Effects of lithology and depth on the permeability of core samples from the Kola and KTB drill holes

Abstract

Permeability measurements were conducted on intact core samples from the Kola drill hole in Russia and the KTB drill hole in Germany. Samples included granodiorite gneisses, basalts and amphibolites from depths up to 11 km. The tests were intended to determine the pressure sensitivity of permeability and to compare the effects of stress relief and thermal microcracking on the matrix permeability of different rock types and similar samples from different depths. The pore pressure Pp was fixed at the estimated in situ pressure assuming a normal hydrostatic gradient; the confining pressure Pc was varied to produce effective pressures (Pe = Pc ‐ Pp) of 5 to 300 MPa. The permeability of the basaltic samples was the lowest and most sensitive to pressure, ranging from 10−20 to 10−23 m2 as effective pressure increased from 5 to only 60 MPa. In contrast, the granodiorite gneiss samples were more permeable and less sensitive to pressure, with permeability values ranging from 10−17 to 10−22 m2 as effective pressures increased to 300 MPa. Amphibolites displayed intermediate behavior. There was an abundance of microfractures in the quartz‐rich rocks, but a relative paucity of cracks in the mafic rocks, suggesting that the observed differences in permeability are based on rock type and depth, and that stress relief/thermal‐cracking damage is correlated with quartz content. By applying the equivalent channel model of Walsh and Brace [1984] to the permeability data of the quartz‐rich samples, we can estimate the closure pressure of the stress‐relief cracks and thereby place bounds on the in situ effective pressure. This method may be useful for drill holes where the fluid pressure is not well constrained, such as at the Kola well. However, the use of crack closure to estimate in situ pressure was not appropriate for the basalt and amphibolite samples, because they are relatively crack‐free in situ and remain so even after core retrieval. As a result, their permeability is near or below the measurable lower limit of our apparatus at the estimated in situ pressures of the rocks.