Geothermal energy，as a clean and low-carbon renewable energy，plays an important role in contributing to the carbon peaking and carbon neutrality goals. Deep carbonate geothermal resources are abundant in China. However，because of high temperature，high pressure，and low permeability，an efficient reservoir stimulation method needs to be adopted，and conductive fractures need to be established to develop such resources. Hydra-jet acid fracturing technology is a kind of stimulation technology integrating abrasive jet perforation，hydraulic isolation，acid etching，and multi-stage and multi-cluster stimulation. Hydra-jet acid fracturing technology can realize fixed-point communications to geological sweet spots and volume fracturing in one trip，which has good applicability to carbonate geothermal reservoirs. In this study，the field test of hydra-jet acid fracturing in the Rongcheng geothermal field of Xiong’an New Area was conducted for the first time. By optimizing the nozzle structure，selecting the corrosion-resistant string materials，designing the pumping program，and simulating the fracture propagation patterns，the hydra-jet acid fracturing tools and operations that can meet the needs of deep carbonate geothermal reservoirs have been developed. The "alternating pumping cross-linked slug and gelling acid" method is adopted to further promote acid etching of distal fractures，fully communicate with natural fractures/vugs，and improve the connectivity between hydraulic and natural fractures，thus increasing the stimulated reservoir volume and enhancing the fracture conductivity，so that efficient reservoir stimulation can be achieved. In this on-site acid fracturing test，the total amount of fluid pumping into the well is 492.65 m3，of which the fracturing fluid is 396.76 m3，and the gelling acid is 95.89 m3. The field application results indicate that the unit water inflow increases from 0.024 m3/(h·m) to 0.288 m3/(h·m) after acid stimulation，approximately 12 times，and the temperature of water at the wellhead increases to 10 ℃. The production enhancement is remarkable. Moreover，the hydra-jet acid fracturing tools developed by the authors are resistant to high temperature，high pressure，wear，and acid corrosion. The research results are expected to provide technical references and thoughts for deep geothermal reservoir stimulation. © 2023 Tianjin University. All rights reserved.

The stimulation efficiency of hydraulic fracturing in coalbed methane (CBM) reservoir are low due to high filtration. In this study, a new compound fracturing method is proposed to improve the CBM reservoir fracturing efficiency combining the advantages of liquid nitrogen (LN2) fracturing and hydraulic fracturing. The compound fracturing starts from LN2 fracturing forming complex fractures and non-filtration zones, follows by injecting nitrogen gas (N2) to displace LN2 near the borehole into the fracture tips and prevent subsequent sand-carrying fluids from freezing in the borehole, and finishes by using sand-fluid mixtures carrying proppant into the frozen coal seam. To test the feasibility, fracturing experiments were conducted to investigate the performances of LN2 compound fracturing. The lab results show that LN2 compound fracturing can create more complex and highly conductive fracture networks than water fracturing and pure LN2 fracturing. Compared with LN2 fracturing, the average fracture aperture, fractures number, and total fracture volume induced by compound fracturing increased by 28%, 10%, and 34%, respectively. Compared with water fracturing, the fractures number, and total fracture volume induced by compound fracturing increased by 55% and 42%, respectively. The reason for the improved conductivity as follows: in the pad stage of the compound fracturing, coal is fractured by LN2 with low viscosity and low temperature, forming a large number of small-scale fractures. In the slurry stage, coal is fractured by water, which further activates the small-scale fractures, increasing the number and aperture of fractures. The key findings obtained in this work is the proposed LN2 compound fracturing method that helps sustainable development of CBM resources in an efficient and environmentally friendly way.

针对干热岩储层改造存在的起裂压力高、裂缝单一和易诱发地震的瓶颈难题，本文结合循环水力压裂和液氮压裂技术优势，探索了一种液氮循环压裂开发干热岩的新思路，即通过“注入—停顿”的方式周期性注入低温液氮，使岩石在交变热应力—流体压力耦合作用下发生疲劳损伤，促进裂缝起裂、转向、分叉进而形成复杂缝网，提高储层改造体积。目前，液氮压裂技术的研究多集中在液氮单次或循环冷却致裂岩石力学机制和液氮压裂造缝机理方面，考虑地应力条件下的液氮循环压裂造缝机理方面的研究未见报道。为了验证液氮循环压裂开发干热岩的可行性，基于自主研发的真三轴液氮循环压裂实验装置，采用可视化材料PMMA（Polymethyl Methacrylate），研究了水平应力差异系数和循环次数的影响规律，揭示了液氮循环压裂裂缝起裂与形态特征，并与清水循环压裂进行了对比。结果表明：在较低的循环次数和循环压力下，液氮循环压裂相对于清水循环压裂可显著降低起裂压力（下降47.1%～71.7%），在交变热应力—流体压力耦合作用下液氮循环压裂易形成以“热应力裂缝+主裂缝”为特征的复杂缝网；液氮循环压裂不易受水平应力差异系数控制，在较大水平应力差异系数下仍能取得较好的造缝效果；液氮循环冷却预处理是液氮循环压裂的关键，增大液氮循环冷却次数可显著降低裂缝起裂压力并形成复杂缝网，当直接采用高压液氮压裂时，起裂压力甚至会超过清水压裂；总体来看，液氮循环压裂相对于清水循环压裂能以较低的循环次数和循环注入压力实现较好的造缝效果，有望为干热岩绿色经济高效开发提供新途径。研究结果可望为液氮循环压裂开发干热岩提供理论依据和实验基础。

Hydraulic fracturing is one of the important stimulation methods to enhance the productivity of coalbed methane (CBM) wells. However, the commonly used water-based fracturing fluids can bring some bottlenecks such as large amount of water consumption, clay-mineral swelling, and poor fracturing performance on ductile coals. Cyclic liquid nitrogen (LN2) fracturing, as a novel nonaqueous stimulation method, has the potential to solve the above problems. In cyclic LN2 fracturing, supercooling LN2 is injected in a cyclic manner [i.e., alternating high injection rate (or pressure) and low injection rate (or pressure)]. Coals will be subjected to cyclic freeze-thaw, stress oscillation, and fatigue damage, which is expected to improve the stimulated reservoir volume. First, laboratory cyclic LN2 fracturing tests were conducted on coal samples with various coal ranks to investigate the fracture initiation/propagation behavior and fracture network patterns. Cyclic water fracturing tests were also conducted as comparisons. Then, computed tomography (CT) scanning and geomechanical/petrophysical properties tests before and after LN2 fracturing were performed to assist in understanding the cyclic LN2 fracturing mechanisms and implications. Finally, to solve the field application concerns, we investigated the possible fracture geometries at the field scale, temperature distribution of LN2 along the wellbore during injection, and the economic feasibility. The key factors affecting the temperature distribution during LN2 transportation along the wellbore were clarified for the first time. The results indicate that cyclic LN2 fracturing shows the potential to decrease the breakdown pressure and produce complex fracture networks. Different coal ranks have different responses to cyclic LN2 fracturing attributed to the variances in natural fracture development and geomechanical/petrophysical properties. Besides, increasing the cycle number is effective in enhancing the cyclic LN2 fracturing performance on coals with relatively higher geomechanical strengths and tighter rock mass. The suggested cycle numbers from low to high for different coal ranks are listed here: low-rank coal < high-rank coal < middle-rank coal. In field applications, gaseous nitrogen (N-2) can be used as the annulus fluid to provide an effective insulation for heat transfer between the low-temperature LN2 and the surrounding environment. In addition, the net present value (NPV) analysis indicates that LN2 fracturing is an economically feasible stimulation method, which can exceed slickwater fracturing in some cases. The key findings are expected to provide preliminary insights into the potential field applications of cyclic LN2 fracturing in CBM or other unconventional oil/gas exploitation.

Hydraulic fracturing using water-based fracturing fluids is widely used in coalbed methane (CBM) reservoir development. However, the stimulation efficiency of conventional hydraulic fracturing in CBM reservoir is low. Liquid nitrogen (LN2) cryogenic fracturing is one possible method to improve the stimulation efficiency. To test the feasibility of the LN2 cryogenic fracturing, laboratory fracturing tests were conducted to investigate the performances of different LN2 cryogenic fracturing methods in this paper. The breakdown pressure and fracture morphology of water fracturing, LN2 direct fracturing, LN2 freeze fracturing, LN2 freeze-thaw fracturing and LN2 compound fracturing were compared. And the mechanisms of different cryogenic fracturing techniques are revealed. The results demonstrate that thermal damage caused by LN2 freezing is the main reason for the complicated fracture pattern in cryogenic fracturing. LN2 freeze-thaw fracturing has the lowest breakdown pressure and can create the most complex fractures among these cryogenic fracturing methods due to the huge thermal damage. LN2 freeze fracturing has the highest breakdown pressure. Because the thermal damage cracks and natural fractures in frozen rock are in freezing shrinkage, which activation are difficult. LN2 compound fracturing is the most potential technologies, which can produce more complex fracture networks than water fracturing and LN2 fracturing. © 2022 Elsevier B.V.

地热作为清洁低碳的可再生能源，对促进实现“双碳”目标具有重要意义．我国深部碳酸盐岩地热资源丰富，但是由于岩体基质渗透率极低，亟需采取高效的储层改造技术建立导流通道才能有效开发．水力喷射酸化压裂工艺是一种集喷砂射孔、水力封隔、酸液刻蚀、多段多簇为一体的增产改造技术，可实现定点沟通地质甜点、一趟管柱体积造缝，对碳酸盐岩热储层具有良好的适用性．首次开展了雄安新区容城地热田水力喷射酸化压裂碳酸盐岩深部地热储层现场试验，通过优化喷嘴结构、优选耐腐蚀管柱材料、针对性设计泵注程序、模拟裂缝扩展形态，研发了能够满足深部碳酸盐岩热储的水力喷射酸压工具和配套工艺，采用“交替泵注交联段塞-胶凝酸”的方式，进一步促进酸液刻蚀远端裂缝，充分沟通天然缝洞，促进水力裂缝与天然裂缝沟通、相交、融汇，从而达到扩大造缝总体积、提高裂缝导流能力的储层高效改造目的．本次现场酸压试验入井总液量为492.65 m~3，其中压裂液为396.76 m~3，胶凝酸为95.89 m~3．试验结果表明：酸压后较酸压前单位涌水量从0.024 m~3/（h·m）提高到0.288 m~3/（h·m），增产12倍，井口水温增加了10℃，增产效果明显；水力喷射酸压工具耐高温、耐高压、耐磨损、耐腐蚀，节约成本，降低作业风险．研究成果可为深部地热储层增产改造提供技术借鉴与思路．

Coalbed methane(CBM) is an important unconventional natural gas. Exploitation of multilayered CBM reservoir is still facing the challenge of low production rate. Radial borehole fracturing, which integrates radial jet drilling and hydraulic fracturing, is expected to create complex fracture networks in multilayers and enhance CBM recovery. The main purpose of this paper is to investigate the mechanisms and efficacy of radial borehole fracturing in increasing CBM production in multiple layers. First, a two-phase flow and multi-scale 3 D fracture network including radial laterals, hydraulic fractures and face/butt cleats model is established, and embedded discrete fracture model(EDFM) is applied to handle the complex fracture networks. Then, effects of natural-fracture nonuniform distribution are investigated to show the advantages of targeted stimulation for radial borehole fracturing. Finally, two field CBM wells located in eastern Yunnan-western Guizhou, China were presented to illuminate the stimulation efficiency by radial borehole fracturing. The results indicated that compared with vertical well fracturing, radial borehole fracturing can achieve higher gas/water daily production rate and cumulative gas/water production, approximately 2 times higher. Targeted communications to cleats and sweet spots and flexibility in designing radial borehole parameters in different layers so as to increase fracture-network complexity and connectivity are the major reasons for production enhancement of radial borehole fracturing.Furthermore, the integration of geology-engineering is vital for the decision of radial borehole fracturing designing scheme. The key findings of this paper could provide useful insights towards understanding the capability of radial borehole fracturing in developing CBM and coal-measure gas in multiple-thin layers.

Coalbed methane (CBM) has emerged as one of the clean unconventional resources to supplement the rising demand of oil and gas. Analyzing and predicting CBM production performance are critical in choosing the optimal completion methods and parameters. However, the conventional numerical simulation has challenges of complicated gridding issues and expensive computational costs. The huge amount of available production data that has been collected in the field site opens up a new opportunity to develop data-driven approaches in predicting the production rate. Here, we proposed a novel physics-constrained data-driven workflow to effectively forecast the CBM productivity based on a gated recurrent unit (GRU) and multilayer perceptron (MLP) combined neural network (GRU-MLP model). The model architecture is optimized automatically by the multiobjective algorithm: nondominated sorting genetic algorithm II (NSGA II). The proposed framework was used to predict gas and water production in synthetic cases with various fracture-network-complexity/connectivity and two multistage fractured horizontal wells in field sites located at Ordos Basin and Qinshui Basin, China. The results indicated that the proposed GRU-MLP combined neural network was able to accurately and stably predict the production performance of CBM fractured wells in a fast manner. Compared with recurrent neural network (RNN), GRU, and long short-term memory (LSTM), the proposed GRU-MLP had the highest accuracy, stability, and generalization, especially in the peak or trough and late-time production periods, because it could capture the production-variation trends precisely under the static and dynamic physical constraints. Consequently, a physics-constrained data-driven approach performed better than a pure data-driven method. Moreover, the contributions of constraints affecting the model prediction performance were clarified, which could provide insights for the practicing engineers to choose which categorical constraints are needed to focus on and preferentially treated if there are uncertainties and unknowns in a realistic reservoir. In addition, the optimum GRU-MLP model architecture was a group of optimized solutions, rather than a single solution. Engineers can evaluate the tradeoffs within this optimal set according to the field-site requirements. This study provides a novel machine learning approach based on a GRU-MLP combined neural network to estimate production perforniances in naturally fractured reservoir. The method is gridless and simple, but is capable of predicting the productivity in a computational cost-effective way. The key findings of this work are expected to provide a theoretical guidance for the intelligent development in oil and gas industry.

Liquid nitrogen (LN2) fracturing is a kind of non-aqueous fracturing technology, which is expected to provide a new and efficient way for coalbed methane (CBM) development. The mechanical properties of coal under LN2 freezing are very important for studying the mechanism of LN2 fracturing. However, most of the current research is limited to studying mechanical properties of rocks after being frozen by LN2 and returned to room temperature. In this paper, the effect of LN2 freezing on the mechanical properties of coal was studied. Uniaxial strength tests and Brazil tests were carried out for dry and water-saturated coal samples with different types and bedding directions. In addition, standard electron microscopy (standard SEM) and cryo-electron microscopy (Cryo-SEM) were used to compare the fracture morphology of coal samples at room temperature and LN2 temperature. The results showed that LN2 freezing can damage and improve the mechanical properties of coal simultaneously. The strength of saturated coal under freezing is higher than that of dry coal, and the filling of ice can enhance the mechanical strength of coal. In addition, the mechanical properties of coal with higher porosity are enhanced more than that of coal with lower porosity under LN2 freezing. The main findings of this study are the keys to the research of LN2 fracturing mechanisms in CBM reservoirs. © 2022 The Authors

Liquid nitrogen (LN2) fracturing has the potential to induce complex fracture networks, avoid formation damage, and eliminate water consumption. However, the flow and heat transfer of nitrogen in fractures and its effect on the fracture generation during LN2 fracturing have not been studied, and the fracturing mechanism remains unclear. In this paper, the nitrogen flow during LN2 fracturing in a straight fracture was simulated. First, a 3D unsteady-state fluid flow and heat transfer model for LN2 fracturing in coalbed methane (CBM) reservoir was developed, which considered the phase transition of nitrogen, thermophysical properties variation of coal and the heat transfer between nitrogen and formation. This model was then validated against published analytical solutions. Subsequently, the model was applied to elucidate the phase distribution of nitrogen and its influence on fracture generation. Finally, the factors that affect the flow and heat transfer of nitrogen were analyzed. The results showed that the nitrogen at the fracture tip was in a supercritical state. Thermal stress had minor effects on the propagation of the main fracture. In addition, fracture aperture, injection velocity, reservoir temperature, injection fluid temperature, fracture propagation pressure and coal cleat porosity could affect the effectiveness of LN2 fracturing in a coal seam. The main findings of this study are the keys to the research of liquid nitrogen fracturing mechanisms in CBM reservoirs.