ABSTRACT:

In view of the physical–chemical properties of CO2, this paper studied the impact of CO2–brine–rock interaction on rock properties and fracture propagation during supercritical CO2 fracturing in Chang-7 tight sandstone. A series of static soaking experiments and true triaxial fracturing experiments were performed to investigate the influence of CO2–brine–rock interaction on rock properties and fracture propagation during supercritical CO2 fracturing. Experimental results showed that calcite, dolomite, K-feldspar, and albite, were variably dissolved during the static soaking experiment. With the increase of reaction time, the number of dissolution pores increased and the pore size enlarged, which caused the enhancement in porosity and permeability (up to one order of magnitude) and the decrement of tensile strength (up to 47%). Compared with slickwater fracturing, supercritical CO2 fracturing reduced the breakdown pressure by 15% and increased the number of fractures. The soaking treatment of the open-hole section of fracturing specimen with CO2-saturated brine reduced the breakdown pressure by 21% and improved the fracture complexity conspicuously in the case of supercritical CO2 fracturing. The obtained results indicate that the physical–chemical properties of CO2 can improve the fracture complexity effectively during supercritical CO2 fracturing in Chang-7 tight sandstone formation.

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