Success Factors in Troll Geosteering

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 17110, “Success Factors in Troll Geosteering,” by S. Leiknes and I. Osvoll, Norsk Hydro ASA, prepared for the 2005 Offshore Technology Conference, Houston, 2–5 May. Copyright 2005 Offshore Technology Conference. Reproduced by permission. Hydro has produced oil from horizontal wells in the thin oil layer of the Troll West field offshore Norway since 1995. The main challenge is to drain the thin target sands efficiently, within a remain-ing oil column less than 20 m thick covering an area of approximately 500 km2. New horizontal producers are continually being drilled. Vertical geosteering is performed on a centimeter scale, and quick decision making within the well teams is needed to optimize the well paths while drilling. Introduction The Troll field is the largest producing oil and gas field on the Norwegian continental shelf. The field is in the northern North Sea on the margin of the Viking graben and is divided by two major north/south trending faults separating the field into three provinces: Troll West oil province, Troll West gas province, and Troll East. Oil has been produced from horizontal wells in the Troll West oil and gas provinces where the initial oil-column thickness was 22 to 27 m and 11 to 13 m, respectively. More than 150 horizontal producers have been drilled in Troll West, and approximately two-thirds of the total oil reserves of 1.4 billion bbl has been produced. The oil column in Troll East is 1 to 4 m thick, and only the huge gas cap has been considered commercial. New horizontal producers are drilled in the progressively thinning oil layer. As shown in Fig. 1, well planning and drilling are increasingly difficult because of the ever-larger number of existing producers, the consequent decrease in remaining targets, and movement of the fluid contacts resulting from production. Collaboration between geophysicists, geologists, and reservoir and production engineers is crucial to meet this challenging task. Reservoir Framework The reservoir consists of stacked, off-lapping, shallow marine sandstones and intervening siltstones mostly in the Upper Jurassic (Oxfordian) Sognefjord formation. Reservoir zones correspond to the finer and coarser portions of the prograding wedges (parasequences) and can be mapped with good confidence over most of the field. For geomodeling and well-planning purposes, the depositional succession is simplified into clean sand and micaceous sands. The clean sands generally are well sorted, with permeabilities ranging from 1 to 30 darcies, while the micaceous sands are characteristically finer and silty, with permeabilities ranging from 10 to several hundred millidarcies.