Cryogenic-Fracturing Treatment of Synthetic Rock With Liquid Nitrogen
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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.
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.
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.
The synthetic samples used in this study are made of concrete and molded into 8×8×8-in. cubes. To better evaluate the temperature field within the samples during experiments, the authors embedded eight thermocouples (TCs) on the diagonals of top surfaces to the depth of 4 in., which is the middle plane of samples.
The cryogenic-fracturing treatment was conducted on two concrete samples (TC1 and TC2). Sample TC1 was initially at room temperature, which is 19°C, while sample TC2 was preheated in an oven to 85°C. Both samples went through 30 minutes of cryogenic-fracturing treatment.
For characterization, the authors used pressure-decay tests as a measurement for average permeability of samples and acoustic measurements before and after cryogenic treatment to obtain a general idea of the extent of cryogenically induced fractures.
The simulation for the cryogenic-fracturing process was based on a modified enhanced-geothermal-system simulator. The modification concerned fracture initiation and propagation. This simulation tool can simulate cryogenic-fracturing processes and predict the distribution of fractures. The approach is discussed in detail in the complete paper.
The basic setup for simulation follows that of the cryogenic-fracturing experiment. The liquid nitrogen is simulated as circulating through the wellbore and cooling the surface of the borehole. The injection pressure is set to be 15 psi, which is the pressure of the liquid-nitrogen tank.
This simulation tool is used to obtain average permeability of samples, by reverse modeling the pressure-decay tests, and to predict the fracture distribution within samples.
Results and Discussion
Sample TC1, under triaxial stresses of 500, 750, and 1,000 psi, was treated with circulation of liquid nitrogen for 30 minutes. The circulation pressure of liquid nitrogen is approximately 15 psi greater than the ambient pressure. Fig. 1 shows the temperature readings from all TCs during the cryogenic-fracturing treatment. Because TC1 and TC8 were not working properly during the experiment, the respective data are not shown. Contrary to expectation, TC6, which was located farther from the wellbore than TC5, actually gave the lowest temperature reading for all diagonal-embedded TCs. This might indicate that there existed some high-conductivity channel through the borehole to locations around TC6.
The acoustic measurement showed a uniform delay caused by triaxial loading. However, other points showed a significant delay, indicating that fractures might be generated in these locations.
The pressure-decay tests before and after the cryogenic treatment showed an improvement in the overall permeability of Sample TC1. The simulation result shows that the average permeability of TC1 before treatment was approximately 1.05×10–2 md. After treatment, the average permeability from simulation was increased to 1.55×10–2 md.
Sample TC2 was also subjected to triaxial stresses of 500, 750, and 1,000 psi. Liquid nitrogen was circulated through its borehole for 30 minutes. However, TC2 was preheated in an oven to 85°C, which made the temperature change more dramatic during the treatment. Therefore, cryogenic-fracturing treatment on TC2 should potentially be more efficient. TC5 and TC8 were compromised and did not provide reasonable readings. The temperature profile indicated a pre-existing fracture on the side of TC6 and TC7.
The acoustic measurement for TC2 showed similar uniform delay caused by triaxial loading, but no significant change from induced fractures.
The pressure-decay tests before and after the cryogenic treatment showed an improvement of the overall permeability of Sample TC2. The simulation result yields an average permeability of TC2 before treatment of approximately 0.130 md. After treatment, the average permeability from simulation was increased to 0.655 md.
Fig. 2 shows a fracture plane of TC2 after injection of dye and breakdown with high-pressure gas. The permeability-enhanced area was dyed red. The shape of this area agrees with the simulation results.
Cryogenic-fracturing technology has been demonstrated to be a promising formation-damage-free and environmentally friendly stimulation technology in laboratory studies. It can achieve reasonable permeability enhancement solely by circulation of liquid nitrogen through the wellbore of synthetic-rock samples. However, without aid of pressure, the extent of permeability enhancement is greatly restricted. The fractures created by cryogenic fluid may not be planar in shape, as in hydraulic fracturing. Instead, the method might evenly increase the permeability in the affected area.
Cryogenic-Fracturing Treatment of Synthetic Rock With Liquid Nitrogen
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