Study on the optimization of timing for horizontal well re-fracturing in tight oil reservoir -- A case study of Baikouquan Formation in North Mahu Oilfield, Junggar Basin, Western China

Abstract There are abundant tight oil resources in China, and “horizontal well + volume fracturing” technology is the one of the main means to develop such reservoirs. The production of tight oil reservoir declines generally rapidly after the initial fracturing, therefore it is necessary to be conducted re-fracturing operation to restore the oil productivity. The determination of the best time of re-fracturing is an important factor to ensure the re-fracturing effect. In this paper, a mathematical model of oil-water phase seepage was established by applying the principle of finite difference method (FDM). A unified grid system was adopted for the division of the matrix and fractures, and the equations were solved by the IMPES method. The reliability of the proposed seepage model gets verified by the historical matching based on the field production data. The oil production after re-fracturing at different moments of time after initial fracturing was predicted. The simulations show that more cumulative oil production can be obtained if re-fracturing is implemented after the fourth year of initial fracturing. During the period after the 4th year of the initial fracturing, the earlier the re-fracturing is implemented, the more the cumulative oil production will be harvested. Considering the cost of re-fracturing operation, the economic benefits of re-fracturing after the sixth year of initial fracturing were evaluated. The results show that the cost can be recovered about one year after the re-fracturing, the net present value (NPV) after four years of re-fracturing reaches 69.95 million yuan, with an output-input ratio of 5.3.