Research on hydrate formation rules in the formations for liquid CO2 fracturing

Abstract

Liquid CO2 fracturing has been widely used for oil and gas field stimulation because of its non-damaging and rapid cleanup properties. However, CO2 near the fracture may form hydrates with in-situ water in the surrounding porous media under low temperature and high pressure conditions, preventing proppant packing and reducing fracture conductivity and reservoir reconstructed effects.In this work, we build a dynamically coupled mass and heat transfer model which describes the unsteady process of CO2 flowing in the fracture and “leaking-off”, or permeating, into the surrounding porous rock. This model considers transition of CO2 phase state, variations of CO2 thermos-physical properties, and the throttling effect. Through combining the model with CO2 hydrate phase equilibrium P-T condition to establish an integrated model, the region of CO2 hydrate formation in the formations fractured can be estimated.The simulation results show that when CO2 flows in the fracture and permeates the surround rock accompanied with CO2 temperature increasing rapidly and pressure decreasing gradually, CO2 will convert from a liquid to a supercritical state. Due to the low temperatures and high pressures of CO2 within surrounding region along the fracture, hydrates may form. Its formation region looks like “a long and narrow bullet”, the length of which is about 35 m in the cases of this paper. In the fractured formations, the hydrate formation region can be enlarged moderately by the CO2 throttling effect. The simulation results also show that fracture conductivity will be considerably reduced by the formation of hydrates. In order to reduce hydrate formation during fracturing, inhibitors such as methanol and NaCl can be injected into the fracturing fluid. Relative to the formation water, the mole fractions of methanol and NaCl should be higher than 20% and 8% respectively in order to successfully suppress hydrate formation.