Accelerated Blowdown of a Strong Water-Drive Gas Reservoir

Abstract

The final blowdown of a Gulf Coast water-drive gas reservoir at a reserves/ production ratio of less than 2 provided an increase in gas recovery. production ratio of less than 2 provided an increase in gas recovery. Pressure/production performance did not conform to conventional tank-type Pressure/production performance did not conform to conventional tank-type material balance predictions because of a large pressure gradient in the water-invaded region. The material balance equation was altered to account for the pressure behind the front and was adapted to an economic optimizer program. Introduction The added recovery benefits attributable to accelerated blowdown of strong water-drive gas reservoirs are well known. The planning for effective blowdown of such reservoirs requires the accurate prediction of reservoir pressure and production performance. Deliverability pressure and production performance. Deliverability maintenance investments must be scheduled well in advance and economic incentives depend on accurate forecasts of gas recovery. Beginning in late 1969, a strong water-drive gas reservoir in the Katy field was blown down at a reserves/ production ratio (R/P) of less than 2. Although gas production ratio (R/P) of less than 2. Although gas recovery during blowdown was more than 30 percent greater than that obtained at a low production rate equivalent to an R/P of 15, reservoir pressure/production performance varied considerably from predictions made performance varied considerably from predictions made using a conventional van Everdingen-Hurst unsteady-state material balance. Reservoir pressures declined more rapidly than predicted, and gas recovery was less than expected. Performance of the reservoir suggested the presence of a substantial pressure gradient in the presence of a substantial pressure gradient in the waterinvaded region, and the conventional material balance used was not capable of modeling it. To obtain accurate predictions, the van Everdingen-Hurst unsteady-state material balance equations were modified to account for higher pressures in the water-invaded region. Using the modified equations, it was possible to match accurately the pressure/production possible to match accurately the pressure/production performance for the Katy reservoir. performance for the Katy reservoir. Katy V-C Reservoir Description and History The Katy V-C reservoir is a uniformly developed sand in the Yequa formation, approximately 40 ft thick, having an original productive area of 7,300 acres. Structurally, the reservoir is an elongate, north-south trending, unfaulted anticline with 105 ft of structural closure above the original gas-water contact at -7,240 ft. Dip on the flanks is a fairly uniform 180 ft/mile. Original gas in place was 330 Bcf. Reservoir rock and fluid data developed from laboratory analysis are shown in Table 1. Early development of the reservoir was designed to supply gas for a small sale and as make-up for fuel and shrinkage incurred in cycling other zones. Fig. 1 shows the completion history. Cycling of the V-C reservoir began in 1950. The cycle program included overinjection during the early years. program included overinjection during the early years. This contributed to a substantial repressuring. The cycle pattern was end-to-end with injection confined to Well pattern was end-to-end with injection confined to Well 4302, completed below the original gas-water contact. A bottom-hole pressure measured in this well, outside the original productive limits and before injection, was 270 psi above the pressure in the uninvaded gas zone as psi above the pressure in the uninvaded gas zone as measured uniformly in three wells active at the time. After cycling was completed, limited gas production was resumed until Sept. 1969 when accelerated blow-down was started. Before blowdown, the production rate had declined below 15 MMcf/D as water influx began repressuring the reservoir (Fig. 2). Cumulative production to Sept. 1969 was 151 Bcf while the reservoir production to Sept. 1969 was 151 Bcf while the reservoir pressure was 2,830 psi. pressure was 2,830 psi. JPT P. 1533