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.

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,结合循环水力压裂和液氮低温压裂技术优势,提出了液氮循环压裂干热岩的储层改造新思路,即通过“注入-停顿”或“高-低排量”交替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。为了验证液氮循环压裂开发干热岩的可行性,本文采用室内实验和数值模拟相结合的方法,以花岗岩为主要岩性,围绕液氮循环压裂花岗岩复杂缝网形成机理开展了深入研究,主要成果如下:基于自主研发的真三轴液氮压裂实验装置,开展了真三轴液氮/清水压裂实验,结合CT(X-ray computed tomography)扫描和冷冻电镜技术,揭示了液氮单次/循环压裂花岗岩的起裂机理和成缝特征。结果表明:液氮循环压裂与清水循环压裂相比可降低破裂压力21.3%～67.2%;清水循环压裂多形成单一主裂缝,液氮循环压裂可形成以“热应力裂缝+主裂缝+分支裂缝”为特征的立体缝网,且裂缝长度、迂曲度、面积、体积分别提升210.8%,19.6%,143.6%,142.6%,裂缝扩展方向不易受地应力控制;增大液氮循环次数、冷却时间和冷却压力,有助于降低破裂压力并形成复杂缝网,但当循环次数为0时(不预冷直接注入高压液氮),仅形成单一裂缝,且破裂压力甚至高于清水,说明通过液氮冷却维持井底低温、促进井周热损伤对提高液氮压裂造缝效果至关重要。基于相场正则化内聚裂缝方法,考虑液氮物性变化和岩石非均质性的影响,建立了液氮循环压裂花岗岩热流固-损伤耦合裂缝扩展模型,并采用真三轴液氮压裂实验数据对模型进行了验证。以损伤比例和起裂/破裂压力为评价指标,揭示了热流固-损伤多场耦合作用下液氮循环压裂裂缝扩展机制和裂缝壁面的损伤特征。结果表明:液氮压裂的损伤比例是清水压裂的1.79倍,液氮压裂的起裂压力和破裂压力分别比清水低80.7%和10.9%;热应力和流体压力作用强度的平衡是液氮压裂花岗岩裂缝分叉与形成复杂缝网的关键,流体压力作用过大,容易形成单一主裂缝,热应力作用过大,易于在井周形成复杂热应力裂缝,对主裂缝的分叉、转向有抑制作用;杨氏模量和热膨胀系数是影响液氮压裂效果的关键物性参数;脉动式循环注入可进一步提升液氮压裂造缝效果,但循环3次后压裂效果提升不明显;在液氮沿主裂缝流动过程中,可形成大量垂直于主裂缝壁面的热应力裂缝。研究成果可望为液氮循环压裂开发干热岩提供实验基础和科学依据。

我国煤层气探明储量以低渗储层居多,压裂仍是主要的增产技术。而常规水力压裂存在滤失严重,压裂裂缝短,支撑剂铺置效果差,压裂液难以返排等问题。为此,本研究提出一种液氮-氮气-水复合压裂提高煤层压裂效果的新思路。首先,向井筒内注入氮气将井底水驱入煤层;然后,注入液氮压裂并冻结含水煤层;随后再次注入氮气将裂缝内液氮驱入煤层深处;最后进行常规水力压裂,注入携砂液支撑裂缝。为揭示煤层液氮复合压裂机理以及探索应用液氮复合压裂提高煤层气井压裂效果的可行性,采用室内实验、理论分析和数值模拟相结合的方式,针对液氮冷冻下煤岩力学性质演化、液氮在裂缝中流动传热特征、液氮复合压裂成缝机理、液氮复合压裂技术适应性评价及液氮复合压裂携砂方式选择开展了研究,主要成果如下:1.实验测量了不同类型煤岩液氮冷冻前和液氮冷冻下的物理力学性质,揭示了液氮冷冻下饱水煤岩力学性质演化机理。结果表明:液氮实时冷冻下煤岩脆性和弹性模量显著提高。液氮冷冻能够对煤岩同时产生强化和损伤作用,饱水煤岩内部孔隙水冻结成冰后对煤岩也产生了损伤和强化双重作用。此外,岩石孔隙度越大,液氮冷冻产生的强化作用越显著。2.建立了液氮缝内流动传热模型,得到了液氮在压裂裂缝内的流动传热特征。结果表明:液氮压裂裂缝尖端处的氮处于超临界态,且该处的超临界氮所处的温度和压力与储层温度和裂缝延伸压力相近。此外,在相同液氮注入体积下,提高注入排量是提高液氮压裂冷冻区域的最优手段。3.开展了真三轴应力下的清水压裂、液氮压裂和液氮复合压裂实验,揭示了煤岩液氮复合压裂的成缝机理。结果表明:复合压裂经历了液氮压裂和清水压裂两个阶段,与清水压裂、液氮压裂相比,复合压裂可以产生更复杂、导流能力更强的裂缝网络。4.开展了不同煤阶煤岩的清水压裂、液氮压裂、液氮复合压裂实验,揭示了不同物性煤岩储层液氮复合压裂的适应性。结果表明:煤岩天然裂缝发育程度和力学性质是影响复合压裂效果的主要原因。在物性类似于沁水高阶煤岩和广胜低阶煤岩的储层中采用液氮复合压裂能大幅提高裂缝复杂度,形成复杂裂缝网络,提高单井产量。5.建立了携砂液在煤岩压裂裂缝中的运移模型,分析了复合压裂采用水力携砂的必要性。结果表明:液氮的携砂能力低于清水,提高流速、降低支撑剂浓度、减小支撑剂粒径、降低支撑剂密度可以改善液氮的携砂效果。以实际压裂施工为例,采用液氮作为携砂液时,需要的施工排量超过9 m3/min,当前现场条件无法达到液氮携砂的施工排量。因此,在液氮压裂后,采用常规水力压裂来提高裂缝宽度和支撑剂铺置效果是首选手段。研究成果揭示了煤岩液氮复合压裂机理,探索了煤层液氮复合压裂的可行性,可望为煤层气高效开发提供一种可行的技术思路。

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.