PORE PRESSURE PROFILES IN FRACTURED AND COMPLIANT ROCKS

Fluid permeability in fractured rocks is sensitive to pore-pressure ch anges. This dependence can have large effects on the flow of fluids th rough rocks. We define the permeability compliance gamma = 1/k(partial derivative k/partial derivative p(p))(pc), which is the sensitivity o f the permeability k to the pore pressure p(p) at a constant confining pressure p(c), and solve the specific problems of constant pressure a t the boundary of a half-space, a cylindrical cavity and a spherical c avity. The result show that when the magnitude of permeability complia nce is large relative to other compliances, diffusion is masked by a p iston-like pressure profile. We expect this phenomenon to occur in hig hly fractured and compliant rock systems where gamma may be large. The pressure profile moves rapidly when fluids are pumped into the rock a nd very slowly when fluids are pumped out. Consequently, fluid pressur e, its history and distribution around injection and production wells may be significantly different from pressures predicted by the linear diffusion equation. The propagation speed of the pressure profile, mar ked by the point where partial derivative p(p)/partial derivative x is a maximum, decreases with time approximately as root t, and the ampli tude of the profile also dissipates with time (or distance). The effec t of permeability compliance can be important for fluid injection into and withdrawal from reservoirs. For example, excessive drawdown could cause near-wellbore flow suffocation. Also, estimates of the storage capacity of reservoirs may be greatly modified when gamma is large. Th e large near-wellbore pressure gradients caused during withdrawal by l arge gamma can cause sanding and wellbore collapse due to excessive pr oduction rates.