The permeability variation of Indiana limestone and Berea sandstone was measured under hydrostatic, triaxial and ko (urnaxial strain) paths. The oscillating pulse technique was found to be fast and convenient way of measuring permeability changes in a variety of rocks, ranging from low to high permeabilities and subjected to stresses in a triaxial cell.
Overburden pressure, tectonic stress and fluid pressure all act upon fluid-saturated reservoirocks. Both the initial and secondary recovery of reservoir fluids, as well as the success of fluid injection program. s, depend upon an understanding of the flow propcrees of reservoir under stress. A very important reservoir property is, of course, the permeability of the reservoir rock. The ability to determine the permeability under different stress levels in a reservoir can be used to study the pore collapse behavior. It is also one of the important parameter which is frequently used as a basic input in numerical studies and hydraulic simulators involving. fluid flow in reservoir rocks. Thus, permeability measurement and its variation under stress in a laboratory under simulated thermomechanical loading and effective stress become very important
The pulse decay method is one of the techniques used to measure the permeability of low permeability rocks. The technique was developed in 1968 (Brace et al.,) and it consists of applying a sudden pressure pulse on the upstream end of a sample and measuring the pressure-time histories in the upstream and downstream reservoirs. The method becomes inconvenient for rocks having very low permeabilities since it requires measurements for very long periods of time.
In this study, an experimental method of measuring permeability of rock samples undergoing deformation m a triaxial cell is developed based on the oscillating pressure pulse technique (Kranz et al., 1990). It consist of applying a pressure pulse of a known frequency and amplitude to the upstream of the sample and measuring the attenuated and delayed cyclic downstream response. Based on the solutions of diffusivity equation with the applied upstream and downstream boundary conditions, permeability and diffusivity can be calculated. The technique ?s programmed in Visual Basic, and is incorporated into a triaxial testing programme (TRIPERM.MAK) to work under the Windows environment (Azeemuddin, 1993). It has been applied to measure the permeability of Indiana limestone as well as Berea sandstone samples updergoing deformation under hydrostatic compression, triaxial compression and uniaxial compression (one-dimensional consolidation) test paths. For comparison, permeability was also measured during hydrostatic compression using the steady state method, in which a steady fluid under a constant pressure head was measured and, using the Darcy's law, permeability was calculated; a good agreement was found.
