Abstract
Shale reservoirs can be stimulated by injecting cryogenic fluids such as liquid nitrogen (LN2) to initiate and propagate fractures and to connect existing natural fracture networks. This research aims to investigate the efficacy of the cryogenic treatments on the porosity, permeability, and mechanical properties (Young's modulus, Poisson's ratio, brittleness, bulk compressibility, and unconfined compressive strength (UCS)) of Marcellus downhole core samples. Three downhole core samples, taken from 1700 m depth, were heated up to the reservoir temperature (66 degrees C). LN2 was then injected into a core holder hosting the core samples. Then, the samples were pressurized at three different pressures of 1.38, 2.76, and 4.14 MPa. Computed tomography (CT) scan, porosity, permeability, and ultrasonic velocity tests were conducted on the core samples prior to and after implementing the cryogenic treatments. The results of the CT-scan analyses, porosity, permeability, and mechanical property measurements revealed that implementing the cryogenic treatments resulted in creating new cracks in all the core samples. The results also demonstrated that pressurizing the injected LN2 at 1.38, 2.76, and 4.14 MPa increased the porosities of the core samples by 1.51%, 2.49%, and 5.01% and significantly increased the permeability by 270%, 388%, and 9482%, respectively owing to the creation of new cracks inside the core samples. Also, the compressional and shear velocities decreased owing to the creation of the new cracks resulted from conducting the cryogenic treatment by up to 2.11% and 0.70%, respectively. As a result, dynamic Young's modulus and Poisson's ratio of the core samples decreased by up to 2.37% and 5.24%, respectively. Moreover, the brittleness and bulk compressibility increased by up to 1.59% and 7.05%, respectively. After implementing the cryogenic treatments, the UCS of the core samples ranged from 34.73 to 2.57 MPa, significantly lower than typical values of UCS for Marcellus core samples (101-170 MPa), owing to the creation of new cracks resulting from implementing the cryogenic treatments. The results revealed that the cryogenic treatments could be potentially implemented in shale fields to increase the permeability, and as a result, increase hydrocarbon production rates.