Estimation of Tertiary Recovery by CO2 Injection - Springer A Sand, Northeast Purdy Unit

Abstract

Introduction The northeast Purdy Springer A sand reservoir is in Garvin County, OK (Fig. 1). The original oil in place (OOIP) was approximately 225 million STB (35.8 × 10 stock-tank m). Primary production from the field began in 1951. The waterflood was developed in three stages from 1960 through 1963. Cumulative oil production through 1977 was 79.5 million STB (I 2.6 × 10 stock-tank M); thus, the possible additional oil available for enhanced recovery is significant. The purpose of this work is to evaluate the potential of tertiary CO2 flooding in the reservoir. The representation of reservoir heterogeneity and the distribution of remaining oil saturation are extremely important to the calculation of oil recovery expected from CO2 flooding. Much effort was devoted to describing the reservoir heterogeneity. The reservoir characteristics were developed from a detailed geological study of depositional environment, log interpretations, core analyses, and pressure transient data. The large number of wells and limited reliable individual-well production data precluded simulation of the entire field. Therefore, a typical-pattern approach using average field properties was selected. This paper first presents the approach used in simulating primary production and the three stages of waterflooding. The method used to evaluate CO2 flooding is presented next, with emphasis on oil recovery sensitivity to the various flood design parameters. These include CO2 slug size, WAG ratio, and well spacing. Field History The initial pressure of the Springer A reservoir was 3,050 psig (21 MPa) at 8,200 ft (2500 m) subsea. The pressure declined to a minimum of about 900 psig (6.2 MPa) in 1964 and then increased as a result of water injection. Current average pressure in the reservoir is about 2,000 psig (13.8 MPa). Initial saturation pressure of the oil was estimated to be 2,785 psig (19.2 MPa), and the initial solution GOR was estimated to be 724 scf/STB (129 std m / stocktank m). The original PVT data are given in Table 1. Current average producing GOR is about 400 scf/STB (71.3 std m /stock-tank M), and the producing WOR is about 10:1. Geological Description The Springer A reservoir contains Lower Pennsylvanian rocks deposited as shallow marine sediments with environmental variations. The sand is a fine-to medium-grained siliceous sandstone dipping approximately 8 degrees southwest. Fig. 2 is a three-dimensional (3D) map showing the reservoir configuration. Data from core analyses indicate numerous tight zones within the gross pay section. Some cores show more than one well-defined pay zone, while others indicate only a single zone. JPT P. 808^