Cryogenic Fracturing of Wellbores Under True Triaxial-Confining Stresses: Experimental Investigation

Abstract

A laboratory study of cryogenic fracturing was performed to test its ability to improve oil/gas recovery from low-permeability reservoirs. Our objective is to develop well-stimulation technologies using cryogenic fluids, e.g. liquid nitrogen (LN) to increase permeability in a large reservoir volume surrounding wells. The new technology has the potential to reduce formation damage caused by current stimulation methods as well as minimize or eliminate water usage. The concept of cryogenic fracturing is that sharp thermal gradient (thermal shock) created at the surfaces of formation rocks by applying cryogenic fluid can cause strong local tensile stress and initiate fractures. We developed a laboratory system for cryogenic fracturing under true-triaxial loading, with liquid nitrogen delivery/control and measurement systems. The loading system simulates confining stresses by independently loading each axis up to about 5000 psi on 8"×8"×8" cubes. Both temperature in boreholes and block surfaces and fluid pressure in boreholes were continuously monitored. Acoustic and pressure-decay measurements were obtained before and at various stages of stimulations. Cubic blocks (8"×8"×8") of Niobrara shale, concrete, and sandstones were tested, and stress levels and anisotropies varied. Three schemes were considered: gas fracturing without cryo-stimulation, gas fracturing after low-pressure cryogen flowthrough, and gas fracturing after high-pressure cryogen flow-through. Results from pressure decay tests show that liquid nitrogen stimulation clearly increases permeability, and repeated stimulations further increase the permeability. Acoustic velocities and amplitudes decreased significantly following cryo-stimulation indicating fracture creation. In the gas fracturing without the stimulation, breakdown (complete fracturing) occurs suddenly without any initial leaking, and major fracture planes form along the plane containing principal stress and intermediate stress directions as expected theoretically. However, in the gas fracturing after cryogenic stimulations, breakdown occurred gradually and with massive leaking due to thermal fractures created during stimulation. In addition, the major fracture direction does not necessarily follow the plane containing principal stress direction, especially at low confining stress levels. In tests, we observed that cryogenic stimulation seems to disrupt the internal stress field. The increase in borehole temperature after stimulation affects the permeability of the specimen. When a stimulated specimen is still cold, it maintains high permeability because fractures remain open and local thermal tension is maintained near the borehole. When the rock warms back, fractures close and permeability decreases. In these tests, we have not used proppants. Overall, fractures are clearly generated by low and high-pressure thermal shocks. The added pressure of the high-pressure thermal shocks helps to further propagate cryogenic fractures 1 2 3 4 5 6 7 8 9 2 generated by thermal shock. Breakdown pressure ...

Propagation of Cryogenic Thermal Fractures from Unconfined PMMA Boreholes

“…Energies 2021, 14, x FOR PEER REVIEW 3 of 17 after breakdown tests [15]. That is, the studies involved no real-time observations of internal cryogenic fracturing processes around the boreholes of 3D specimens, an approach similar to the CT-scan monitoring of hydraulic fracturing by de Borst et al [27].…”

“…Phase transition during the fracturing process and the low fluid viscosity of LN can facilitate fracture propagation and network generation [11]. The fracability of shale increases after LN pretreatment, especially under high confining pressure conditions, and after LN treatment, the strength and brittleness of shale decrease, which reduces the initiation and propagation pressure of reservoir stimulation [3,14,15].…”

“…Although studies have performed the visualization of thermal crack propagation in simple 1D or 2D geometries [21][22][23], none have attempted to explain the process of cryogenic fracturing propagating over time from borehole geometry, which is relevant to well stimulation. While reporting some cryogenic fracture patterns, notably tortuosity, the studies inferred them from boundary observations [24][25][26] or post-mortem observa-tions after breakdown tests [15]. That is, the studies involved no real-time observations of internal cryogenic fracturing processes around the boreholes of 3D specimens, an approach similar to the CT-scan monitoring of hydraulic fracturing by de Borst et al [27].…”

Effects of Pressurizing Cryogenic Treatments on Physical and Mechanical Properties of Shale Core Samples—An Experimental Study

“…Due to the instant difference in the temperatures, a rapid increase in the tensile stress occurs and as a result, new cracks are created [24]. When injecting LN 2 into reservoirs, it vaporizes at a liquid-to-gas expansion ratio of 1:694 (at atmospheric pressure and 20 • C), which pressurizes the formation, propagates the fractures deeper, and connects pre-existing fractures networks [25].…”

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

“…Cryogenic fracturing is a stimulation technology that uses cryogenic fluids like liquid nitrogen to fracture unconventional oil and gas reservoirs and it reduces the formation damage, as it is a waterless technology [17][18][19]. Smith [20] also reported that using the Earth's geothermal energy to warm cryogenic flood fluids injected into unconventional reservoirs causes an increase in reservoir pressure and this rise in pressure results in the production of fluids from the reservoirs.…”