Experimental study of sandstone stress sensitivity under different fluids: Characteristics and mechanisms

Stress sensitivity is the phenomenon of permeability variation that caused by pore throat changes, opening or closure of fractures under forces, which is an important parameter in reservoir evaluation. Fractured sandstones under various fluids will produce different stress sensitivity, which can greatly influence oil and gas develop-ment. However, little is known on this topic currently. Hence, in this study, characteristics and mechanisms were investigated based on a series of experiments and theoretical analysis. The results indicate that: I, compared with unfractured rocks, fractured rocks show a stronger stress sensitivity. The permeability shows an obvious "decrease initially steep and finally gentle" pattern with the increase of effective stress. II, from purified water to low NaCl/CaCl2 mineralization fluids, the permeability damage of fractured rocks decreases gradually with the increase of mineralization degree; however, it increases with mineralization degree when rocks are under high mineralization fluids, and the damage degree is also high. The permeability damage degree is generally low when fractured rocks are under the action of kerosene. Mechanism analysis indicates that: I, fractured rocks have both fracture and pore systems, whose opening and closure can produce stronger stress sensitivity and more obvious nonlinear variation of permeability than unfractured rocks. II, under purified water conditions, the expansion of clay minerals is an important indicator and reason for the permeability damage. III, under low fluid minerali-zation degree conditions, the reaction between fluids and minerals will destroy rock structures, increase fracture aperture, and produce secondary pores, and finally decrease the permeability damage to a certain extent. IV, under high fluid mineralization degree conditions, crystals may be deposited, blocking up pores/fractures and producing high permeability damage. V, there are no reactions between minerals and kerosene, hence, the permeability damage degree is generally low. The results are expected to provide references for the exploration and development of fractured tight reservoirs in Kuqa Depression.