Integrated Investigations on the Refracturing Process in Tight Oil Reservoirs

Abstract

ABSTRACT Refracturing is one of the most effective methods to solve the problem of rapid production decline and low recovery in tight oil reservoirs, especially in the late production stage. The decrease of pore pressure and changes of in-situ stresses during early production might result in inefficient refracturing due to the high leak-off or fracture reorientation. Thus, the volume and timing of fluid injection is important for the effectiveness of refracturing. To properly predict the geometries of re-fractured hydraulic fracture, it is necessary to consider both the stresses and pore pressure changes before refracturing. For Changqing oil field, both new and existing fractures are fractured during the refracturing process. It is also important to understand which kind of fractures should be re-fractured to bring greater production improvement. To investigate the above problems, an integrated simulation procedure is conducted to optimize the refracturing processes, which couples the initial fracturing, production, refracturing and post production after refracturing with a 4D geomechanical model. The comparisons between fracturing existing and new fractures, and optimal fluid injected volume are discussed. According to the 4D geomechanical model results, after the initial fracturing and production, the pressure around the hydraulic fractures is gradually recovered with fluids injection. When the amount of fluid injected is increased to 400 m3 per 100 meters, the pressure can be well recovered and the pressure depletion during the well shut-in process is small. Moreover, for the area with high remaining oil saturation after liquid injection, it is beneficial to select the old perforations with good pressure and stress recovery and perforate new fractures in the well section without obvious stress variations to improve the efficiency of refracturing. The integrated numerical simulation of the refracturing can provide important guidance for the future designs of refracturing in tight oil wells. INTRODUCTIONF After a period of production in oil and gas wells, a pressure depletion zone will gradually form around the hydraulic fractures, which will change the magnitude and orientation of in-situ stresses and even inverse the direction of minimum and maximum horizontal stresses. The decrease of pore pressure could results in the loss of fracture conductivity, resulting in rapid decline of oil production (Yi and Sharma, 2016). Refracturing, which injects a large volume of fluid into the depleted reservoir again, can effectively supplement the pressure depletion, increase the fracture complexity and conductivity, hence maintaining the stable production of oil wells.(Shah et al, 2017 and Ren et al, 2020) Typically, three kinds of stimulation strategies are used for fractured wells to enhance the production, including direct refracturing, huff and puff and fluids injection before refracturing.