基于应力增速与应力梯度的水压致裂应力扰动评价研究

Evaluation of stress disturbance during hydraulic fracturing based on stress increasing rate and stress gradient

水压致裂的应力扰动评价是动力灾害煤岩层水压致裂的关键科学问题。提出了基于应力增速、应力梯度的水压致裂应力扰动评价方法。采用大尺寸真三轴水压致裂应力扰动物理模型实验,分析了水压致裂应力扰动的大小、特征及其影响因素。水压致裂过程中当泵注排量相同时,同一时间内距离水压裂缝越近,平均孔隙压力增速及孔隙压力梯度越大,孔隙压力增速和孔隙压力梯度按指数规律衰减。相同位置处,泵注排量越大,平均孔隙压力增速和孔隙压力梯度越大。开泵瞬间会产生很高的骨架应力增速,同时在致裂过程中,水压力的大幅度波动也会产生较大的骨架应力增速,且波动幅度越大,产生的骨架应力增速也越大。随着泵注排量的增大,开泵瞬间水压裂缝两侧骨架应力增速峰值也相应增加。泵注排量相同时,越靠近水压裂缝,骨架应力梯度越大,随着距离水压裂缝越远,应力梯度逐渐变小。初步给出了不同工程背景下水压致裂应力扰动效应的控制原则,在工程施工中,应根据具体的工程背景,基于应力扰动的评价,采取相应的针对措施,做到趋利避害。

The evaluation of stress disturbance during hydraulic fracture is a key scientific problem of hydraulic fracturing in coal and rock formations with dynamic disasters. A method for evaluating hydraulic fracturing stress disturbance based on stress growth rate and stress gradient is proposed.Large-scale physical model experiments of stress disturbance during hydraulic fracturing is performed under true triaxial stress. The size, characteristics, and influencing factors of stress disturbance during hydraulic fracturing are analyzed. When the pumping rate is the same, the closer to the hydraulic same time, the greater the average pore pressure growth rate and pore pressure gradient, and the pore pressure growth rate and pore pressure gradient decay exponentially. At the same location, the greater the pumping rate, the greater the average pore pressure growth rate and pore pressure gradient. At the moment when the pump is turned on, a high matrix stress growth rate will be generated. Meanwhile,large fluctuations in water pressure will also produce a larger matrix stress growth rate during hydraulic fracturing. The greater the fluctuation range of water pressure, the greater the matrix stress growth rate.With the increase of pumping rate, the peak value of the matrix stress growth rate on both sides of the hydraulic fracture also increases correspondingly. When the pumping rate is the same, the closer to the hydraulic fracture, the greater the matrix stress gradient. The further the distance from the hydraulic fracture,the smaller the matrix stress gradient. The control principles of stress disturbance effect during hydraulic fracturing under different engineering backgrounds are initially given. In engineering construction, corresponding measures should be taken according to the specific engineering background and the evaluation of stress disturbances, so as to seek advantages and avoid disadvantages.