Abstract

A review is given of the problems of sea water injection in North Sea reservoirs. The multiplicity of filtration systems used or being considered and their elation to water quality are discussed. Initial results are given for an experimental programme attempting to quantify and analyse the impairment of permeability in core plugs resulting from water permeability in core plugs resulting from water injection. Significant impairment was recorded due to core plugging even with dilute systems of fine particles. particles

Introduction

The development costs of oil reservoirs in the North Sea are on such a scale that it is necessary to ensure maximum initial production rates and high early recovery to provide economic returns on the heavily front-loaded capital investment. Pressure maintenance by water injection is therefore an essential feature of many development plans.

Reviews of a number of North Sea developments were presented recently at Europec (Figure 1). For example, presented recently at Europec (Figure 1). For example, BP's Forties Field which was discovered in 1970 has been on production since mid 1975 and sea water injection commenced in late 1976. This major field has a production of 500,000 BOPD (3312 m /h) and a water injection of 450,000 BWPD (2981 m /h), reduced from the originally planned 600,000 BWPD (3975 m /h) on the evidence during planned 600,000 BWPD (3975 m /h) on the evidence during the three years of production of a significant natural water drive. This water injection system has been extensively described and is referred to later.

The Shell/Esso development of the Brent Field also includes seawater injection at an early stage. This field, which was discovered in deeper more northerly waters in 1971, is structurally more complex than Forties and the water is to be injected downdip in the dipping unconformity trap. Small scale production from Brent commenced in 1976 but full-scale production and water injection is not expected until this year, 1979, when all the production and transport facilities should be completed. Optimum production offtake will be in the order of 600,000 BOPD (3975 m /h) with a planned maximum water injection capacity of about 1 million planned maximum water injection capacity of about 1 million BWPD. Shell/Esso are simultaneously developing other fields in the Brent area and these will also have water injection schemes. The overall Brent complex plan powerfully demonstrates the complications of developing a number of related major offshore fields and the susceptibility to delay of overall production targets when individual developments within such a plan are delayed.

The Mobil Beryl 'A' Field which was discovered in 1972 in a complex faulted structure started producing in 1976. Pressure maintenance by injection of both associated gas and Pressure maintenance by injection of both associated gas and seawater is planned. Gas re-injection commenced in 1977 and seawater injection is due to start shortly. No details have been released of the water injection system.

The BNOC Thistle Field which was discovered in 1973 started producing in 1976. A field peak rate of 216,000 BOPD (1431 m3/h) is planned and pressure maintenance will be by seawater injection supplemented by gas re-injection. A planned water injection building up to 250,000 BWPD (1656 m3/h) should commence early 1979.

The Occidental Piper Field which has been on production since late 1976 is currently flowing at 300,000 BOPD (1987 m3/h). Water injection which commenced early 1978 is at present at 190,000 BWPD (1258 m3/h) and is planned to build present at 190,000 BWPD (1258 m3/h) and is planned to build up to 250,000 BWPD (1656 m3/h). Using accurate pressure build-up surveys aquifer influx rates have been determined at an early stage in the field development.

The Shell/Esso Auk Field is an interesting non-typical North Sea field where oil was discovered in 1971 in a thin poorly defined Zechstein carbonate structure. Production poorly defined Zechstein carbonate structure. Production commenced in 1975 and initial development plans called for pressure maintenance by seawater injection since a pressure maintenance by seawater injection since a communicating aquifer was not expected.

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