Relation of Gas-well Spacing to Ultimate Recovery

This paper embodies the results of theoretical studies concerning gasreservoirs, especially the effect of drilling programs of various intensitiesupon pressure depletion and ultimate recoveries. The primary objective has beento determine the net quantity of recoverable gas that could be obtained as aresult of each additional well, in comparison with the gas that might besecured from a single well ideally placed with respect to both acreage andgeological structure. No method is yet known whereby the relative merits ofintensive and sparse drilling can be compared experimentally; a reservoir musteither be drilled closely or developed with wide spacing, but both programscannot be conducted simultaneously. Hence, the analytical method seems to bethe only approach to the problem. While the present state of our theoretical knowledge does not offer a precisesolution to the problem of gas flow in sand bodies, it takes such appropriatecognizance of the limitations of the methods used as to restrict any answerobtained to the realm of engineering accuracy. In solving the problem for theaverage gas field, it was found that practically the same answers resulted asto pressure distribution, regardless of the size of the drainage radius, eventhough the latter was assumed to vary from a few feet as a minimum to that ofthe entire reservoir as a maximum. Such limits represent extreme possibilities;therefore, the practical conditions that are associated with normal developmentprograms, in which wells are placed in the center of tracts varying in sizefrom 40 to 640 acres, will of necessity result in less extreme variations thanthose indicated in the tables accompanying this paper. Stated more concisely, the amount of gas unrecovered at abandonment (of the particular field used forillustrative purposes) will lie somewhere between 4.6 and 6 per cent of thetotal original content, no matter how intensively the field is drilled. Gas reservoirs may be divided into two general types, those in which the energyis intrinsic, and those of which the energy is extrinsic. T.P. 868