Pore structure alteration induced by CO2-brine-rock interaction during CO2 energetic fracturing in tight oil reservoirs

摘要

CO2 energetic fracturing is an important technique for developing tight oil resources by creating complex fractures and enhancing formation pressure. The physical properties of host rocks may be changed by CO2-brine-rock interaction in the soaking stage of CO2 energetic fracturing. However, the pore structure alteration behavior and mechanism during CO2 energetic fracturing are still unclear. To address this problem, this work conducted static soaking experiment under reservoir temperature-pressure conditions and employed multiple techniques to comprehensively characterize the pore structure. The results show that CO2-brine-rock interaction generated many large pores (10-50 mu m) due to the dissolutions of carbonate and feldspar, numerous tiny intragranular pores (1-6 mu m) and some microfractures in clays. The T-2 distribution could be applied to characterize the pore size distribution of tight sandstones when the pores saturated with brine. Some secondary clay particles and debris dispersed into the brine caused by CO2-brine-rock interaction. These particles may block extremely small pore throats during flowing back. These blockages weakened the exchange between the small and the large pores, characterized by single-peak and insignificant three-peak T-2 distributions transformed into apparent dual-peak T-2 distributions. After soaking the rock with CO2-saturated brine for 168 h at 20 MPa and 80 degrees C, the maximum pore throat radius increased four-fold, and the average pore throat radius increased by 86.5% from 0.089 mu m to 0.166 mu m, indicating a significant increase in the pore connectivity. Increasing the soaking pressure or CO2 concentration can transform small pores into larger pores and can open bedding planes significantly, thus increasing the porosity and permeability of tight sandstones.