Effects of cyclic cryogenic treatment on rock physical and mechanical properties of Eagle Ford shale samples - An experimental study

摘要

Unconventional reservoirs have ultra-low permeability, and the combination of horizontal wells and multistage transverse hydraulic fracturing have unlocked economical hydrocarbon production from such formations. Fracturing fluid involves large volumes of water that is partially recovered during the flowback stage, and the rest remains in the formation damaging the matrix-fracture interface. In this paper, an alternative waterless stimulation technique has been studied. A cryogenic fluid, liquid nitrogen (LN2), was injected into cores from Eagle Ford formation. The effects of cyclic LN2 on rock physical and dynamic elastic properties were examined. Two cycles of injecting LN2 into cores at the reservoir temperature and pressurizing at different pressures (1.38, 2.76, 4.14 MPa) were implemented. Before and after each cycle, computerized tomography scan, porosity, permeability, and ultrasonic velocities tests were conducted on the samples. The results demonstrated that cyclic cryogenic treatment increases porosity and permeability and decreases ultrasonic velocities of the samples resulting from inducing new cracks and extending and widening the existing ones. Permeability enhancement, which results from the creation of high-permeable pathways allowing the gas to move rapidly across the tested samples, was noticeable (up to 2 orders of magnitude). Whilst, comparing to permeability, the porosity improvement was not significant (up to 26%) owing to the small increase in pore volume compared to the sample's bulk volume. The results showed a direct relationship between injection pressure and porosity and permeability enhancements after the completion of each cycle. Moreover, the reductions in ultrasonic velocities and dynamic elastic properties were insignificant compared to the changes in the porosity and permeability. Additionally, porosity and permeability enhancements and velocities reduction mainly occurred after completing the first cycle. This is because of lowering the thermal stress on samples, which results in reducing the possibility of creating new cracks or propagating and widening the current ones.