Cryogenic-Fracturing Treatment of Synthetic-Rock With Liquid Nitrogen

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 185050, “Experimental Study and Modeling of Cryogenic-Fracturing Treatment of Synthetic-Rock Samples With Liquid Nitrogen Under Triaxial Stresses,” by B. Yao and L. Wang, Colorado School of Mines; T. Patterson, Devon Energy; T.J. Kneafsey, Lawrence Berkeley National Laboratory; and X. Yin and Y. Wu, Colorado School of Mines, prepared for the 2017 SPE Canada Unconventional Resources Conference, Calgary, 15–16 February. The paper has not been peer reviewed. Cryogenic fracturing is a waterless stimulation technology that uses cryogenic fluids to fracture unconventional oil and gas reservoirs, and, to date, the underlying mechanism has been investigated rarely and is often understood poorly. 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 process and mechanism of cryogenic fluids in creating fractures in synthetic-rock samples. Introduction Traditional hydraulic fracturing relies on mostly water-based fracturing fluids and usually consumes a tremendous amount of water. The usage of water not only can cause potential formation-damage issues but also can place a significant stress on local water resources and the environment. Thus, waterless stimulation technologies, especially cryogenic fracturing, are being developed to solve these issues. Cryogenic fracturing uses cryogenic fluids such as liquid nitrogen to fracture unconventional oil and gas reservoirs. Fractures will be induced by the dramatic change of temperature when a warm body, such as reservoir rock, is exposed to a frigid fluid, such as liquid nitrogen. Although the feasibility of cryogenic-fracturing technology has been demonstrated by both laboratory experiment and pilot field tests, the mechanism behind it was rarely investigated and is poorly understood. A preliminary test involved a series of experiments investigating patterns of surface fractures and the effect of cryogenic treatment after submerging concrete-rock samples into liquid nitrogen. Computed-tomography scans showed that the fracture penetrated into the center of the block after 30 minutes of submersion. Further cryogenic-fluid-injection tests on concrete and shale samples have demonstrated the efficacy of permeability enhancement around the wellbore by injecting liquid nitrogen at both low and high pressures. Experimental Approach To conduct a cryogenic-fracturing treatment in a manner similar to that seen in field applications, the authors created a testing environment with sample sizes (8×8×8 in.) between the core and the real-reservoir scales. Liquid nitrogen was circulated into cubic samples through 6-in.-deep boreholes drilled from the top of the samples and cased with stainless-steel tubing by epoxy for the top 2 in. The confining stresses were applied on each sample by a triaxial-loading system that has the capability of providing a true triaxial-stress condition with different stresses along three different axes. The liquid nitrogen was drawn from a tank, injected directly into the wellbore, and then vented through the annulus space to the environment. The purpose of liquid-nitrogen circulation is to achieve a better cooling effect on the openhole section of the wellbore. Pressure, temperature, and stress data were collected during the experiment.