Reduction in Permeability With Overburden Pressure

Oil bearing rocks, usually found at depths of 2,000 to 10,000'Ft, areelastically deformed by overburden pressure. Although the change in porositywith pressure in a number of example rocks has been shown to be small, it wasthought that overburden pressure might have a significant effect onpermeability. This note presents results of an investigation to determine themagnitude of the change in permeability with pressure. In routine core analysis, permeabilities are measured on rock samples whichare not under overburden pressure. If permeability measured in this way differsfrom permeabilities measured under overburden pressure, a systematic error isintroduced into well productivity calculations. The experiments described inthis report were made to determine the magnitude of the change and thus to givean estimate of the error introduced into the calculations. The error introducedby the neglect of the change in permeability when overburden pressure isremoved may account for part of the difference between well productivitycalculated from core analysis and the actual well productivity. The results obtained in this investigation have only qualitativesignificance because of the difficulty in reproducing in the laboratory thestress conditions on the rock buried in the earth. The assumption which isusually made is that the rocks in the earth are under a uniform pressure equalto the weight per unit area of the vertical overburden column from the rock inquestion to the surface of the earth less the pressure of the liquid in thepores of the rock. For an overburden consisting of sandstones and shales (ofassumed average specific gravity of 2.3) and for a liquid pressure equal to thepressure in a salt water column (of specific gravity 1.0) reaching from therock to the surface, the resulting net pressure on the rock is approximately0.56 psi per ft of depth. Experimental Method Clean, dry sandstone core plugs one in. in diameter and about three in. longwere mounted in a copper foil jacket as shown in Fig. 3 or molded in a Lucitejacket as shown in Fig. 4. All cores except those labeled "2" in Fig. 1and "8" in Fig. 2 were measured in the copper foil jacket. Cores"2" and "8" were measured' in the Lucite jacket. T.N. 147