The Bachaquero Study - A Composite Analysis of the Behavior of a Compaction Drive/Solution Gas Drive Reservoir

Abstract

Merle, H.A.,* SPE-AIME, Shell Internationale Petroleum Mij. Kentie, C.J.P., Shell Internationale Petroleum Mij. van Opstal, G.H.C., Koninklijke/Shell Exploratie en Produktie Laboratorium Schneider, G.M.G., Shell Internationale Petroleum Mij. Compaction of reservoir rock can constitute an important drive mechanism in oil reservoirs. This mechanism may not be apparent until reservoir pressure declines below a certain threshold pressure. This review of a pressure declines below a certain threshold pressure. This review of a study of a large Venezuelan heavy-oil reservoir includes consideration of related surface subsidence phenomena. Introduction With some 18 billion bbl of oil initially in place, the Bachaquero Post-Eocene reservoir is one of the largest heavy-oil fields on the Bolivar Coast in Venezuela. The most typical feature of Bachaquero is the occurrence of substantial surface subsidence, locally exceeding 370 cm and indicating appreciable reservoir compaction. A study was made to analyze the subsidence/ compaction phenomena and the interaction of compaction with other prevailing drive mechanisms. Highlights of the study include (1) calculation of areal distributions of reservoir compaction from measured surface subsidence, using overburden deformation theory; (2) analysis of compaction/reservoir-pressure relationships; and (3) replay of past production performance with a special reservoir simulator. The study illustrates the existence of only threshold reservoir pressures below which appreciable rock compressibilities become evident. Compaction and solution gas drive are shown to act as the main production mechanisms, with their relative contributions varying considerably over the reservoir depending on local thresholds and oil viscosities. Bachaquero Post-Eocene Reservoir Reservoir Description As shown in Fig. 1, the Bachaquero Post-Eocene reservoir is a gently dipping monocline bounded by a major fault in the west, a nonconformity against a tectonic high in the northwest, and stratigraphic pinchouts toward the north and east. In the south the formation is water-bearing. The shore line of Lake Maracaibo forms the boundary of the land from which Compania Shell de Venezuela has been producing. Most of the lake area has been operated by Creole Petroleum Corp., while Mene Grande Oil Co. has produced from four lake parcels along the shore. The field is drilled with some 2,200 wells in a hexagonal pattern, with a basic well spacing of 231 m. The reservoir consists of unconsolidated sands interspersed with many clay lenses. Net sand thickness increases from northeast to southwest, up to some 107 m near the western boundary fault. Porosities are between 30 and 40 percent. Permeabilities are generally high, varying from 500 to 3,500 md in the well developed parts - that is, the lake area and the proven land area (Fig. 1). Oil gravity varies significantly over the reservoir, ranging from 12 degrees to 14 degrees API on land, and from 13 degrees to 18 degrees API under the lake. Initially, no fee gas was present in the reservoir. At the start of production the crude was at bubble point in the northwestern part of the field, with increasing undersaturation up to some 600 psi being evident toward the southeast. JPT P. 1107