Influence of the Pore Structures on Stress Sensitivity of Tight Sandstone Reservoir

Abstract

The percolation of oil and gas in tight sandstone reservoir is different from conventional reservoir due to its tight lithology, tiny pore throat, Jamin effect and strong stress sensitivity. To well study the influence of pore structures on stress sensitivity of tight sandstone reservoir, the pore structures of Chang-6 reservoir were characterized using automated scanning electron microscope (QEMSCAN), casting lamella, laser scanning confocal microscope (LSCM), field emission scanning electron microscope (FESEM) and mercury intrusion porosimetry. The results indicated that the reservoir had low average surface, complex pore structure and strong heterogeneity. Intragranular and intergranular dissolved pores which were mainly potassium feldspar dissolved pores were developed in the reservoir, and some chlorites were developed around these pores. The contact types between grains were dominantly line and point-line contact; the sorting of pores and throats was fair. The ratio of micro-pores was extremely low, while the ratios of submicro-pores and nano-pores were up to 40.66% and 59.10%, respectively. Before the effective stress was increased to 20 MPa, nano-pores which were abundant in Chang-6 reservoir would be closed firstly, resulting in lager reduction of permeability. As the effective stress was increased, the reduction of permeability got smaller. As the effective stress was reduced, the equivalent liquid permeabilities of cores were increased because the chlorite developed around the pores enhanced the compression resistance of grains, and the final recovery rate was in the range of 78% to 92%.