Reservoir Matrix Porosity at The Geysers from Core Measurements

Abstract

Matrix porosities of Geysers reservoir rocks are quite low compared to most geothermal reservoir rocks. Porosities measured on nine cores of reservoir graywacke range from 0.6% to 5.8%, with an average of 2.3%. Measured porosities of four graywacke cores from outside the reservoir have a narrower range, from 0.9% to 2.3%. Porosities of three cores of felsite intrusion range from 1.7% to 4.2% with average background values expected around 2%. Much of the porosity in the cores is fracture related which leads to dependence of measured porosity on sample size. Larger samples include more widely spaced fracture sets and hence yield higher porosity value. Actual bulk reservoir porosities are there fore thought to be somewhat higher than measured core porosities. The distribution of porosity in the reservoir graywacke appears to be related to two factors: vertical depth and distance above the felsite intrusion. A model ahs been constructed to describe the distribution of porosity in the Geysers reservoir based on these two parameters. In the model porosities are highest in shallow reservoir that is far above the felsite intrusion. From there porosities decrease downward and toward the edges of the reservoir where the reservoir is deeper.