The Longest Horizontal Well Drilled in The U.A.E., Geosteered in a Thin Calcareous Dolomite Sublayer

Abstract

September 2012, ZADCO drilled the longest well to date in the U.A.E. to test the productivity and injectivity of a low permeability, carbonate reservoir in a giant Middle East oil field. The extended reach well was drilled to a total measured depth of 24,390 ft., with a horizontal length of 11,015 ft.The borehole target zone was a~3.5 ft. thick calcareous dolomite sublayer sandwiched between upper and lower~4 ft. thick limestones. The critical factors for success were: (1) LWD tools capable of differentiating calcareous dolomite from the nearby limestones; and (2) LWD sensors close to the bit as possible to enable prompt geosteering reactions to possible lateral facies changes. Challenges were: (1) the capability of the jackup rig to drill the required distance; (2) minimizing hole undulations; and (3) remain within the 3.5 ft. target zone. Concerns were raised about the use of nuclear chemical source density tools and the risk of a lost nuclear source in hole should the drill pipe become stuck. To address these concerns, the second half of the lateral section was steered without a chemical nuclear source in the downhole tool string.A LWD formation pressure test tool was run post drilling to evaluate formation mobility. The result supported well placement in the higher permeability dolomite layer which was later validated by production and injection results. This paper illustrates good geosteering practices in a challenging long horizontal well with carefully selected LWD program to achieve the target and reduce risks.