Wireline Borehole Images–LWD Well Placement Integration to Determine Faults in Awali Field, Bahrain

Abstract

The Awali field is a mature carbonate oil field in Bahrain. It is a giant gentle limb of anticline compartmentalized by numerous normal faults. The reservoir is intersected by normal faults that create half-graben reservoirs. Characterizing these faults through borehole data is necessary to understand the reservoir compartmentalization and improve the geologic model, which was built from seismic data of poor vertical resolution. Fault structure was characterized by the deep directional resistivity of a logging-while-drilling (LWD) tool used to geosteer horizontal wells through faulted zones. The LWD tool detected the faults, determined their throw and estimated its extent, and mapped the bed boundaries up and down within its depth of investigation. With this information, engineers were able to place wells over horizontal sections ranging from 1,000–3,500 ft [305–1,067 m] MD within the target zone and stay just 4–14ft [1.2–4.3 m] below the roof (i.e. the top of the Kharaib carbonate reservoir). The high-resolution wireline borehole image logs recorded after the well reached TD provided further confirmation of fault presence and strike and dip angle. On borehole image logs, normally the faults are indicated by fracture association and drag pattern on nearby structural bedding dips or by abrupt facies or textural changes. Most faults in the Awali field have NW–SE trends with high dip angle; they are considered as a normal fault regime forming the graben and half-graben reservoir traps. Combining the complementary LWD well-placement results and high-resolution borehole image log interpretation enabled us to determine the fault type and its orientation.