ABSTRACT: Reservoir Evaluation for Horizontal Drilling

HURLEY, NEIL F., Marathon Oil Company, Littleton, CO

Horizontal drilling is here to stay! The technique is being used more every day to obtain incremental oil recovery from naturally fractured reservoirs. Success has not been universal, however, as selection criteria for candidate reservoirs are not well established. An integrated analysis of many parameters will improve the rate of success. Subsurface mapping and examination of production data are important in the early phases of candidate selection. Production data include decline curves, bottomhole pressures, water-cuts and gas-oil ratios, interference tests, and tracer surveys. In some fields, structure and bed-curvature maps may be used to help characterize natural fractures. In pressure-depleted reservoirs, horizontal drilling generally yields good results where the water drive is weak to nonexistent, and gravity drainage is an important energy mechanism. Factors such as thickness of the oil column and lease considerations are locally important.

Core, outcrop, and petrophysical analyses can then be used to measure fracture aperture, strike, dip, intensity, length, height, and style of intersection. The maximum horizontal in situ stress direction, which is related to the preferred orientation of open, permeable fractures, can be determined from a number of techniques including borehole breakouts. Approaches such as stochastic modeling of 3-D fracture networks can then forecast the likelihood of fracture/borehole intersections. Finally, post-mortem analyses that employ gamma-ray and borehole-imaging logs can be used to reconstruct the borehole profile and characterize natural fractures encountered by the well.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)