DESIGN, EXECUTION, AND EVALUATION OF ACID TREATMENTS OF NATURALLY FRACTURED CARBONATE, OIL-RESERVOIRS OF THE NORTH-SEA

This paper discusses damage removal in naturally fractured carbonate, oil reservoirs of the central North Sea, located in the vicinity of sa lt domes. The reservoir rock is the well-known Tor chalk, the mechanic al properties of which have been greatly modified by the proximity of diapirs. The matrix rock is much harder (similar to a limestone) and e xtensively fissured. Wells drilled through these reservoirs suffer fro m very severe damage as a result of the invasion of the natural fractu res by large amounts of drilling mud. Productivity is restored by high -rate damage-removal treatments alternating stages of crosslinked gel, customized acid formulations, and ball sealers. Pre-and postjob well performances demonstrate huge productivity-index increases as a result of the cleanup of the natural fissures. This paper emphasizes mud-los s survey during drilling and perforation strategies; candidate selecti on, i.e., pre-and postjob productivity based on both rock (matrix and fissures) and damage properties; treatment execution (as per actual No rth Sea jobs); and treatment evaluation and diversion efficiency.