煤岩定向井水力裂缝起裂及非平面扩展实验

Experimental investigation on fracture initiation and non-planar propagation of hydraulic fractures in coal seams

基于真三轴压裂装置对天然煤岩开展室内压裂模拟实验,对比了直井和定向井两种井型压裂水力裂缝的起裂特征及扩展形态,主要研究了不同相对方位角条件下,地应力、煤岩割理对定向井压裂水力裂缝非平面扩展规律的影响。实验结果表明,水力裂缝的整体扩展方式由相对方位角、割理和地应力状态共同决定:随着相对方位角的增加,水力裂缝形态复杂程度、泵注压力以及裂缝延伸压力增加;当水力裂缝沿着与井筒斜交方向起裂时,受割理影响在扩展过程中裂缝转向并发生扭曲,引起近井筒区域裂缝形态复杂化,起裂点附近出现多裂缝。相比直井形成的"工"或"十"型裂缝形态,近井筒主裂缝扭曲扩展并伴随着多次级裂缝的复杂裂缝形态,是造成煤层定向井压裂水力裂缝延伸困难的主要因素。

True tri-axial test system was deployed for fracturing simulation of coal outcrops to investigate the initiation and propagation of hydraulic fractures in vertical and directional wells. The influences of in-situ stress and cleats on non-planar propagation of hydraulic fractures in directional wells under different relative azimuths were analyzed. The test results show that the general propagation pattern of hydraulic fractures is jointly controlled by azimuth, cleats and in-situ stress. As the relative azimuth increases, the hydraulic fractures become more complicated in geometry and subject to increasing pumping pressure and propagation pressure. If the hydraulic fractures are initiated along a direction skewed with wellbore, the effect of cleats would alter the extension path and appear distortion of hydraulic fractures, inducing more complicated fracture geometry near the wellbore, with many fractures at the initiation point. Compared with vertical wells where 1-shaped or X-shaped hydraulic fractures are formed, directional wells often have twisty propagation of dominant fractures near the wellbore and presence of multi-level fractures, which impede the further extension of hydraulic fractures in coal seams.