Numerical simulation of prepad CO₂ energized fracturing based on experimental data in Mahu conglomerate tight formation

ABSTRACT: Pre injection of CO₂ before main fracturing can effectively increase oil production and store CO₂. SEM scanning showed that complex cross fracture structure is formed on the rock surface, and the permeability of the core is significantly improved after CO₂ soaking. According to the experimental data and production history fitting, the permeability of fractures and matrix affected by CO₂ are established. The temperature of CO₂ along wellbore was calculated to determine the phase state of CO₂. We compared the effects of CO₂ injection amount, shut in time and injection mode on fracturing stimulation. The results show that: for Mahu conglomerate reservoir in Xinjiang, pre injection of CO₂ can increase oil production by more than 15% compared with direct fracturing in the first year; continuous injection is recommended to developing the reservoir, which can contact larger reservoir area; single fracture injection of 60 tons CO₂ and shut in time of 15 days can achieve good results in reservoir development. For refractured well, CO₂ + fracturing fluid is preferred due to low viscosity of CO₂ and weak capability to propagate the fracture.