Transient Pressure Behavior in Vertically Fractured Reservoirs

The transient pressure behavior of a welt which produces a single compressible fluid through a single-plane 1 • eni-cal fracture has been investigated mathematically. Tire fracture is assumed to possess infinite flow capacity, to be of limited radial extem, and to penetrate the producing formation completely in the vertical direction. Previous studies of vertically fractured wells have been concerned primarily with production rate performa11Cl! or semisteady-' state pressure behavior. This study was undertaken to ascertain the influence of i • ertical f ract11res 011 t/'a11sie11t pressure tests such as pressure build-ups and flow tests. In a vertkally fractured ,\')'Stem, flow i11 the region nearest tire fracture is practically linear, whereas farther away from tire ftact11re esselltially Niel/al flow prevqils. Th11s, tra11sie11t' p1·ess11re analyses hasecl 011 radial f /ow theory are somewhat iilaccurate. As fract11re .pe11etratlon increases radially, kh values calculated from pl'essure build-up and flo.w test c:u/'ves become increasingly larger tha11 tme values. Failure to consider the ef feet of fracture penetl'at/011 also introduces inaccuracies into tire calculatio11 of fl'acture length from tire apparelll skin factor and into the determlnat/011 of average reservoir pressure. If the total length of the fracture is 20 per cent, or greater, of the drainage radius of the jWell, corrections must he made to pressure build-up and lffow test results. Methods for correcti11g such /'esults are discussed in this paper. For wells with prefracturing pressure bufld-up or flow test data, It is possible to estimate fracture length by comparison with post fracturing build-up ' or flow test _, r!Jults. Ill new wells or wells without prefrac-fllring build-up or 1 'fow test data, fracture length must be estimated to co/'rect the values obtal11ed from analysis of pressul'e tests after fracturing. Fracturing efficiency calculations · should be made whenever possible to provide an estimate of fracture length.. Ta~les of the dimensionless pressure drop as a function of tilne and'/l'acture penetration are included in this papel''. Using th~e ~alues should permit analysis of other types of transient pres.sure behavior in ve/'fically fractured wells. 1 ltJTRODUCTION Hydraulic fractmi,ng has been used quite. successfully for over a decade as a completion and stimulation tech