Evaluation of recoverable reserves based on dynamic data for horizontal wells in unconventional oil reservoir

Unconventional reservoirs are often exploited by horizontal well volumetric fracturing. Recoverable reserves are an important basis for evaluating the development effect of oil reservoirs. In this study, an evaluation index of recoverable reserves was proposed which based on the mathematical statistics method. A mathematical model for the index and the standardized calculation process were established. The dynamic data in the study was from the RA tight reservoir of Songliao basin. The results show that: (1) Horizontal wells production can be categorized into three distinct groups based on daily oil production and cumulative oil production (N_p), showing no obvious "L"-shaped decrease. (2) A power function is presented by the correlation between N_p logarithm (logN_p) and production time, with a distinct inflection point in the curve. As the evaluation index of recoverable reserves, the N_p corresponding to the inflection point is taken. (3) After verification, the derivative value of logN_p corresponding to the inflection point is approximately 0, and the derivative parameter m represents the characteristics of different regions. Statistically, the study area m is 0.0003. (4) The recoverable reserves evaluation indexes of all known horizontal wells that can reflect the real production capacity were calculated. The recoverable reserves of the three blocks in the study area were successively S1 block, S3 block and S2 block from high to low. (5) According to the relationship between cumulative oil production, initial daily production and the evaluation index, the evaluation index is only suitable for the characterization of recoverable reserves in the middle and late production of tight oil wells, but not suitable for the characterization of recoverable reserves in the early production of wells.