A reservoir simulation study of a water-drive gas reservoir, the Soehlingen Schneverdingen Sandstone, was carried out in order to estimate its gas in place and to determine production strategies which optimize the ultimate recovery.
The reservoir study was performed with Mobil’s black oil simulator, ALPURS, and followed closely the representation set forth by the log analysts and production geologists, which determined the geologic zonations and many of the reservoir parameters and trends. During the history-match phase, the critical match parameters proved to be the vertical communication and the aquifer size in comparison to the gas-cap size. The vertical communication could be specified from a coning study of a well producing water, and from diverging shut-in pressures which developed between two wells in the field. The aquifer si?e in comparison to the gas-cap size could be determined within limits from the differing pressure developments at the producing wells.
The prediction phase of the study revealed the somewhat surprising result of gas pockets being left behind even by structurally-high wells. This is a consequence of the water front cusping into the wells before the pressure level in the gas zone could be completely reduced. The thin layers of tight vertical permeability also contribute to this effect by reducing communication within the gas cap.
Several alternative production strategies were investigated in detail, such as accelerating the gas production to "outrun" the water encroachment, producing large amounts of water in one well after it waters out to slow the water movement to the other wells, and varying the location and the timing of additional wells. The accelerated production and other timing effects do not appear in this case to provide a significant improvement in the ultimate recovery. Water production, however, would be beneficial under certain circumstances, provided that the water could be economically disposed of. After accounting for the technical feasibilities as seen today, the various management proposals could be evaluated for the future development of the field.