ABSTRACT:

To improve the complexity of hydraulic fracture (HF) in tight reservoirs, the characteristics of HFs created by different fluids in dissimilar geological reservoirs should be investigated. In this study, laboratory true triaxial fracturing experiments were conducted on three tight sandstones (from He-8, Shan-1, and Chang-7 formations in Ordos Basin, China) by using slickwater, liquid CO2 (L-CO2), and supercritical CO2 (SC-CO2). Experimental results show that L-CO2 and SC-CO2 can improve the fracture complexity to a limited extent in homogenous tight sandstones (He-8 and Shan-1) compared with slickwater. However, the fracture complexity can be significantly enhanced using CO2 fracturing in the layered tight sandstone (Chang-7). The resulting fracture using slickwater is simple and straightforward with relatively smooth surfaces. L-CO2 fracturing generates curved macro and micro multibranch fractures with some shattered particles on the uneven surfaces. SC-CO2 fracturing can induce a complex fracture network with crushed surfaces and many tortuous microfractures. The fracture width created by slickwater fracturing is the largest, followed by that created by L-CO2 fracturing, and that created by SC-CO2 fracturing is the smallest. The obtained results may provide a laboratory research basis for the fracturing fluid selection and fracturing treatment design in different tight sandstone reservoirs.

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