Modeling a Repressured Waterdrive Gas Reservoir
- A.K. Moltz (Exxon Co. U.S.A.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- April 1993
- Document Type
- Journal Paper
- 314 - 318
- 1993. Society of Petroleum Engineers
- 5.1.5 Geologic Modeling, 4.6 Natural Gas, 1.6 Drilling Operations, 1.6.9 Coring, Fishing, 5.6.1 Open hole/cased hole log analysis, 5.5.8 History Matching, 4.1.4 Gas Processing, 5.2.1 Phase Behavior and PVT Measurements, 2.4.3 Sand/Solids Control, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc)
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Reservoir modeling confirmed reserve losses occurring in a waterdrive gas reservoir to justify blowdown project initiation. The material-balance models initially used could not match the water influx and repressuring observed in the reservoir. This paper shows that material-balance calculations need to be modified to account for trapped-gas compression that occurs with repressuring, and demonstrates the use of a "simple" numerical simulation model to predict performance. Numerical simulation enabled us to match reservoir behavior successfully, quantifying the effects of trapped-gas compression, and to identify the importance of critical blowdown rates and timing for optimizing reserve recovery from this repressuring waterdrive gas reservoir.
The Tom O'Connor 5,100 Ft Sand is a gas reservoir located in Refugio County, TX. This fairly homogeneous Frio sandstone contains dry gas with a moderate directional waterdrive from the southwest. Fig. 1 gives a composite log and a structure map of the reservoir. At the start of this study, the reservoir was already in the later stages of depletion, having recovered 85% of the original gas in place.
During a period of low gas market demand, the reservoir experienced significant water advancement and gas-zone repressuring from a lengthy production curtailment by the purchaser. Gas-zone pressures increased from 800 to 1,100 psig during this period (see Fig. 2). The operator successfully demonstrated that substantial reserve losses were occurring and obtained a special gas allowable to blow down the reservoir .
Reservoir modeling was performed to confirm and quantify the gas reserve losses resulting from production curtailment and to predict the additional recovery that would result from blowdown initiation. Numerical simulation was used effectively to model the historical performance of the reservoir and to determine the optimum blowdown rate to maximize recovery.
A simple tank-type model was developed with constant reservoir rock and fluid properties. The model incorporated radial flow from one central producer and contained a limited, directional aquifer as indicated by performance. Fig. 3 gives the reservoir input properties and model layout. Fig. 4 shows historical water encroachment and remaining free-gas volumes determined from known gas/water contact locations based on drill wells and gamma ray/neutron log surveys. Model accuracy was enhanced by the lengthy performance data available, which allowed excellent model calibration through history matching.
|File Size||429 KB||Number of Pages||5|