The effect of combined proppants upon the fracture conductivity in tight gas reservoirs

Sand and ceramic proppant are the most commonly used materials to keep fractures open during hydraulic fracturing. Ceramic proppant has higher hardness and sphericity than sand, but it is much more expensive. To reduce the cost, sands are pumped at the beginning to replace a portion of ceramic proppants, while ceramic proppants are pumped at the end to support the fracture outlet where the effective closure stress is large. However, the effective conductivity of the propped fracture varies with the ratio of these two types of proppants, and it is worthy of laboratory investigation to determine the optimal substitution ratio of sands for fields with different effective closure stresses. In this work, the fracture conductivity with various ratios of sands to ceramic proppants is evaluated by an API standard Fracture Conductivity Evaluation System (FCS-842) under different effective closure stresses. Experimental results show that the fracture conductivity of the propped fracture decreases with the effective closure stress due to the crushing of proppants, while the decreasing rate of fracture conductivity is proportional to the ratio of sands to ceramic proppants within the propped fracture. Two empirical models are further derived from the results, which can be used to forecast the performance of fracture conductivity at different effective closure stresses and sand ratios. The findings of this work can guide people to optimize the sand ratio in the slurry when hydraulically fracturing the reservoirs at different depths with different effective closure stresses.