Experimental Investigation of Cryogenic Fracturing of Rock Specimens Under True Triaxial Confining Stresses

Abstract

Abstract We have performed a laboratory study of cryogenic fracturing for improving oil/gas recovery from low-permeability shale and tight reservoirs. Our objective is to develop well stimulation techniques using cryogenic fluids, e.g. liquid nitrogen (LN) to increase permeability in a large reservoir volume surrounding wells. The new technology has the potential to reduce formation damage created by current stimulation methods as well as minimize or eliminate water usage and groundwater contamination. The concept of cryogenic fracturing is that sharp thermal gradient (thermal shock) created at the rock surface by applying cryogenic fluid can cause strong local tensile stress and initiate fractures. We developed a laboratory system for cryogenic fracturing under true triaxial loading, with a liquid nitrogen delivery/control and measurement system. The loading system simulates confining stresses by independently loading each axis up to about 5000 psi on 8"×8"×8" cubes. Both temperature in boreholes and block surfaces and fluid pressure in boreholes were continuously monitored. Acoustic and pressure-decay measurements are obtained before and at various stages of stimulations. Cubic blocks (8"×8"×8") of Niobrara shale, concrete, and sandstones have been tested, and stress levels and anisotropies are varied. Three schemes are considered: gas fracturing without cryo-stimulation, gas fracturing after low-pressure cryogen flow-through, gas fracturing after high-pressure flow-through. Pressure decay results show that liquid nitrogen stimulation clearly increases permeability, and repeated stimulations further increase the permeability. Acoustic velocities and amplitudes decreased significantly following cryo-stimulation indicating fracture creation. In the gas fracturing without the stimulation, breakdown (complete fracturing) occurs suddenly without any initial leaking, and major fracture planes form along the plane containing principal stress and intermediate stress directions as expected theoretically. However, in the gas fracturing after cryogenic stimulations, breakdown occurred gradually and with massive leaking due to thermal fractures created during stimulation. In addition, the major fracture direction does not necessarily follow the plane containing principal stress direction, esp. at low confining stress levels. In tests, we have observed that cryogenic stimulation seems to disrupt the internal stress field. The increase of borehole temperature after stimulation affects the permeability of the specimen. While a stimulated specimen is still cold, it keeps high permeability because fractures remain open and local thermal tension is maintained near the borehole. When the rock becomes warm again, fractures close and permeability decreases. In these tests, we have not used proppants. Overall, fractures are clearly generated by low and high-pressure thermal shocks. The added pressure of the high-pressure thermal shocks helps to further propagate cryogenic fractures generated by thermal shock. Breakdown pressure is significantly lowered by LN stimulation with breakdown pressure reductions up to about 40% observed.