Synergistic Evaluation of a Complex Conglomerate Reservoir for EOR, Barrancas Formation, Argentina

Abstract

Summary An engineering/geological study of the Top Red Conglomerate (TRC) section of the Barrancas formation, Mendoza area, Argentina, was conducted (1) to evaluate historical waterflood performance and recovery efficiency and (2) to develop a reservoir description and predictive model for later use in evaluation of reservoir response to EOR process applications. Original oil in place (OOIP) in the TRC reservoir was about 400 million STB [616 ⨯ 10–6 stock-tank m3]. The field had produced about 154 million STB [24.5 ⨯ 10–6 stock-tank m3] or 38.5% OOIP through 1980 and is under consideration for application of a caustic flooding EOR process. The TRC shows extremely large variations in permeability, both areally and vertically, owing to its origin as the uppermost part of a thick, alluvial fan, braided channel sequence of sediments. Porosity and permeability development in these rocks is governed primarily by the abundance of detrital clays. Reservoir quality also is reduced somewhat in localized areas by the presence of calcite and zeolite cements and by authigenic clays. An abundance of chemically reactive minerals in the formation poses a significant potential for formation damage and/or adverse reactions with injected EOR chemicals. A geological description of layering and areal variability in the reservoir was developed and used to guide the application of a black oil simulator to two cross-sectional models. Simulation of waterflood performance indicated good vertical sweep efficiency near injection wells, with less efficient sweep farther away owing to gravity segregation and an adverse mobility ratio. A preliminary screening and feasibility study evaluated several EOR processes for recovering the oil left after waterflooding. Caustic flooding appeared to be the most feasible EOR process for application in this reservoir. The mineralogy, chemical reactivity, and cation exchange capacity (CEC) of representative core samples were examined as a part of the EOR feasibility screening. The understanding of reservoir performance, distribution of remaining oil in place, reservoir heterogeneity, and chemical reactivity of the formation obtained during this study provided the basis for a reservoir model to be used in subsequent predictions of performance under EOR operations. Introduction The Mendoza contract area operated by Argentina-Cities Service Development Co. is about 50 miles [80 km] southeast of the city of Mendoza in the Cuyo basin of western Argentina (Fig. 1). The major producing formation in this study is the TRC interval of the Juro-Cretaceous Barrancas formation. The Punta de las Bardas (PB), Vacas Muertas (VM), and Gran Bajada Blanca (GBB) fields are contiguous and form a single, common reservoir in the TRC. Fig. 2 gives the stratigraphic nomenclature in the area. Development of the TRC reservoir began in 1959. The field produced under primary depletion, assisted by a natural water drive, until 1967, when it was recognized that the natural water drive was not strong enough to sustain continued fluid withdrawal from the reservoir. At this point, a pressure-maintenance program was implemented by injecting water into most of the wells located near the original water/oil contact. Individual well production performance has been controlled primarily by sand quality, structural location, completion intervals, and proximity to local permeability barriers in the reservoir. Material balance calculations indicate that the aquifer is of only limited influence in reservoir pressure maintenance. The initial pressure in the TRC reservoir was about 2,600 psi [17.93 MPa]. The pressure declined to a minimum of about 850 psi [5.86 MPa] by 1967–68, and then increased as a result of water injection pressure maintenance. The current reservoir pressure is about 1,500 psi [10.34 MPa]. Fig. 3 shows a waterfront advance map superimposed on a structure contour map of the TRC reservoir. The waterfronts shown represent advancement of a 40% water-cut front through time. The map shows that water advancement has been controlled by the structural configuration of the reservoir and by the presence of permeability barriers, particularly the large area near the center of the reservoir where the TRC interval is missing. It is unclear whether this area resulted from erosion of the TRC or from nondeposition; however, it does form an effective local barrier to water influx. JPT P. 295^