Montrose Field Reservoir Management

Abstract

ABSTRACT The Montrose Field, one of the earliest oil discoveries in the UK sector of the North Sea, was put on production in 1976. The Field is operated by Amoco for the Amoco/Gas Council Group, a group of companies comprised of the British Gas Corporation, Texas Eastern and Amerada Hess in addition to Amoco. Extensive reservoir pressure data collection from the start of production enabled the Amoco/Gas Council Group to develop a reservoir depletion policy that was by no means obvious from reservoir studies completed prior to the start of production and is believed will result in the maximum recovery of oil. The reservoir pressure data gathered included conventional bottomhole shut-in pressure buildup surveys plus, in all wells drilled since 1977, Repeat Formation Tester(RFT) pressure surveys; a tool which has only recently been found to be a strong aid in reservoir evaluation and in developing a sound reservoir management policy. INTRODUCTION The Montrose Field is located in 9l.4m (300 feet) of water in the UK sector of the North Sea approximately 209 kilometres (130 miles) east of Aberdeen, Scotland (see Figure 1). The Field was discovered in 1971 with the drilling of Well 22/18-2 and overlaps two UK Blocks (22/17 and 22/18) both of which are licensed to the Amoco/Gas Council Group. A centrally-located, 24 wellslot steel platform was installed in 1975 and to date 20 wells, 16 producers and 4 water injection wells, have been drilled. Production started mid-1976 and in 1979averaged 4452 cubic metres of oil per day (28,000 bopd) and 238 cubic metre of water per day (1,500 bwpd). A water injection program was initiated in March,1978 with an injection capacity of 7950 cubic metre per day (50,000 bwpd). Oil is transported from the Field by tankers loaded offshore via two loading buoys, each of which are connected to the platform by a 1.6 kilometre(one-mile) long subsea pipeline. Neither the loading buoys nor the platform contain significant oil storage facilities and thus whenever weather conditions become too severe for the tanker to remain moored to the buoy, production is shutdown. Production downtime averaged 20 percent in 1979. The Montrose Field reservoir is a sequence of deep water turbidite sandstones and minor shales of Paleocene age. â?¢The structure is a low-relief double anticline with local structural highs and lows related primarily to the areal distribution of sandstone and shale in the top parts of the Paleocene Sand. The reservoir oil column is underlain by water. RFT pressure measurements in wells drilled after the start of production indicate that vertical communication exists between the aquifer and the oil column. The water injection programme was designed for injection into the aquiferbeneath the oil column, rather than along the edge, in the belief that better pressure maintenance and a higher waterflood sweep efficiency would result. Subsequent pressure surveys and the results of a reservoir simulation study have increased the Group's confidence in this decision, and reservoir performance to date has substantiated the correctness of the injection policy. This paper reviews the reservoir management policy formulated for the Montrose Field and, in particular, the importance of RFT reservoir pressure data in the making of the policy.