Effect of Vertical Fractures on Reservoir Behavior- Results on Oil and Gas Flow

Abstract

A homogeneous and uniform cylindrical reservoir containing oil andgas is fractured vertically on completion and is produced at a constantbottom-hole pressure. The fracture has an infinite flow capacity,is of limited lateral extent and is bounded above and below by theimpermeable strata defining the vertical extent of the reservoir.Results show that such a fractured reservoir can be represented bya reservoir of circular symmetry having very nearly the same productionhistory. The well radius of this circular reservoir is about 1/4the fracture length and is essentially the same as that obtainedpreviously for a single fluid of constant compressibility. At thesame value of cumulative oil production, gas-oil ratios of fracturedreservoirs producing at constant terminal pressure are larger thanthose of reservoirs having no fractures. This leads to more inefficientuse of the reservoir energy in fractured wells and results in lowerreservoir pressures for the same cumulative oil production. The reductionin operating life due to fracturing a reservoir is not as great asthat for a slightly compressible fluid. This difference can be accountedfor by the lower reservoir pressure in the fractured reservoir andits adverse effect on the average mobility and compressibility ofthe oil. As anticipated. the reduction in operating life increasesas the reservoir permeability decreases. The type of results presentedin this report can be used to determine the economic attractivenessof fracture treatments per se, to select the initial spacing to beused in developing a field, and to compare the relative merits offracturing available wells and in fill drilling.