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Yang, Ruiyue (China University of Petroleum) | Chunyang, Hong (China University of Petroleum) | Huang, Zhongwei (China University of Petroleum) | Wen, Haitao (China University of Petroleum) | Li, Xiaojiang (Sinopec Research Institute of Petroleum Engineering) | Huang, Pengpeng (China University of Petroleum) | Liu, Wei (China University of Petroleum) | Chen, Jianxiang (China University of Petroleum)
Summary Multistage hydraulic fracturing is widely used in developing tight reservoirs. However, the economic and environmental burden of freshwater souring, transportation, treatment, and disposal in hydraulic fracturing operations has been a topic of great importance to the energy industry and public alike. Waterless fracturing is one possible method of solving these water-related issues. Liquid nitrogen (LN2) is considered a promising alternate fracturing fluid that can create fractures by coupled hydraulic/thermal loadings and, more importantly, pose no threats to the environment. However, there are few laboratory experiments that use LN2 directly as a fracturing fluid. In this work, we examine the performance of LN2 fracturing based on a newly developed cryogenic-fracturing system under true-triaxial loadings. The breakdown pressure and fracture morphologies are compared with water fracturing. Moreover, fracture-initiation behavior under cryogenic in-situ conditions revealed by cryo-scanning electron microscopy (cryo-SEM) is presented, and the role of thermal stress is quantified by a coupled thermoporoelastic-damage numerical simulation. Finally, the potential application considerations of LN2 fracturing in the field site are discussed. The results demonstrate that LN2 fracturing can lower fracture initiation and propagation pressure and generate higher conductive fractures with numerous thermally induced cracks in the vicinity of the wellbore. Thermal gradient could generate enormously high-tensile hoop stress and bring about extensive rock damage. Fracture-propagation direction is inclined to be influenced by the thermal stress. Furthermore, phase transition during the fracturing process and low fluid viscosity of LN2 can also facilitate the fracture propagation and network generation. The key findings obtained in this work are expected to provide a viable alternative for the sustainable development of tight-reservoir resources in an efficient and environmentally acceptable way. Correction Notice: This PDF has been updated to correct the original submission date in the provenance. No other information was changed.
Cha, Minsu (Texas A&M University) | Alqahtani, Naif B. (King Abdulaziz City for Science and Technology) | Yao, Bowen (Colorado School of Mines) | Yin, Xiaolong (Colorado School of Mines) | Kneafsey, Timothy J. (Lawrence Berkeley National Laboratory) | Wang, Lei (Colorado School of Mines) | Wu, Yu-Shu (Colorado School of Mines) | Miskimins, Jennifer L. (Colorado School of Mines)
Summary 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 and minimize or eliminate water usage. The concept of cryogenic fracturing is that a sharp thermal gradient (thermal shock) created at the surfaces of formation rocks by applying cryogenic fluid can cause strong local tensile stress and start fractures. We developed a laboratory system for cryogenic fracturing under true-triaxial loading, with LN-delivery/control and -measurement systems. The loading system simulates confining stresses by independently loading each axis up to approximately 5,000 psi on 8×8×8-in. cubes. Temperature in boreholes and at 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-in.) 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 flow-through, and gas fracturing after high-pressure cryogen flow-through. Results from pressure-decay tests show that LN stimulation clearly increases permeability, and repeated stimulations further increase the permeability. Acoustic velocities and amplitudes decreased significantly after 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 because of thermal fractures created during stimulation. In addition, the major fracture direction does not necessarily follow the plane containing the 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 generated by thermal shock. Breakdown pressure is significantly lowered by LN stimulation, with observed breakdown-pressure reductions up to approximately 40%.
Yang, Ruiyue (China University of Petroleum) | Hong, Chunyang (China University of Petroleum) | Huang, Zhongwei (China University of Petroleum) | Yang, Zheqi (China University of Petroleum) | Huang, Pengpeng (China University of Petroleum)
ABSTRACT: Coalbed methane (CBM) is an important global energy resource. However, caused by its low permeability, reservoir stimulation methods must be taken. Cyclic liquid nitrogen (LN2) fracturing is an innovative fracturing technique to induce fracture networks by means of cyclic thermal loading on coal. To study the feasibility of cyclic cryogenic fracturing on CBM reservoirs, fracture initiation behavior of coals subjected to cyclic LN2 freeze-thaw treatment is investigated by laboratory experiments. The coal rocks were first treated by different cycles of LN2 freeze-thaw process, then were fractured under tri-axial stresses. The results indicate that cyclic LN2 freeze-thaw treatment can decrease the breakdown pressures of coals depending on the cycle numbers, with observed breakdown-pressure reductions up to 41% after 4 cycles. The major reason could be attributed to a major weak plane being formed with numerous thermal cracks inside the coal samples during the thermal fatigue loading. When the confining stress level is low, the major fracture is followed by the thermal shock. On the contrary, if a coal sample is subjected to higher stress level, the directions of the major fracture is controlled by the minimum horizontal stress principle. The key findings are expected to provide theoretical guidance for CBM development.
Coalbed methane (CBM) is well recognized as one of valuable energy resources (Moore 2012; Huang et al. 2014). Caused by its low permeability (less than 0.001 mD) (Ren et al. 2014), reservoir stimulation methods must be taken in order to produce gas at economic rates from a coal bed. However, conventional hydraulic fracturing using massive amounts of water-based fracturing fluids is unfavorable in the arid regions and also can damage the groundwater reservoirs (Javadpour et al., 2015). Hence, researchers developed a series of waterless fracturing technique, such as CO2 fracturing, liquified petroleum gas (LPG) fracturing and liquid nitrogen (LN2) fracturing, etc.
Yang, Ruiyue (China University of Petroleum) | Huang, Zhongwei (China University of Petroleum) | Wen, Haitao (China University of Petroleum) | Bi, Jianfei (China University of Petroleum) | Pang, Zhaoyu (China University of Petroleum)
ABSTRACT: Coalbed methane (CBM) reservoirs are naturally fractured formations with high permeable cleats surrounding the coal matrix. Fracturing fluid leak-off is often encountered during hydraulic fracturing process in CBM reservoirs. Liquid nitrogen (LN), one kind of super cooling and environmentally friendly fluid, is expected to be used as a temporary-blocking agent. When water in the cleats encounters with LN, the cleats can be quickly frozen and plugged. This process also can be called as LN-ice temporary blocking. To understand the temporary-blocking capability of LN and prove the feasibility of this technique quantitively. We conduct laboratory experiments to test the breakdown pressure of water-saturated coal rock samples treated with LN. The results indicate that the breakdown pressure for a LN ice-temporary-blocked coal specimen increases significantly compared with an untreated specimen. In this work, the increasing rate is approximately 103% and 31% for water fracturing and gaseous-nitrogen fracturing, respectively. Furthermore, the LN treated specimen shows a primary fracture on the surface or around the perforated area, rather than plenty of micro-fractures appeared in the untreated specimen. The key findings obtained in this study is expected to provide an experimental based guidance in the development of LN-ice temporary blocking technique.
Coalbed methane (CBM), as one of the unconventional gas resources, has been an important energy supply. Caused by its low permeability, hydraulic fracturing has become a significant stimulation method to exploit the coalbed methane reservoirs. However, coalbed is naturally fractured formations, lots of fracturing fluid could be loss into the natural fracture system. The fracturing fluid invasion could directly affect the fracture geometry, fracture-closure time, and proppant distribution (Ribeiro and Sharma, 2012). Besides, fracturing fluid leak off could increase the pumping rate, contaminate the formation, wellbore instability, etc. (Feng and Gray, 2018). Hence, hydraulic fracturing technology has low efficiency in the development of coalbed methane reservoirs.
To solve this problem, researchers and field trials have developed various lost-control additives/composites, such as polymers, gels, fibers, granular grains, etc (Dorman and Udvary, 1996; Xue et al., 2015). These additives can control the leak-off of most of hydraulic fracturing fluid. However, most of them are chemical products, which could contaminate the reservoir formation and underground water. The principles of designing a temporary-blocking-agent are: (1) prevention of loss circulation; and (2) minimization of formation damage (Dorman and Udvary, 1996).
Jiang, Long (China University of Petroleum, East China) | Cheng, Yuanfang (China University of Petroleum, East China) | Han, Zhongying (China University of Petroleum, East China) | Gao, Qi (China University of Petroleum, East China) | Yan, Chuanliang (China University of Petroleum, East China) | Wang, Guihua (China University of Petroleum, East China) | Wang, Huaidong (CCCC Marine Construction & Development Co., LTD) | Fu, Lipei (Changzhou University)
Abstract With the application of hydraulic fracturing in unconventional gas reservoirs, large-scale fracturing operations have resulted in water shortage and environmental pollution. Many scholars and governments gradually began to explore many waterless fracturing technologies in recent years, such as high-energy gas fracturing, liquefied petroleum gas fracturing, nitrogen foam fracturing, liquid/supercritical CO2 fracturing, and cryogenic fracturing using liquid nitrogen (LN2), etc. As a waterless fracturing technology, LN2 fracturing can significantly improve the coalbed methane and shale gas wells production, greatly improving the effectiveness of reservoir stimulation. But the efficiency mechanism and influencing factors of LN2 fracturing were still not clear in well bottom conditions. To investigate the influence of LN2 treatment on pore characteristics and carrying capacities of anisotropic shale, the shale from the Longmaxi formation in Chongqing, China, was selected as samples for LN2 treatment experiment. A series of permeability, ultrasonic wave, nuclear magnetic resonance (NMR) and triaxial compression experiments were conducted on different bedding shales. In pore structure tests, the increasing range of permeability is 8.01% – 74.36%, and the P-wave velocity decreases by 4.06% – 16.08%; in NMR tests, the morphology of transverse relaxation time (T2) distribution curves for LN2 treatment samples was significantly different than that of the original sample. Moreover, the change of the saturated sample is greater than that of the dry sample. In triaxial compression tests, the compressive strength, elastic modulus and brittleness of the shale were generally lower for all treated samples with LN2. The study results indicate that LN2 can cause serious irreversible damage to the internal structure of shale, which helps to open natural fractures and decrease the initiation pressure of reservoir stimulation. In cryogenic fracturing using LN2, it is of great significance to generate the complex fracture networks and improve the fracturing performance. This fracturing technology plays an important role in the development of unconventional natural gas with a bright future.