PERMEABILITY AND POROSITY OF THE ILLINOIS UPH-3 DRILLHOLE GRANITE ANDA COMPARISON WITH OTHER DEEP DRILLHOLE ROCKS

Permeability, porosity, and volumetric strain measurements were conduc ted on granite cores obtained at depths of 0.7 to 1.6 km from the Illi nois UPH 3 drillhole at effective confining pressures from 5 to 100 MP a. Initial permeabilities were in the range of 10(-17) to 10(-19) m(2) and dropped rapidly with applied pressure to values between 10(-20) a nd 10(-24) m(2) at 100 MPa, typical of other deep granite core samples . These values are several decades lower than equivalent weathered sur face granites at comparable effective confining pressures, where weath ering products in cracks and pores inhibit crack closure with applied pressure. Permeabilities of the Illinois cores were inversely related to sample,depth, suggesting that stress relief and thermal microfractu res induced during core retrieval dominated the fluid now. Thus these samples provide an upper bound on in situ matrix permeability values. A comparison of core permeability from UPH 3 and other deep drillholes shows that stress relief damage can often dominate laboratory permeab ility measurements. We conclude that it may be difficult to make meani ngful estimates of in situ permeability based on either borehole sampl es (possible damage during retrieval) or surface-derived analogs (alte red by weathering). Volumetric strain determined from porosity measure ments was compared with differential strain analysis (DSA) data report ed by other investigators on samples from the same depths in the drill hole. Our strain measurements (0.002 to 0.005 at 100 MPa) were nearly twice as large as the DSA values, Probably because of the crack-enhanc ing effects of fluids present in our samples that are absent in the dr y DSA cores, as well as other time-dependent deformation effects. This difference in observed strain magnitudes between the two measurement methods may be an important consideration if strain and/or porosity da ta from deep core samples are used in models of stress, fluid circulat ion, and excess fluid pressure generation in the midcrust.