Optimization of proppant size for frac pack completion using a new equipment

It is critical to optimize the proppant size for an effective sand control in weakly consolidated formations and to obtain high productivity of medium-to-high permeability formations. Compared with the gravel pack completion, the frac pack has a completely different sand control mechanism and pressure field on the proppant. To achieve effective sand control and stimulation, a treatment should be designed to obtain the maximum conductivity and optimize the proppant size for sand production. At present, there are no unified criteria for proppant-size selection for the frac pack; neither is the equipment for conducting such experiments available. In order to experimentally study the frac pack, we designed a special equipment called the “Frac Pack Sand Control Model” for the experiments in which the proppant size was matched with the formation particle size. By measuring the fracture conductivity, sand production rate, and the size of the particles produced, we investigated the effect of fluid viscosity, flow rate, median grain diameter of formation sand, and proppant concentration on sand control and conductivity. We also studied the proppant embedment in unconsolidated formations and the effects of combinations of proppants of different sizes on sand control and conductivity. Experimental results show that the optimal proppant size for the frac pack is 6.8–9.7 times that of formation sands. Compared with proppant of a fixed size, a combination of proppants of variable sizes is preferred for a better sand control or higher conductivity, depending on the conditions of a particular formation. In a weakly consolidated formation, proppant embedment can be very serious, and becomes even more serious with an increasing proppant size. While ensuring the optimal size of proppant, i.e. D50=(6.8–9.7)d50, a high proppant concentration is needed to obtain effective sand control in a frac pack. The proppant concentration not less than 15 kg/m2 is usually recommended in a frac pack. The finding in the research is significant for both theoretical study and field engineering.