Simulation of a Partial Pressure Maintenance Gas Cycling Project with a Compositional Model, Carson Creek Field, Alberta

Abstract

A model study of gas cycling retrograde condensate reservoirs was conducted to compare these three schemes: continued cycling with low rate gas sales, cycling with normal rate gas sales, and depletion without further cycling. Results show that after six years of cycling at dewpoint pressure, partial cycling with normal rate gas sales is the most economical of the schemes studied. Introduction The Carson Creek field is a retrograde gas condensate field of Central Alberta, discovered in 1957. The field encompasses about 18,000 acres and contains the Carson Creek Beaverhill Lake A Pool and the Carson Creek Beaverhill Lake B Pool. The original raw gas in place in the two pools was about 400 Bcf. The field was not produced until Dec., 1962, when gas cycling operations began. Original cycling operations consisted of processing between 70 and 90 MMcf/D raw gas to extract about 4,000 B/D condensate, and reinjecting all residue gas into the reservoirs. During July, 1963, the cycling plant also began processing from 5 to 10 MMcf/D solution gas from the adjacent Carson Creek North oil field. Residue gas from this stream was used as make-up gas and also was injected into the Carson Creek reservoirs to fully replace reservoir withdrawals. During July, 1966, deep cut facilities were installed at the plant to extract LPG. Plant liquids production since then has averaged about 6,400 B/D. Gas sales of about 5 MMcf/D from the plant began in July, 1967, and were increased to 15 MMcf/D in Nov., 1968. To the end of 1968, the cycling operation had recovered about 26 percent of the original in-place pentanes-plus and 12 percent of the original in-place LPG from the reservoirs. It was proposed to increase gas sales above 15 MMcf/D and to continue cycling by only partially replacing the reservoir withdrawals. Under such a scheme the raw gas plant throughput rate would be unchanged, thereby maintaining liquid production at a high rate. Residue gas to meet the gas sales contract rate would be sold and the amount remaining after sales would be reinjected. The reservoir pressure decline resulting from increasing the gas sales rate would cause retrograde condensation to occur and possibly reduce the recoverable reserves of natural gas liquids. It was expected, however, that much of the condensed liquids could be revaporized as a result of the continued injection of lean gas. A study was therefore undertaken to analyze the reservoir performance through its history and to predict liquid recoveries, taking into account the effect predict liquid recoveries, taking into account the effect of revaporization of retrograde liquids in the reservoir The study included analysis of three schemes of future operation:continued cycling with low rate gas sales as in the present scheme,partial cycling with nominal rate gas sales, andreservoir depletion with normal rate gas sales and no further cycling. Laboratory experiments were performed to evaluate the revaporization characteristics of the Carson Creek retrograde liquid by lean injection gas. A two-dimensional, three-phase compositional model was developed to simulate reservoir performance, taking into account revaporization in the reservoir. This paper discusses the study and presents both the results paper discusses the study and presents both the results of the history match and the predicted performance for the three schemes of future operation investigated. JPT P. 38