Phase Prediction of Supercritical Carbon Dioxide and its Application in Fracturing Oil Wellbores

In recent oil and gas exploration, the most reservoirs are low permeability with abundant reserves. Conventional mining of low permeability reservoir is commonly utilizing the hydraulic fracturing technology, whereas, it encounters various technical issues, such as clay expansion and water lock damage. Using the fluid of supercritical carbon dioxide (S-CO2) to exploit the low permeability oil and gas reservoirs is attracting more attention. The implementation of S-CO2, without liquid phase, can help avoid the aforementioned problems. Nevertheless, the phase change of CO2 during fracturing is complicate, and it is difficult to accurately predict the CO2 phase transition. In this work, first, the physical properties of S-CO2 were analyzed by the Span-Wagner model and Vesovic model. Next, S-CO2 was applied to a typical oilfield, and an unsteady coupling model of heat transfer and pressure drop was developed. Then the staggered grid method and iteration procedures were used for numerical solutions, and the temperature and pressure distributions of wellbores were investigated. The results indicate that the temperature control of a wellbore is the key to the phase prediction of S-CO2; CO2 within the single-diameter pipeline below 2300 m can maintain the supercritical state, while CO2 within the stepped pipeline can maintain the supercritical state at the depth of 2280 m. Moreover, compared with the single-diameter pipeline, the bottom pressure of the stepped pipeline is lower and the bottom temperature is higher. By analyzing the flow and heat transfer of S-CO2 in the wellbores, the phase state of S-CO2 was well predicted, which is helpful to improve the exploring performance of low permeability oil and gas reservoirs.