煤岩水射流径向钻孔控制压裂机理研究

Severe problems exist in present coalbed methane fracturing stimulation in China, including inadequate vuggy connectivity in zone far from wells and severe sand plug in the near well-bore area, as well as limited stimulated volume. As a result, the bottleneck of low production rate and high cost per well has long been unresolved. Integrated hydra-jet radial drilling and fracturing technology is proposed in the present research. Several Radial wellbores of certain length and direction are initially drilled by high pressure jet in coal bed, followed by hydra-jet fracturing in the well bore resulting in complex three dimensional fracture networks. Focusing on the key scientific problem of generation mechanism and depiction of fracture networks, the research attempts to uncover characteristic of induced stress distribution during hydra-jet drilling and fracturing. Also, experiments will be conducted to simulate crack patterns in different conditions of coal rock property, in-situ stress, and redial well bore. Based on the fracture initiation mode with hydra jet drilling and fracturing obtained, mechanism about fracture initiation and competing propagation under competing controlling factors of natural weak planes and crustal stresses will be illustrated by experiment. Numerical simulation will be conducted to explore depiction approach of multiple fracturing cracks and obtain controlling factor in the generation of three dimensional fracture networks under competing parameters. Perspective research results can offer theoretical basis for developing new stimulation methods in coalbed methane development, achieving breakthrough in economical coalbed methane exploration.

水力压裂是目前我国煤层气开采的首选措施，但常规压裂受地应力和岩性控制易形成短宽缝，导致远井地层沟通不足、近井地层砂堵严重等问题，改造体积有限。本项目提出水射流径向钻孔控制压裂的新思路，即：在煤层水射流钻成不同空间方位、数量、长度的径向孔，利用孔间应力干扰一体化压裂控制形成三维复杂裂缝网络。围绕径向孔间应力相互作用对裂缝扩展的影响这一关键科学问题，计划开展如下研究：径向孔眼分布对初始应力场的影响规律研究, 揭示孔间应力干扰作用与应力场重构特征；室内实验探究不同径向孔、岩性、地应力及压裂液等参数的煤岩水射流径向钻孔控制压裂人工裂缝形态，搞清不同起裂模式下的裂缝扩展行为，揭示基于孔间干扰应力场与天然弱面竞争的裂缝起裂和多裂缝扩展机理；开展裂缝扩展数值方法研究，确定水射流径向钻孔控制压裂形成裂缝网络的主控因素。研究成果可为形成一种煤层气增产新方法提供理论依据，同时为页岩气等非常规开采提供方法储备。

煤层气的高效开发利用既可增加清洁能源供应，又可有效预防煤矿瓦斯事故。目前，水力压裂是我国煤层气开采的首选措施，但往往压裂后远井地层沟通不足、近井地层砂堵严重，整体改造体积有限。为此研究团队提出水射流径向钻孔控制压裂方法用于煤层气增产改造新思路。围绕煤岩水射流径向钻孔控制压裂孔间应力干扰、地应力场重构与煤岩水射流径向钻孔控制压裂裂缝起裂、扩展机理等关键科学问题开展系统研究，形成以下成果：1）建立了煤岩水射流径向钻孔控制压裂孔间应力干扰的数学模型与有限元模型，揭示了径向孔间应力干扰作用规律与应力场重构特征，发现井眼方位是影响径向孔间应力场的关键因素。2）考虑煤岩的非均质性，建立了煤岩径向水平井压裂裂缝起裂数值模型，揭示了地应力与天然弱面竞争控制径向井压裂煤岩的裂缝起裂机理，发现垂向主应力值越大越有利于形成裂缝网络。3）开展了径向井压裂室内实验，揭示了径向井压裂煤岩的多裂缝竞争扩展机理。4）构建了径向井压裂煤岩的裂缝扩展离散元模型，确定了地应力状态、径向井长度、压裂液粘度及储层杨氏模量等为影响煤岩水射流径向钻孔控制压裂裂缝扩展的主控因素。项目研究证实了水射流径向钻孔控制压裂新方法用于煤层气增产改造的可行性。研究成果在《Fuel》、《Journal of Petroleum Science and Engineering》、《International Journal of Heat and Mass Transfer》、《天然气工业》等行业主流期刊发表论文26篇（SCI收录18篇，TOP期刊7篇），授权国家发明专利5件，获省部级及行业协会科技成果一等奖3项。