Influence of effective stress on the acoustic velocity and log-derived porosity

Experimental studies indicate that when effective stress increases, compres sional wave velocity in porous rocks increases. Reservoir pressure reductio n, resulting from hydrocarbon production, increases effective stress. For a rock with a given porosity the sonic log may show decreasing values as the pressure in the reservoir decreases. This in turn may lead to underestimat ion of the actual porosity of the reservoir rocks in low pressure reservoir s. The range of such underestimation for liquid saturated reservoirs may no t be significant, but since the influence df effective stress on velocity i ncreases as fluid saturation changes to gas, porosity underestimation by co nventional velocity-porosity transforms for gas bearing rocks may increase. Examples are taken from partially depleted gas reservoirs in the Cooper ba sin, South Australia. The stress dependent nature of velocity requires that the in situ pressure condition should be considered when the sonic log is used to determine the porosity of gas producing reservoir rocks.