Acoustic Emissions as a Tool for Hydraulic Fracture Location: Experience at the Fenton Hill Hot Dry Rock Site

Abstract

Summary The Delta South field is situated on an anticlinal high, positioned between two major growth faults. The field is in an offshore environment in the Niger Delta area. The G-2 and G-3 reservoirs contain about 90% of the field's reserves. Primary production from these reservoirs is by gravity drainage and gas-cap expansion. The effect of water influx on the total recovery mechanism has been minimal. Reservoir engineering studies indicate primary depletion will permit the recovery of 30% of the original oil in place (OOIP). The installation of a waterflood pressure maintenance project should allow the ultimate recovery to be increased to approximately 50% of OOIP. Introduction The Delta South field was discovered in May 1965 and is situated approximately 3 miles offshore the coastline of Bendel State, Nigeria (Fig. 1). The field is in a complex of 10 offshore fields operated by Gulf Oil Co. (Nigeria) Ltd. (GOCON) in conjunction with the Nigerian Natl. Petroleum Co. (NNPC), which is the majority partner. Fig. 1 shows that the field is adjacent to the GOCON/NNPC Escravos River Tank Farm. The field is in a nearshore surf area, and the water depth varies from 10 to 16 A. Production from the field is derived principally from the G-2 and G-3 sands. The G-2 reservoir is at an average depth of 8,920 ft subsea and is underlain by the G-3 reservoir. The two sands are separated by an intervening 100-ft-thick shale member that excludes pressure and fluid communication. The two reservoirs are similar in structure and rock and fluid properties. The Delta South field commenced production in March 1968. The production of 21 % of OOIP from G-2 by March 1978 had resulted in a reservoir pressure decline of 33%. The production of 29% of OOIP from G-3 during this same time interval resulted in a reservoir pressure decline of 39%. Reservoir rock and fluid property data acquisition commenced in 1967 when Delta South Well 8 was cored with a rubber sleeve core barrel. In 1972 Delta South Well 13 was cored. Reservoir oil fluid sampling was done in 1968 and again in 1979. The initial sampling was a prelude to simulation studies carried out at the Gulf Research and Development Co. facilities in 1970. The results of these and subsequent simulation studies are presented in a separate paper (Part 2). The studies have indicated the need for the installation of a water injection project in the field. The necessary approvals have been obtained, and project start-up was scheduled for late 1981. This paper describes the Delta South field, the producing history, the expected results, the injection/producing method, and the facilities to be installed. Part 2 describes the reservoir simulation work and the results of those investigations. Field Description The Delta South G-2 and G-3 sands were deposited dun ing mid-Miocene time. These sands are part of an extensive barrier bar system related to the ancient Niger River. Since Eocene time, the depositional forces prevalent at the confluence of the Niger River and the Bight of Benin were dominated by waves and tides. These types of deltaic deposits often produce reservoir rock of excellent quality. First, wave and tidal current velocities are relatively constant, which results in a fairly uniform grain-size distribution. Second, the constant reworking of the sediments tends to remove clay particles. Third, the sand bodies tend to have a sheet on blanket geometry (sands can be correlated easily for at least 4 miles along strike and 2 miles along the dip). JPT P. 141^