The First Long-Term Horizontal-Well Test in the Troll Thin Oil Zone

Abstract

Summary An 11-month horizontal well production test was performed offshore Norway inthe giant Troll gas field to prove performed offshore Norway in the giant Trollgas field to prove possible thin oil zone reserves. The oil column in the testarea possible thin oil zone reserves. The oil column in the test area is only 22 m, and the 500-m horizontal well was positioned 4 m from the water zone and18 m from the gas zone. The well, tied into production and testing ship Petrojarl 1, was put on production in production and testing ship Petrojarl 1, was put on production in Jan. 1990, The initial oil rate from the horizontalwell was more than four times higher than that of a vertical well in the samearea, Test results show that horizontal wells represent a viable technology foreconomic oil production from the thin oil zone in the Troll field. Introduction The Troll field, located below 300 m of water offshore Norway (Fig. 1), contains 0- to 26-m-thick oil rims sandwiched between a large gas cap and anactive aquifer. In the westernmost fault block of the field, the Troll West oilprovince, the oil zone is the thickest, 22 to 26 m. The oil in place (OIP) ofthis province is estimated to be 121 × 10(6) stock-tank m3. The oil in the Troll West oil province is located in high-quality sands withpermeabilities from 3,000 to 10,000 md. The oil production is limited by gasconing, resulting in rapidly production is limited by gas coning, resulting inrapidly decreasing oil rates. Developments by vertical wells therefore haveconsistently exhibited marginal economy. Horizontal wells are known to improve well productivity and to reduce water-and gas-coning problems. The Helder field, offshore The Netherlands, wasredeveloped in 1987–88 with 10 horizontal wells. Results show improvedvolumetric sweep, reduced water coning, and productivities up to 20 timeshigher than for vertical wells. In the Prudhoe Bay field in Alaska, manyhorizontal wells have been drilled. A 5-mm-long horizontal well in this fieldyielded productivities 1.5 to 3 times that of vertical wells. The wells alsoare successful in reducing gas coning. In the Troll field, pretest simulation studies indicated that developmentbased on 500-m horizontal wells could offer an economically attractive oildevelopment. Because of the 300-m water depth, highly unconsolidated reservoirrock, and a thin oil column, however, development with horizontal wells wasconsidered a high-risk project. To confirm the horizontal-well potential and long-term behavior, and therebyreduce the risk involved, a decision was made in June 1989 to perform an8-to-12-month production test with a 500-m horizontal well in the oil zone ofthe Troll West oil province. province. The well, operated by Norsk Hydro A.S., was completed in Dec. 1989, and production from the test ship began in Jan.1990. When the test concluded after 11 months, the well had produced 1 043 000stock-tank m3 of oil and the water cut had stabilized at about 35%. Background The Troll field covers 700 km2. The estimated gas in place (GIP) and OIP are1670 × 10(9) std m3 and 615 × 10(6) stocktank m3, respectively (Fig. 2). Twomain north/south faults divide the field into three provinces: Troll West oilprovince, Troll West gas province, and Troll East (Fig. 2). The Troll West oilprovince is province, and Troll East (Fig. 2). The Troll West oil province isinto the Southern and the Northern oil provinces (Figs. 3 and 4). Concession Block 31/2 (Fig. 2), operated by A/S Norske Shell, was awarded in1979. Blocks 31/3, 31/5, and 31/6 were awarded in 1983 with shared operatorshipbetween Norsk Hydro A.S., Den norske Stats Oljeselskap A.S., and Saga Petroleum A. S. Phase 1 of the Troll development, as proved in 1986, calls for gas Phase 1 of the Troll development, as proved in 1986, calls for gas production to startfrom Troll East in 1996. Before the horizontal test production to start from Troll East in 1996. Before the horizontal test well, 26 exploration wells hadbeen drilled within the four blocks; hydrocarbons were encountered in 22 wells. Two-dimensional seismic surveys Cover all four blocks. To select the horizontalwell location accurately, a 3D seismic survey was carried out during the summerof 1989. Reservoir Description The Troll field is contained within three easterly tilted fault blocks(Figs. 2 and 3). The reservoir interval is of Middle to Upper Jurassic Age, andthe reservoir sediments consist of clean, medium to coarse sand interbeddedwith micaceous and silty, very fine to fine sand. These sediments are generallyvery poorly consolidated. A typical feature of the reservoir sediments in the Troll field is thenumerous calcite-cemented layers occurring in all lithologies in all wells. There are two classes of calcite-cemented layers: extensive (up to severalkilometers) wide sheets found at the boundaries of the geological zones and theless extensive (1 to 100 m) calcite-cemented layers located within thegeological zones. JPT p. 914