Case History and Appraisal of the West Buffalo Red River Unit High-Pressure Air Injection Project

Abstract

Abstract The West Buffalo Red River Unit (WBRRU) project is one of the three High-Pressure Air Injection (HPAI) projects in the Buffalo Field. It produces from the Red River reservoirs, which are deep (~8,400 ft), thin (~15 ft), hot (215 °F), low permeability (~10 md) carbonates. The unit comprising 4,560 acres with 15 producing wells was formed in July 1987 and began air injection operations four months later. Before unitization the oil production was declining steadily but the response to air injection has resulted in a significant production increase over its historical decline. While unit production in July 1987 was 200 BOPD, a peak production rate of 500 BOPD was achieved in January 1990 with 15 producing wells, after the injection of 2 BSCF of air. As of December 2006 about 23 BSCF of air had been injected, for a cumulative oil production of 2.3 MMSTB since the beginning of injection. This paper reviews the production and economic performance of the WBRRU high-pressure air injection project to date. The production performance analysis includes estimates of incremental oil recovery due to air injection using conventional decline curve analysis, the expected ultimate recovery using the volume-burned method, and a discussion on how air utilization has changed over time. A detailed economic evaluation is carried out using different economic yardsticks such as net present value and incremental rate of return. A discussion on the impact of oil price on the economic profitability of the project is presented as wells as an estimate of the minimum oil price required for attractive economic results. To date, the HPAI project in WBRRU continues to be a technical and economic success. In year 2005 old vertical producers were re-entered to drill horizontal laterals and take advantage of the great reservoir response to injection. Introduction The WBRRU project is one of the three HPAI projects currently in operation in the Buffalo field1–2, which is situated in the southwest edge of the Williston Basin as illustrated in Fig. 1. The WBRRU is one of the six units in the Buffalo field1–2 and is located in Townships 20–21N, Ranges 3–4E of Harding County, South Dakota, in the western area of the field as shown in Fig. 2. The unit produces from the Red River carbonate reservoir of Ordovician age that occurs at an average depth of 8,400 ft. The Red River is the deepest formation in which widespread commercial production has been found in the Williston Basin. The recovery mechanism under primary production was by liquid expansion and rock compaction. Little gas was produced so solution gas and gas expansion are not significant contributors to the drive mechanism. Ultimate primary recovery is estimated to be 6.5% of the original oil in place. Several different improved oil recovery techniques were considered. Waterflooding was ruled out primarily because of the anticipated limited water injectivities and also because of high water saturations. The cumulative reservoir voidage of approximately 2.2 million barrels could not be replaced within a reasonable length of time; the best estimates indicated it would take over three years to attain fill-up assuming an average injection rate of about 200 BWPD for 10 injectors. Gas and/or carbon dioxide (CO2) were ruled out mainly due to the excessive high cost of both gas and CO2.3 The process finally selected was high-pressure air injection primarily because of the successful performance of the two HPAI projects located northeast of the proposed WBRRU in the same field, the one in the Buffalo Red River Unit (BRRU)4–6 and the other in the South Buffalo Red River Unit (SBRRU)5–6. The WBRRU comprising 4,560 acres with 15 producing wells was formed in July 1987 and began air injection operations four months later. Before unitization the oil production was declining steadily but the response to air injection has resulted in a significant production increase over its historical decline. While unit production in July 1987 was 200 BOPD, a peak production rate of 500 BOPD was achieved in January 1990 with 15 producing wells, after the injection of 2 BSCF of air through 7 injection wells. As of December 2006 about 23 BSCF of air had been injected, for an incremental oil production of nearly 2 MMSTB.