Cryogenic fracturing is a waterless stimulation technology that uses cryogenic fluids to fracture unconventional oil and gas reservoirs, which, to date, are rarely investigated and poorly understood. This study aims to investigate the efficacy and feasibility of cryogenic fracturing technology in enhancing the permeability of unconventional reservoir rock analogs. Laboratory cryogenic fracturing experiments and finite difference modeling are integrated to reveal the processes and mechanism of cryogenic fluids on creating fractures in synthetic rock samples.
The 8-inch cubic rock samples were initially prepared by embedding eight tiny thermocouples on their diagonals to the depth of 4 inches, so real-time temperature distribution in the samples can be monitored during the experiments. Confined with true tri-axial stresses, liquid nitrogen was injected into the borehole to crack the synthetic rock samples at low pressure (~15 psi) and reservoir temperature (~85 °C) via a tubing-casing type wellhead. Before and after the liquid nitrogen stimulation, acoustic measurements and pressure decay tests were carried out to evaluate the fracture generation and permeability enhancements of the rock samples. The experimental processes were modeled using TOUGH2-EGS by integrating the Mogi–Coulomb failure criterion into the fracture generation module.
Comparison of pre- and post-stimulation pressure decay curves showed significant permeability enhancements of the synthetic rock samples after liquid nitrogen stimulation. Delay of acoustic signal arrivals measured on rock faces indicated that multiple fractures have been created inside the rock samples. Temperature profiles recorded during the liquid nitrogen stimulation mapped the temperature distribution through the rock samples. Rock properties were measured and input into the modified TOUGH2-EGS model to simulate the experiments, the results from modeling successfully reproduced the experimental results, in terms of temperature profiles, a general fracture morphology, and pressure decay curves.
Cryogenic stimulation is proved to be capable of generating fractures and enhancing the matrix permeability in the near-wellbore area at low injection pressure. The controlling factors in affecting the cryogenic fracturing effect are captured by the modified finite difference model, providing a useful tool for design and predication of field-scale applications.