Coupled Physical-Chemical Effects of CO2 on Rock Properties and Breakdown During Intermittent CO2-Hybrid Fracturing

This paper introduces a new intermittent CO2-hybrid fracturing design for stimulating tight sandstone reservoirs by fully utilizing the coupled physical-chemical effects of CO2. The design consists of (1) injecting pure CO2 to create a complex fracture network; (2) soaking the well for several days or weeks, and (3) pumping a CO2-/water-based slurry to enhance the complexity of fracture network more extensively. In order to examine the feasibility of the design, the coupled physical-chemical effects of CO2 on rock properties and breakdown were investigated by conducting laboratory static soaking experiments and fracturing experiments on the layered Chang-7 tight sandstones. Experimental results show that calcite and dolomite were first dissolved, followed by K-feldspar and albite, while quartz and clays were slightly eroded during the static soaking experiments. With the increase of soaking time, the number of dissolution pores increased and the pore size enlarged, which caused the enhancement in porosity and permeability (up to 749%) and the decrement of tensile strength (up to 47%). Compared with slickwater fracturing, supercritical CO2 (Sc-CO2) fracturing (physical effect) and intermittent CO2-hybrid fracturing (coupled physical-chemical effects) reduced the breakdown pressure by 15.0% and 33.4%, respectively. Sc-CO2 fracturing tended to create more spatially dispersed fractures with the fractal dimension (D-f) ranging from 2.2653 to 2.2719 than the single fracture created by slickwater fracturing (D-f ranging 2.1302-2.1369). Notably, intermittent CO2-hybrid fracturing enhanced the fracture complexity (D-f ranging 2.3772-2.3915) conspicuously in comparison with Sc-CO2 fracturing. The obtained results indicate that the coupled physical-chemical effects of CO2 can improve fracture complexity significantly during intermittent CO2-hybrid fracturing in Chang-7 formation.