Alvord [3000 ft Strawn] LPG Flood Design and Performance Evaluation

Abstract

Summary Mitchell Energy Corp. has implemented a liquefied petroleum gas (LPG)/drive-gas miscible process in the Alvord (3000 ft Strawn) Unit in Wise County, TX, utilizing the U.S. DOE tertiary incentive program. The field has been waterflooded for 14 years and was producing near its economic limit at the time this project was started. This paper presents the results of the reservoir simulation study that was conducted to evaluate pattern configuration and operating alternatives to maximize LPG containment and oil recovery performance. Several recommendations resulting from this study were implemented for the project. On the basis of model predictions, tertiary oil recovery is expected to be between100,000 and 130,000 bbl [15 900 and 20 700 m3]or about 7% of the oil originally in place (OOIP) in the unit. An evaluation of the project performance through Dec.1981 is presented. This portion of the paper was written after drive-gas injection had just been initiated and represents only a preliminary evaluation of the project. In July 1981 the injection of a 16% hydrocarbon pore volume (HCPV) slug of propane was completed. Natural gas is being used to drive the propane slug. A peak oil response of 222 B/D [35.3 m3/d] was achieved in Aug.1981 and production has since been declining. This compares with a peak rate of 400 B/D [63.6 m3/d] during the waterflood, and an oil rate just prior to initiation of LPG injection of 7 B/D [1.1 m3/d]. The observed performance of the LPG flood indicates that the actual tertiary oil recovered will reach the predicted value, although the project life will be longer than expected. The results presented in this paper indicate that, without the DOE incentive program, the economics for this project would still be uncertain at this time. Introduction The Alvord (3000 ft) is approximately 10 miles [16 km]north of the town of Bridgeport in Wise County, TX(Fig. 1). The Alvord (3000 ft) Unit has been successfully waterflooded and was producing near its economic limit in Dec. 1979. A tertiary flood then was considered as an alternative to abandoning the field. Of the tertiary methods available, micellar flooding was eliminated from serious consideration because freshwater was not available and, according to published screening parameters, the calcium and magnesium concentrations were too high in the reservoir and supply water. CO2 was ruled out because of the question of miscibility at this depth and lack of supply. The LPG/drive-gas miscible process was chosen because miscibility could be achieved and convenient sources of LPG and natural gas were available. Economic risks were reduced because the DOE tertiary incentive program was available at that time. The LPG/drive-gas process has produced favourable results in waterflooded reservoirs in the past. The objective of this process in a waterflooded reservoir is to connect the residual oil with the injected LPG and displace it to the producing wells. The water must be displaced by the developing oil bank, the oil must be miscibly displaced by the LPG, and the LPG must be miscibly displaced by the drive gas. The mobility ratio is unfavorable at all flood fronts, and one should expect viscous fingering of the LPG into the oil bank and viscous fingering of the drive gas into the LPG bank. This is an important phenomenon and results in the rapid dissipation of small LPG slugs. When the reservoir is horizontal, the slug dissipation is faster because of stratification and gravity override. When the LPG slug is dissipated, the process reverts to an immiscible gas drive. In the early history of LPG flooding, the use of slugs that were too small probably prevented many projects from being as successful as expected. Several publications indicated that LPG slug sizes, on the order of 10 % HCPV or less, would be adequate for most reservoir situations. Later, others reported that because of viscous fingering, stratification, and gravity override, larger LPG slugs were needed. From the onset of this project it was felt that a successful project required a large LPG slug (about 20%HCPV). It was also recognized that a pattern flood configuration and operating procedure would have to be adopted that would prevent the mobilized oil and the displacing LPG solvent from migrating downdip into the aquifer. Thus a short simulation study was undertaken before embarking on the field project to evaluate alternative flood patterns and operating procedures and to estimate project recovery performance. General Field Description The field is in the central portion of the Fort Worth basin. It produces from an upper Pennsylvanian Strawn series sandstone, the Bryson sand, which was deposited by fluvial processes. A porous sand isopach of the Bryson sand is presented in Fig. 2. JPT P. 119^