Borehole stability technology includes chemical as well as mechanical methods to maintain a stable borehole, both during and after drilling. Drilling fluids range from water to oil to complex chemical systems with properties designed for specific site conditions to aid the drilling process. Drilling fluids perform the following functions: carrying cuttings out of the hole, cleaning the bit, cooling and lubricating the bit, providing buoyancy to the drill string, controlling formation fluid pressures, preventing formation damage, and providing borehole support and chemical stabilization.
Most borehole stability and drilling fluids-related problems can be handled with present technology in relatively easy, well-defined environments, provided stringent quality control measures are maintained. Nevertheless, severe, complex drilling situations and formations still present serious challenges to economically viable drilling.
Borehole failures are an increasing concern due to an ''explosion" in drilling horizontal wells in unique geological environments (e.g., Hanford site) as well as drilling "difficult" hydrocarbon reservoirs. Difficult reservoirs include, for example, unconsolidated or poorly consolidated sediments, shales, complex reservoir geometries, naturally fractured reservoirs, and overpressured reservoirs. This had led to greater research emphasis related to the stability of circular rock openings (International Society for Rock Mechanics, 1987; National Research Council, 1993). Borehole stability has been the subject of several recent research papers (Guenot, 1987; Maury, 1987; Maury and Sauzay, 1987; Santarelli and Brown, 1987; Bjarnasson and others, 1988; Roegiers and Detournay, 1988; Beus and Dar, 1989; Ewy and Cook, 1989; Haimson and Herrick, 1989; Périé and Goodman, 1989), and current borehole design criteria
have been reassessed (Detournay and Fairhurst, 1987; Gill and Ladanyi, 1987; Gumusoglu and others, 1987; Műhlhaus, 1987; Lin and Fairhurst, 1988; Vardoulakis and Papanastasiou, 1988; Papanastasiou and Vardoulakis, 1989).
The goal of this research is to provide an efficient basis to obtain a near-gauge opening that can be maintained easily for the long term with minimal environmental impact, disturbance of the drilling objectives, and drilling costs. In some industries (e.g., petroleum and environmental), the emphasis on the near-gauge opening is to minimize the costs of drilling, evaluating, and cementing the hole, whereas for other applications this could very well be seen as an attempt to mobilize the residual rock strength in order for it to participate in the overall support requirements. Long term implies no "unexpected" maintenance costs for the life of a project.
Efforts and progress are being made by several organizations and have led to new, important, proprietary, or patented technologies usually available for license, but rarely utilized by large numbers of laboratories or applied in the field. For example, one oil company has recently developed an extensive shale data base along with a patented method to determine in situ shale strengths from correlations with index properties derived from simple measurements on rock cuttings. Included in this data base are correlations to assist in providing proper mud weight guidelines for several high-angle oil and gas well projects.
When considering areas in which improvements could be made to avoid stability or environmental problems associated with the use of drilling fluids, the following are selected research needs (in no particular priority order):
Rock-fluid interaction: A better understanding of this coupled phenomenon may lead to greater penetration rates.
Flow balance measurements: Unexpected loss of drilling fluids can lead to catastrophic failures; hence, any advanced warning, especially in geothermal environments, will be beneficial. Also, the development of drilling fluids that result in zero fluid loss, no matter what the formation characteristics, would be breakthrough.
Air-based systems: Such systems could decrease formation damage and address some of the environmental concerns, provided dust can be adequately controlled.
Heterogeneous media: Substantial changes in formation characteristics affect drilling orientation and efficiency. Sampling in such environments is also challenging.
Hostile environments: Permafrost and hot formations, for example, adversely affect the penetration rate.
Environmentally acceptable drilling fluids: Nontoxic, oillike fluids to avoid undue formation damage are needed.
Formation characterization: Characterization methods are needed for environmental applications, especially contaminant detection and measurement. Characterization methods are also needed for shales because a majority of hydrocarbon well footage is drilled in shaley lithologies. This includes both pertinent rock properties and reservoir characteristics, such as the existing stress field and pore pressure.
Reliable stability models: Stability models, especially models that couple thermal, porous, mechanical, and chemical phenomena, are needed.
Priorities for R&D
From the above list, six important research areas could yield significant advances and benefits. As with most engineering disciplines, there exists a wide gap between R&D developments and field applications. In addition to new research, efforts should be made to transfer some of the existing technologies that could immediately improve problems encountered in borehole stability.
Formation characterization and validation: Formation characterization should be undertaken during the drilling process, integrating the information obtained from drilling data, various types of logs, cutting data, seismic information, surface measurements, structural geology, and in situ stress measurements. The critical parameters that should be determined are the following:
rock strength, usually at in situ conditions of pressure and temperature; an attempt could be made to determine its variation during the drilling process due to progressive unloading of the rock;
formation constitution, because some minerals such as smectite could cause time-dependent instabilities due to environmental changes;
permeability, because it governs potential inflow (production) and outflow (loss of drilling mud) conditions (it should be realized that for heavily fractured reservoirs, in which secondary permeability is greatly affected by the applied stresses, care should be taken when assessing fluid transmissivity from borehole geometries);
pore pressure, because it affects safety (formation fluid and gas kicks) and influences all hydraulic and formation constitutive effects;
stress state, which controls most mechanisms;
abrasivity, which influences drillability; and
discontinuities, because they introduce major heterogeneities in all aspects.
Rock alteration: A very limited amount of research has been undertaken on the chemomechanical weakening of rocks. The underlying mechanisms are still very much disputed; therefore, the potential for defining and controlling this rock-fluid interaction has not been fully realized. The same is true for drilling in very sensitive formations for which specific inhibitive fluids could be developed. Finally, injection of "active" fluids at the rock face to increase penetration rate and decrease bit wear has not been pursued aggressively. One might envision two flow loops, a primary one with the purpose of transporting the cuttings and a secondary one to help the drilling process.
Improved stability models and rock data (shale): Existing design codes for stability models are quite conservative. For example, linear elasticity would predict that some vertical and many directional boreholes drilled for hydrocarbon exploration and production should be unstable when, in fact, many are stable. The present limitation is due not only to the models used, but also to the poor knowledge of the constitutive behavior of the formation. In addition, time-dependent failures that occur occasionally (e.g., during the production phase of a reservoir) are almost never predicted, pointing to the need for introducing solutions that take into account the coupling between fluid flow and rock deformation. Such poroelastic efforts are currently being pursued. In all cases, the design codes need to be user friendly and easy to apply.
Drilling techniques for environmental remediation boreholes: Drilling techniques for environmental remediation are evolving as emphasis is placed on removing subsurface contamination from Superfund and other contaminated sites. Current approaches are somewhat limited, but the industry is borrowing some existing petroleum technology. The development of new technology or the adaptation of existing technologies from other industries is required to significantly expand current economics. Expanded and improved technology is necessary for horizontal wells; reverse circulation; dry-air drilling; and drilling waste/effluent minimization, recovery, and disposal.
Methods for drilling and stabilizing heterogeneous formations: Heterogeneous formations present special and expensive difficulties both in drilling and in maintaining a stable borehole. Formations that contain both hard (or abrasive) and soft components drill very slowly with severe demands on bits. Fractured or heavily jointed rock causes more complex loading of the borehole that requires, in many instances, the development of more sophisticated numerical modeling to predict borehole behavior.
Drilling fluids: Drilling fluids technology is a moving target due to rapidly expanding needs, demands, and restrictions such as environmental remediation, air drilling, severe temperature conditions, increased lubricity requirements, restrictions on oil-base systems, discharge limitations, and horizontal and extended reach drilling.
For example, oil-base drilling fluids have come under severe regulatory restrictions. Effective alternatives have been developed, but they are expensive. To comply with new government regulations restricting the use of some technologies or practices, drilling organizations have responded well by developing acceptable alternatives; however, these solutions usually have substantial added costs and limitations that are sometimes prohibitive.
Fluid development needs encompass the design of new environmentally acceptable water-base fluids and oil-like systems that will provide alternatives to oil-base devices. Such new fluids should provide superior filtration control to minimize fluid invasion and damage to permeable zones. They should possess good mudcake properties to provide needed filtration control and prevent differential-pressure sticking of the drill pipe against the borehole wall. The new fluids should provide adequate holecleaning capabilities in horizontal or high-angle wells. Improvements in
understanding the fundamentals of cutting transport flow visualization, airfoam behavior, and fluid viscoelastic behavior will aid that process.
Beus, M. J., and Dar, S. M., 1989, Three-dimensional FEM analysis to scale field measurements from deep mine accessways: inRock Mechanics as a Guide for Efficient Utilization of Natural Resources, Proceedings 30th U.S. Symposium on Rock Mechanics, Morgantown, W. Va., p. 783-790.
Bjarnasson, B., Ljunggren, C., and Stephansson, O., 1988, New developments in hydrofracturing stress measurements at Lulea University of Technology: Proceedings 2nd Workshop on Hydraulic Fracturing Stress, Measurements (HFSM'88), University of Minnesota, v. 1,p. 113-140.
Detournay, E., and Fairhurst, C., 1987, Two-dimensional elastoplastic analysis of a long cylindrical cavity under non-hydrostatic loading: International Journal of Rock Mechanics, Mineral Science, and Geomechanical Abstracts, v. 24 (4),p. 197-211.
Ewy, R. T., and Cook, N. G. W., 1989, Fracture processes around highly stressed boreholes: Proceedings 12th Annual Energy-Sources Technical Conference, Houston, ASME, New York, p. 63-70.
Gill, D. E., and Ladanyi, B., 1987, Time-dependent ground response curves for tunnel lining design: Proceedings 6th ISRM Congress, Montreal, Rotterdam, Balkema, p. 917-921.
Guenot, A., 1987, Contraintes and ruptures autour des forages petroliers: Proceedings 6th ISRM Congress, Montreal, Rotterdam, Balkema, p. 109-118.
Gumusoglu, M. C., Bray, J. W., and Watson, J. O., 1987, Analysis of underground excavations incorporating the stain softening of rock masses: proceedings 6th ISRM Congress, Montreal, Rotterdam, Balkema, p. 923-928.
Haimson, B. C., and Herrick, C. G., 1989, Borehole breakouts and in-situ stress: Proceedings 12th Annual Energy-Sources Technical Conference, Houston, ASME, New York, p. 17-22.
International Society for Rock Mechanics, 1987, Failure mechanisms around underground excavations: Commission Report, Proceedings
6th ISRM Congress, Montreal, Rotterdam, Balkema, p. 1119-1128.
Lin, D., and Fairhurst, C., 1988, Static analysis of the stability of three-dimensional block systems around excavations in rock: International Journal of Rock Mechanics, Mineral Science, and Geomechanical Abstracts, v. 25,p. 139-147.
Maury, V., 1987, Observations, recherches et resultats recents sur les mecanismes de ruptures autour de galeries, isolees—Report of the ISRM Commission on Failure Mechanisms Around Underground Excavations: Proceedings 6th ISRM Congress, Montreal, Rotterdam, Balkema, p. 1119-1128.
Maury, V., and Sauzay, J. M., 1987, Borehole stability: case histories, rock mechanics approach and results: Proceedings SPE/IADC Conference, New Orleans, SPE 16051, Richardson, Tex., SPE, p. 11-24.
Mühlhaus, H. B., 1987, Stability of deep underground excavations in strongly cohesive rock: Proceedings 6th ISRM Congress, Montreal, Rotterdam, Balkema, v. 2, p.1157-1161.
National Research Council, 1993, Stability, Failure, and Measurements of Boreholes and Other Circular Openings: Washington, D.C., National Academy Press, 92 pp.
Papanastasiou, P., and Vardoulakis, I., 1989, Bifurcation analysis of deep boreholes—II. Scale effect: International Journal of Numerical Analysis and Methods of Geomechanics, v. 13, p.183-198.
Périé, P. J., and Goodman, R. E., 1989, Evidence of new failure patterns in a thick-walled cylinder experiment: Proceedings 12th Annual Energy-Sources Technical Conference, Houston, New York, AMSE, p. 23-27.
Roegiers, J. C., and Detournay, E., 1988, Consideration on failure initiation in inclined boreholes: Proceedings 29th U.S. Symposium on Rock Mechanics, University of Minnesota, p. 461-469.
Santarelli, F. J., and Brown, E. T., 1987, Performance of deep wellbores in rock with a confining pressure-dependent elastic modulus: Proceedings 6th ISRM Congress, Montreal, Rotterdam, Balkema, p. 1217-1222.
Vardoulakis, I., and Papanastasiou, P., 1988, Bifurcation analysis of deep boreholes—I. Surface instabilities: International Journal of the Numerical Analysis and Methods of Geomechanics, v. 12,p. 379-399.