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Pore Volume Compressibility

The standard test for pore volume compressibility is a hydrostatic test. In that test a saturated rock of known initial pore volume is confined and the pore fluid is held at constant pore pressure. The confining pressure is ramped up to a specified level at a slow rate (quasi-static) and the volume of extruded pore fluid is continuously measured. The extruded fluid represents the pore volume change, or when divided by the original volume, the pore strain. A non-linear function of the effective stress (confining pressure minus the constant pore pressure) is fit to the pore strain and the derivative of that curve is the pore volume compressibility. Porous materials (i.e. rocks) have four compressibilities that are "thermodynamic analogs" of compressibility in solids. These are: bulk compressibility at changing hydrostatic stress and constant pore pressure, bulk compressibility at changing pore pressure and constant hydrostatic stress, pore compressibility at constant pore pressure and changing hydrostatic stress and the pore compressibility at constant hydrostatic stress and changing pore pressure. The four are related through the grain compressibility. Thus, given the pore volume compressibility at constant pore pressure and changing hydrostatic stress and the grain compressibility (for example, the weighted average of the compressibility quartz and feldspar in sandstone) the other three can be calculated.

Compressibility under uniaxial stress
This test does not have a "thermodynamic analog" in the sense of the four compressibilities mentioned above; compressibility is basically a hydrostatic concept. However, the uniaxial compressibility or compaction can be measured and is obviously of interest to oil companies studying reservoir compaction drive and subsidence. The boundary conditions are more difficult to maintain in these tests. The uniaxial stress pvc is done in a similar way to the hydrostatic pvc at constant pore pressure, however in this case the confining pressure is held constant and uniaxial stress (differential stress along the long axis of the cylindrical sample) is applied. The change in pore volume is measured and a curve can be fit to the pore strain to compute the compressibility. . This test is similar to a standard triaxial test (uniaxial stress test) with the addition of pore volume monitoring and computation of compressibility from the pore strain. The test also yields Young?s modulus and Poisson?s ratio since in this test the sample is typically instrumented with strain gauges. The gauges are required to monitor the rock for potential shear failure under uniaxial stress. Rocks typically don't fail under hydrostatic stress, so in the normal pvc test we usually do not instrument the rock with strain gauges.

Compressibility under uniaxial strain
In this test we maintain a condition of zero radial strain on the rock cylinder boundary with a special radial strain gauge. The gauge is in a feedback loop that controls the confining pressure to prevent any change in the diameter of the rock. As the confining pressure changes the axial stress also must be maintained. Again, if constant pore pressure is maintained the pore strain can be measured and compressibility computed. Tests of this nature are often referred to as Ko (K-naught) tests.

Pore pressure depletion test
In this test we maintain a boundary condition of zero radial strain while changing the pore pressure and maintaining a specified axial stress condition, usually constant axial stress (meaning that the differential axial stress must change as the confining pressure changes in maintaining the radial strain). The boundary conditions are computer controlled via feedback loops on the radial gauge.

With all of these tests, the consolidation of the rock will impact the results. Poorly consolidated rocks are much more difficult to test and the data have lower reliability. Shear velocities are difficult if not impossible to obtain in unconsolidated rocks.

All of the tests are performed in servo-controlled systems that maintain appropriate temperature and stress boundary conditions on the sample. The ranges available typically cover those found in most reservoirs.