The Effect of Thermal Shocking with Nitrogen Gas on the Porosities, Permeabilities, and Rock Mechanical Properties of Unconventional Reservoirs

Abstract

Cryogenic fracturing is a type of thermal shocking in which a cold liquid or gas is injected into a hot formation to create fractures. Research has shown that like traditional hydraulic fracturing, cryogenic fracturing could improve oil/gas recovery from unconventional reservoirs. Research has also shown, though, that, unlike traditional hydraulic fracturing, which uses water-based fluids, cryogenic fracturing limits and can even heal damage that is near the wellbore. Previous studies on thermal shocking, however, have generally examined only a few parameters at a time. To provide a more complete overview of the process, this study examines the effects of thermal shocking with low-temperature nitrogen gas on the porosities, permeabilities, and rock mechanical properties of unconventional reservoirs. Three cycles of thermal shocking were applied to a core sample and an outcrop sample from an unconventional reservoir. Each sample was heated at 82 °C for 1 h, and then nitrogen at −18 °C was injected at 6.89 MPa for 5 min. The porosities and permeabilities of the cores and the velocities at which ultrasonic waves travelled through them were measured both before and after each thermal shock. The results strongly suggest that the thermal shocking produced cracks. The porosity increased by between 1.34% and 14.3%, the permeability increased by between 17.4% and 920%, and the average P-wave velocity decreased by up to 100 m/s. From the reduction in P-wave velocity, it was determined that the brittleness ratio increased by between 2 and 4 and the fracability index increased by between 0.2 and 0.8.

Numerical simulation on hydraulic fracture propagation in laminated shale based on thermo-hydro-mechanical-damage coupling model

“…When the temperature exceeds 400 C, macroscopic cracks appear on the shale surface under water cooling. Elwegaa et al (2019a) carried out liquid nitrogen cooling experiments for shale at 82 C. The results show that thermal shock can improve shale brittleness, stimulate shale initiation, and expand the existing fractures in shale, indicating that thermal shock is beneficial to improving hydraulic fracturing performance.…”

“…When the temperature exceeds 400°C, macroscopic cracks appear on the shale surface under water cooling. Elwegaa et al. (2019a) carried out liquid nitrogen cooling experiments for shale at 82°C.…”

Coupling Relationship between Lithofacies and Brittleness of the Shale Oil Reservoir: A Case Study of the Shahejie Formation in the Raoyang Sag

“…Owing to the variations in the sedimentary environment, the mineral composition, structure, and organic content of CFSR are highly heterogeneous, which is not conducive to selecting fracturing intervals [4][5][6][7][8]. One of the vital construction factors affecting the recoverability of shale oil is hydraulic fracturing treatment [9][10][11][12]. Fracability is a measure of the difficulty of a shale reservoir to be effectively reformed and indicates whether a reservoir may produce a larger reformed volume by fracturing [13,14].…”

Propagation of Cryogenic Thermal Fractures from Unconfined PMMA Boreholes

“…The shale pore structure undergoes significant alteration after freeze-thaw cycles, and some micro-pores are aggregated to form micro-and macro-cracks [3]. Although LN cooling increases permeability [2,5,6], it is less effective for sandstone than it is for other sedimentary rocks [7,8]. The surface micro-structure of rocks raises the Leidenfrost point, thus shortening the duration of film boiling and increasing the rate of cooling.…”