porosity systems, its limitations in describing the behaviour of some complex fractured reservoir systems have been observed. It is now well known that almost all fracture systems are much more complicated than the suggested model of three orthogonal sets of uniform fractures. One of the most important factors that has been identified as a necessary addition to improve the overall description of such complex reservoirs is the definition of a nine-component permeability tensor for the fracture system. This tensor is used to model fluid flow in complex reservoirs with multiple zones of directional permeability, where the orientation and magnitude of the principal permeabilities may vary between different zones in the reservoir. Snow(2, 3) studied the convenience of using mathematical equivalents of parallel plate openings to simulate fractures dispersed in orientation, distributed in aperture, and of arbitrary spacing. Models for fractured media which contained any number of planar conductors of any orientation and any fine aperture were presented. A key assumption was that all of the conduits have smooth parallel plane walls of indefinite extent (infinite fractures), and an rbitrary aperture. As a result, a permeability tensor could be obtained by superposition of contributions due to the fractures, and due to the permeable matrix. Long et al.(4, 5) addressed the more realistic scenario of finite or discrete fracture systems, where properties such as shape, orientation and location of the fractures in an impermeable matrix were considered to be random variables.

fax 01-972-952-9435. AbstractA study of cuttings transport at intermediate inclinations using aerated fluid, to determine the amount of solids that exist in the wellbore and minimum flow requirements for 'clean-hole' condition is presented. The experimental program included over 300 tests, performed with a large-scale facility (flow loop 100-ft in length, with 8" OD casing and 4.5" OD drillpipe). The angles of test section inclination were 30º, 45º and 60º from vertical. Four pipe rotational speeds (0, 40, 80, and 110 rpm) were used for different liquid and gas flow rate combinations. New correlations were found to estimate the required critical gas flow rates for hole cleaning at specified liquid flow rate and drill pipe rotation combinations, and to predict volumetric cuttings concentration as a function of air and water flow rate, drill pipe rotational speed and inclination angle.

Summary A study of cuttings transport at intermediate inclinations using aerated fluid, to determine the amount of solids that exist in the wellbore and minimum flow requirements for "clean-hole" condition, is presented. The experimental program included more than 300 tests, performed with a large-scale facility [100-ft-long flow loop with 8-in. outer diameter (OD) casing and 4.5-in.-OD drillpipe]. The angles of test section inclination were 30°, 45°, and 60° from vertical. Four pipe-rotational speeds (0, 40, 80, and 110 rpm) were used for different liquid-and gas-flow-rate combinations. New correlations were found to estimate the required critical-gas-flow rates for hole cleaning at specified liquid-flow rate and drillpipe-rotation combinations, and to predict volumetric cuttings concentration as a function of air and water flow rate, drillpipe-rotational speed, and inclination angle. Introduction In recent years, two goals of oil and gas production companies have been to develop new methods to improve hydrocarbon recovery in mature areas and to exploit new low-pressure/ low-permeability reservoirs. The use of underbalanced and near-balanced drilling techniques has found applications for these particular cases. Cuttings transport is one of the major factors affecting cost, time, and quality of directional wells. The significant advantages related to drilling with aerated fluids are reduced by inefficient cuttings transport to the surface. Specifically, these advantages depend on understanding the interaction between fluids and the drill cuttings. Cuttings transport with multiphase fluids is dominated by many variables, and the interaction of all of these variables adds complexity to this subject. Because of this, an experimental approach has been selected to accomplish this investigation. The understanding of cuttings transport with multiphase fluids is very limited because the majority of research in cuttings transport has been conducted with conventional drilling fluids. The study of three-phase flow is relatively new, and there are not enough studies that consider the transport of solids with Newtonian gas/liquid mixtures, pipe rotation, and the slip between phases. This paper reports on an experimental study of hole cleaning with aerated fluids at intermediate hole angles considering drillpipe rotation. The main results of this work are experimental data, empirical correlations, and observations. It is intended to serve as a guide to the current technology, explaining how and why the effect of pipe rotation is related to the cuttings concentration in the wellbore.

Drilling on top of the Mesa in the Piceance Basin presents a significant loss circulation and stuck pipe challenge to operators wanting to exploit the huge gas reserves in the area. Operators have experienced losses that exceed 4000 barrels of mud when the intermediate section is drilled using conventional techniques. This is due to a combination of natural fractures and weak rock. Various strategies have been deployed to tackle the problems, including under-balance drilling operations and Direction Casing While Drilling (DcWD). A new technique described in this paper is now the best practice for ConocoPhillips in the area. It involves acquiring realtime circulating density (ECD) measurements and control of mud weight in the annulus, using direct air injection through a parasite aerating string (PAS).During the development stages of this new process an annular pressure sub (APWD) was run to gather diagnostic data. Analysis of the data shows conventional drilling practices often yield up to 3 ppg variation in circulating density exposed to the formation. Analysis of the data also suggests there is a fracture reopening gradient of approximately 8.3 ppg and there are huge circulating density variations during connections. The new strategy shows these wells can be drilled with an ECD in the range of 5-7 ppg using conventional water based mud systems. This strategy allows wells with a very narrow mud weight window to be drilled safely. This simple approach avoids the use of complex multiphase models, giving the flexibility to quickly deploy the technique to the well site without the need for expert personnel. The information enables the driller to control and keep the ECD within recommended limits, delivering a safe and productive well.The alternative approach of using conventional well design techniques would result in multiple casing strings and cost overruns, while more advanced techniques such as DcWD and under-balance drilling would require specialized equipment and crews. This new technique uses existing and common drilling technologies along with new software tools for geomechanics analysis and drilling surveillance to achieve excellent results. This paper presents a simple risk management technique using today's conventional technologies to successfully manage loss circulation risk in the Piceance basin.

Drilling on top of the mesa in the Piceance basin presents a significant lost-circulation and stuck-pipe challenge to operators wanting to exploit the gas reserves in the area. Operators have experienced losses that exceed 4,000 bbl of mud when the intermediate section is drilled using conventional techniques. This is because of a combination of natural fractures and weak rock. Various strategies have been deployed to tackle the problems, including underbalanced-drilling (UBD) operations and directional casing while drilling. A new technique implemented by an operator in the Piceance basin is described in this paper; it involves acquiring real-time equivalent-circulating-density (ECD) data and control of mud weight in the annulus by use of direct air injection through a parasite aerating string (PAS).During the development stages of this new process, a real-time annular-pressure sensor was run in the bottomhole assembly (BHA) to gather diagnostic data. Analysis of the data shows conventional drilling practices often yield up to 3 lbm/gal variation in ECD exposed to the formation. The analysis also suggests that there is a fracture-reopening gradient of approximately 8.3 lbm/gal and that there are significant ECD variations during connections. The new strategy shows that these wells can be drilled with an ECD in the range of 5-7 lbm/gal using conventional water-based-mud systems. This strategy allows wells with a narrow mud-weight window to be drilled without significant mud loss to the formation. This approach avoids the use of complex multiphase models, and the downhole ECD can be displayed on the driller's console in real time from the data measured by the annular-pressure sensor and sent through the mud-pulse-telemetry measurement-while-drilling (MWD) tool. Alternatively, if an annular-pressure sensor is not included in the BHA, the downhole ECD can still be estimated accurately by use of a simple spreadsheet calculation (Scott 2009). This gives a measure of flexibility in deploying the technique to the wellsite. Expert personnel were used in the initial diagnostic stages to establish the procedures and validate the effectiveness of the technique. After that, they will not normally be required at the wellsite to manage the process in subsequent wells because the ECD data from the annularpressure sensor can be understood by the driller, and the alternative spreadsheet solution, if used, can also be managed by the wellsite supervisor to calculate a reliable downhole ECD measurement. The ECD data enable the driller to control and keep the annular pressure within recommended limits, ensuring that lost circulation risk is reduced and wellbore stability is maintained.The alternative approach of using conventional well-design techniques would result in multiple casing strings and in cost overruns, while more-advanced techniques such as directional casing while drilling and UBD would require specialized equipment and crews. This new technique uses existing and common drilling technologies along with new software tools...