Ness Horizontal-Well Case Study

Abstract

Summary This paper reviews the reservoir management work that led to the recommendation to drill the Ness field horizontal well. It also discusses the poor well performance seen almost immediately after production began and the reasons for that well performance revealed by a postdrill reservoir simulation. Finally, the atypical logging responses observed in the well are discussed. Introduction The Ness field has contributed significantly to the oil production volumes and revenue for the partners in the Mobil-operated Block 9/13 in the U.K. sector of the North Sea. When the horizontal well was drilled, the partners were Amerada Hess, Enterprise Oil, North Sea Holdings, and Mobil. The field was originally brought on stream in Aug. 1987 at a rate of 17,500 BOPD [2780 m3/d oil]. At the end of 1988, production rates began to decrease because of water production, which was thought to be caused by water coning. by Summer 1989, it was apparent that additional production wells had to be drilled to maintain a plateau production rate. Alternatively, the partnership would have to accept an extended production life associated with increasing water production rates production life associated with increasing water production rates and lower ultimate recovery. The initial field development wells were drilled and completed with vertical-well technology. Both vertical and horizontal wells were considered for the additional wells. In particular, the expected performances of both well types were particular, the expected performances of both well types were compared on the basis of their impact on the long-term development program for Ness. program for Ness. This paper addresses the reservoir management questions raised and decisions made that led to the recommendation to drill the Ness horizontal well. Further, explanations are offered for the causes of the unusual logging-tool responses seen in horizontal wells, specifically from measurement-while-drilling (MWD) technology. Background Discovery and Geologic Description. The Ness field was discovered in May 1986 by the Exploration Well 9/13b-28A (S28A), which encountered 152 ft [46.3 m] of hydrocarbon-bearing Beryl formation sandstone. It was later completed as a development well. A second development well, Well 9/13b-31Z (S31Z), was completed in Aug. 1987 and had 106 ft [32.3 m] of Beryl formation pay. Figs. 1 and 2 show the field and wells, respectively.