The influence of stress path on compressibility and permeability of an overpressurised reservoir sandstone: Some experimental data

Abstract

Laboratory experiments were performed to measure how the compressibility and permeability of an overpressurised quartz/feldspar-rich reservoir sandstone depend on the stress path induced by a production-induced decrease in hydrocarbon pressure. Two stress paths were followed in the experiments: one involved a uniaxial compaction with strain only along the cylindrical axis of the sample (i.e. no radial strain), and the other involved a simultaneous axial and radial compaction due to an isotropic increase in effective stress.The experimental results demonstrate a strong influence of compaction mode (i.e. stress path) on compressibility. Firstly, at a given effective axial stress, the uniaxial compressibility is a factor of between 2 and 10 higher than the axial compressibility observed during an isotropic increase in effective stress. Secondly, at around 60 MPa effective axial stress, the uniaxial compressibility shows a marked non-linear increase with increasing effective stress (compaction weakening). In contrast, the axial compressibility obtained in experiments with an isotropic increase in effective stress is much lower and increases roughly linearly with effective axial stress. Microstructural analysis revealed that the compaction weakening is probably due to intergranular fracturing and transgranular fracturing, particularly of quartz grains near partly hydrolysed feldspars.The horizontal permeability decreased by 5% to 10% of the initial permeability with every 10-MPa decrease of the reservoir fluid pressure, independent of stress path. This result agrees with data in the literature on isostatic compaction/permeability experiments, but it disagrees with the data of Rhett and Teufel (1992), who observed a permeability increase during uniaxial compaction of reservoir sandstones. A possible explanation for this discrepancy is that Rhett and Teufel probably measured the permeability parallel to the long dimension of preferentially oriented stress-induced microcracks, whereas in the present study the permeability was measured in a direction orthogonal to the long dimension of most microcracks.