Mechanical and Chemical Effects of Pore Fluids on Rock Properties1
The disposal of wastes underground involves injection of fluids into a rock mass that, through natural processes, has probably attained a state of at least metastable equilibrium. Injected fluids tend to disrupt this equilibrium mechanically and chemically. The primary question concerns what effect this disruption will have on the rock body. Fracturing, faulting, or the reactivation of old faults could lead to earthquakes and the creation of fractures that may provide permeability channels through which injected fluids could escape from the intended disposal beds. Accelerated compaction, brought about by chemical weakening of the load-bearing framework of the rock or by reduction of fluid pressures upon withdrawal, can cause surface subsidence.
At depths of 3-4 km, most rocks are brittle, and fracture and rigid-body rotation are the dominant mechanisms of deformation. The most important physical parameter in this regime is the effective confining pressure Pe, defined as the difference between the confining pressure Pc and the pore-fluid pressure Pp. Both the breaking strength and the compaction of rocks are dependent upon the magnitude of Pe, regardless of the absolute values of Pc and Pp. Increases in PP produced by injection of fluids decrease the normal stresses but do not change shear stresses across potential failure surfaces. The result could be fracturing, faulting, or the reactivation of preexisting faults. Decreases in Pp produced by withdrawal of fluids from a reservoir can lead to compaction and surface subsidence as the Pe is increased.
The second important aspect of the problem is the role of the pore-fluid chemistry. Significant reductions in rock strength have been shown to result from lowering of the surface energy of solids that occurs as a result of adsorption of pore fluids and associated modification of bonding. Triaxial-compression tests on sandstone show that the coefficient of internal friction is not altered by pore solutions of 0.002 to 2 ppm of FeCl3. The lowering of the breaking strength is due rather to the reduction of the intragranular cohesive strength as a function of the concentration of the electrolyte solution.
The frictional characteristics of already-broken rocks may be significantly altered by the introduction of surface-active fluids. In light of manmade earthquakes, such as those near Denver, Colorado, the influence of pore-fluid pressure and chemistry suggests that more sophisticated tests of rock properties should be made if problems caused by unexpected rock failure are to be eliminated.