EFFECTS OF LITHOLOGY AND DEPTH ON THE PERMEABILITY OF CORE SAMPLES FROM THE KOLA AND KTB DRILL HOLES

Permeability measurements were conducted on intact core samples from t he Kola drill hole in Russia and the KTB drill hole in Germany. Sample s included granodiorite gneisses, basalts and amphibolites from depths up to 11 km. The tests were intended to determine the pressure sensit ivity of permeability and to compare the effects of stress relief and thermal microcracking on the matrix permeability of different rock typ es and similar samples from different depths. The pore pressure P(p) w as fixed at the estimated in situ pressure assuming a normal hydrostat ic gradient; the confining pressure P(c) was varied to produce effecti ve pressures (P(e) = P(c) - P(p)) of 5 to 300 MPa. The permeability of the basaltic samples was the lowest and most sensitive to pressure, r anging from 10(-20) to 10(-23) m2 as effective pressure increased from 5 to only 60 MPa. In contrast, the granodiorite gneiss samples were m ore permeable and less sensitive to pressure, with permeability values ranging from 10(-17) to 10(-22) m2 as effective pressures increased t o 300 MPa. Amphibolites displayed intermediate behavior. There was an abundance of microfractures in the quartz-rich rocks, but a relative p aucity of cracks in the mafic rocks, suggesting that the observed diff erences in permeability are based on rock type and depth, and that str ess 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 th e Kola well. However, the use of crack closure to estimate in situ pre ssure was not appropriate for the basalt and amphibolite samples, beca use they are relatively crack-free in situ and remain so even after co re retrieval. As a result, their permeability is near or below the mea surable lower limit of our apparatus at the estimated in situ pressure s of the rocks.