A Numerical Simulation of Kaybob South Gas Cycling Projects

Abstract

In this study of the relative performance of downdip, line-drive, and crestal injection schemes, a three-dimensional, three-phase model was used to predict project performance and two-dimensional areal and cross-sectional models were used for supplementary calculations. The results of the study made it clear that a different scheme would have to be used with each of the two units in question. Introduction The Kaybob South field is located approximately 160 miles northwest of Edmonton, Alta., Canada. The Beaverhill Lake A pool was discovered in 1961, and subsequent development resulted in the formation of three Unit areas (Fig. 1). In the first of these Units (Unit 1), a gas cycling scheme was started in 1968. This was necessary because the reservoir fluid exhibits retrograde condensation and is extremely rich in condensate and sulfur. (The simulation of a gas cycling project for Unit 1 has already been discussed in a paper that presents a concise description of the development history, geology, data preparation and predicted performance of the area.) predicted performance of the area.) Two subsequent Units (Nos. 2 and 3) were formed to the south of Unit I as a result of additional drilling. Governmental authorities approved cycling schemes with throughputs of 170 MMcf/D and 445 MMcf/D for Units 2 and 3, respectively, with an initial voidage replacement of about 52 percent of withdrawals. To predict better the performance of these two Units predict better the performance of these two Units under a broader range of cycling pattern alternatives, more sophisticated mathematical models were required than those used for Unit 1. The inclusion of gravity override effects in this new work caused a greater areal dispersal of the injected dry gas fronts. To ensure that the results of this study were true to reality, a group of associated studies was performed before the proposed cycling alternatives were investigated. The results of these auxiliary studies provided:interference and water influx information,pressure drawdown restrictions due to water coning,pseudo relative permeability relationships,reasonably sized grid spacings, anddata on vertical communication and gravity effects. These results were incorported into the main study of cycling pattern alternatives. We shall describe here the work performed and present the results obtained. Reservoir Description Following the formation of Unit 1, additional drilling defined the eastern and southern limits of the reservoir. The Beaverhill Lake gas pool extends approximately 32 miles and currently contains 57 gas wells in the area defined as Units 2 and 3. In this area the reservoir can be geologically divided into two stages of reef growth. The two stages have distinctive lithologies and are separated by a correlatable green shale marker that averages about 5 ft in thickness and is referred to as the "B" shale marker (Fig. 2). The porous layer beneath this shale consists of a medium to coarsely crystalline dolomite. The upper porous unit is distinguished by a fine to medium porous unit is distinguished by a fine to medium crystalline matrix. Initial formation pressure data indicate that a gradient exists from north to south across the pool. This pressure gradient, along with a depressed gas-water contact in most of the Unit 3 area, led to the postulation of interreef interference. There appears to be communication with the Pine Creek and the Pine Northwest fields, as illustrated in Fig. 2. JPT P. 1253