Cryogenic fracturing for reservoir stimulation – Laboratory studies

While hydraulic fracturing has revolutionized hydrocarbon production from unconventional resources, waterless or reduced-water fracturing technologies have been actively sought due to concerns arising from the heavy use of water. This study investigates the feasibility of fracture stimulation by using cryogenic fluids to create a strong thermal gradient generating local tensile stress in the rocks surrounding a borehole. Cracks form when the tensile stress exceeds the material׳s tensile strength. This mechanism has not been exploited in the context of stimulation and may be used to fracture reservoir rocks to reduce or eliminate water usage. This paper reports initial results from a laboratory study of cryogenic fracturing. In particular, we have developed experimental setups and procedures to conduct cryogenic fracturing tests with and without confining stress, with integrated cryogen transport, measurements, and fracture characterization. Borehole pressure, liquid nitrogen, and temperature can be monitored continuously. Acoustic signals are used to characterize fractures before and after the experiments. Cryogenic tests conducted in the absence of the confining stress were able to create cracks in the experimental blocks and alter rock properties. Fractures were created by generating a strong thermal gradient in a concrete block semi-submerged in liquid nitrogen. Increasing the number of cryogenic stimulations enhanced fracturing by both creating new cracks as well as widening the existing cracks. By comparing the cryogenic fracturing results from unstressed weak concrete and sandstone, we found that the generation of fractures is dependent on the material properties. Water in the formation expands as it freezes and plays a competing role during cryogenic cooling with rock contraction, thus is an unfavorable factor. A rapid cooling rate is desired to achieve high thermal gradient.