褐煤煤层原位注过热水蒸汽控制压裂机理研究

In china, it is rich in lignite coal resource. However, the factors such as high moisture content, low calorific value, short spontaneous combustion period greatly restrict the extensive development and utilization of lignite. The technology of in-situ vapor-injected dewatering and quality improvement of coal will be a new development trend in the extensive development and utilization of lignite for its outstanding advantages such as simple process, large processing capability, low investment. The key step to the technology is to construct horizontal fractures in lignite coal seam via hydraulic fracturing. In view of the fracturing effect of superheated vapor pressure and pyrolystic effect of vapor temperature on coal, hydraulic fracturing via injecting superheated vapor of 600 degrees Celsius into lignite coal seam is an effective method. In the program, the research methods such as field research, testing, theoretical analysis and numerical simulation will be taken to carry on the following researches. The evolution of physico-mechanical properties of lignite will be measured in coupled thermo-pyrolystico-mechanical processes; Fracture opening and fluid flow for only one unconnected fracture, mechanism of hydro-fractures initiation and propagation, and control conditions of forming horizontal hydro-fractures will be studied in coupled pyrolystico-mechanical processes in the program. The mathematical model on hydraulic fracturing via injecting superheated vapor into in-situ lignite coal seam will be developed in coupled ground stress (mechanical), vapor pressure (hydraulic), thermal stress (thermal) and pyrolysis (chemical) processes abbreviated THMC. According to the result of numerical simulation, pyrolystic deformation of coal seam will be gotten. The law of fluid flow, heat and mass transfer in the hydro-fractures will be revealed. In the light of the research results, the technical scheme will be proposed for hydraulic fracturing via injecting superheated vapor into in-situ lignite coal seams, and it will supply technical guide for the key step to the technology of in-situ vapor-injected dewatering and quality improvement of lignite coal.

我国褐煤资源极为丰富，但含水率高、发热量低、发火期短等问题制约了褐煤的大规模开发与利用。原位注热脱水与提质改性技术工序简单、能力大、投资少，是褐煤大规模开发与利用技术发展的新方向。该技术的关键步骤是水力压裂建造水平裂缝。考虑过热水蒸汽对煤具有压裂和热解双重作用，采用过热水蒸汽（600℃）实施压裂以形成水平裂缝是有效方法。项目采用现场调研、试验研究、理论分析与数值模拟相结合的研究方法，研究三轴应力-热应力-热解作用下褐煤物性参数演化规律；研究三轴应力-热解作用下人工预制单一未贯通裂缝的张开度与渗流特性演化规律、褐煤煤层压裂裂缝起裂扩展机制及形成水平裂缝的控制条件；建立地应力-蒸汽压力-热应力-热解（THMC）耦合作用下褐煤煤层原位注过热水蒸汽控制压裂数学模型，通过数值模拟，分析压裂过程中煤层热解变形、裂缝渗流与传热传质规律，为褐煤原位注热脱水与提质改性工程的关键步骤实施提供技术支持。

我国褐煤等低变质程度煤储量极为丰富，如果能通过地下处理将煤中的有机成分提取出来，不仅节约开采成本，而且一定程度上缓解油气紧张问题。但褐煤含水率高、发热量低、发火期短等问题制约了褐煤的大规模开发与利用。原位注热脱水与提质改性技术工序简单、能力大、投资少，是褐煤大规模开发与利用技术发展的新方向。该技术的关键步骤是水力压裂建造水平裂缝。考虑过热水蒸汽对煤具有压裂和热解双重作用，项目采用过热水蒸汽（T<600℃）对褐煤煤层实施压裂以形成水平裂缝。通过本项目得出如下结论：.（1）褐煤地层受长期地质作用及地层自身地质结构因素影响，局部地区地应力场以静水应力场为主。.（2）褐煤热解引起内部孔隙结构发生显著变化，低温阶段(T＜300℃)，由于煤中水分和自由气体的散失而产生大量裂纹；在高温阶段(T＞300℃)，有机质的热解导致煤中大量孔隙裂隙的形成，称这种因热解作用导致煤等一类富含有机质的岩石发生破坏的现象为热解破裂。与无机岩石的热破裂过程相比，煤的热解破裂在破裂机理、裂纹起始位置、裂纹形态方面具有显著的独特性。.（3）三轴应力-热应力-热解耦合作用下褐煤弹性模量和渗透率随温度非单调变化，均表现为先增大后降低。弹性模量在200℃达到最大值，渗透率在100-150℃达到最大值。.（4）三轴应力-热应力-热解耦合作用下褐煤单一裂缝渗透率随温度变化和完整试样相同，先增大后降低，100℃-150℃增至最大值。.（5）THMC耦合作用下褐煤煤层原位注过热水蒸汽控制压裂过程中，压裂连通时，距离注热井20m范围内蒸汽温度高于300℃；在地层的垂直剖面上，温度场分布形态为一椭圆形。压裂－热解过程中，裂缝内高温流体存在明显的温度梯度，裂缝前端主要是由压裂引起的张开度，高于300℃的裂缝后端是压裂与煤热解的复合作用引起的张开度；在注热井和生产井未连通前，压裂－热解通道上的裂缝张开度比较小，随着两井贯通，压裂通道上的裂缝张开度迅速增加，此时已形成对流加热，高温蒸汽能够迅速地加热煤层。