活性水压裂液对高煤阶煤岩力学性质的影响

Effect of Active-water Fracturing Fluid on the Mechanical Properties of High-rank Coal Rock

煤层气低产井重复压裂增产效果差异大,裸眼压裂水平井井眼容易垮塌,需要研究压裂液对煤岩力学性质及其对重复压裂及水平井稳定性的影响。通过煤岩样品单轴压缩试验和三轴压缩试验,研究了压裂液对煤岩抗压强度、弹性模量及变形特征的影响,并探讨了其对重复压裂和水平井井眼稳定性的影响。结果表明:煤岩弹性模量和抗压强度随压裂液浸泡时间增加而持续降低,塑性特征更加明显,重复压裂容易形成短宽缝,压裂效果较差,因此需要快速返排压裂液;浸泡后煤岩的弹性模量随围压的增加先增加后降低,围压6 MPa左右时,煤岩弹性模量最大,因此埋深700~800 m时,重复压裂形成长裂缝的可能性较大;浸泡后煤岩的抗压强度随围压的降低而降低,且围压越小,降低幅度越大,因此,储层埋深越浅,浸泡后储层裂缝闭合程度越强,渗透率降低程度大,越需要尽快排出压裂液。

Re-fracturing and horizontal well are important ways to improve the single well production and the recovery and to realize efficient development of CBM reservoir,while the stimulation effect of re-fracturing is significant and the borehole collapse has restricted the application of horizontal well.So it is important to research the effect of fracturing fluid on the mechanical properties of coal rock and on the re-fracturing effect and the horizontal well stability.In this paper,the effect of drilling fluids on the compressive strength and elastic modulus of coal rock was studied through uniaxial compression test and tri-axial compression test of coal rock samples,as well as the influences of which on re-fracturing effect and the borehole stability.The results showed that the immersion of coal rock in fracturing fluids resulted in the decrease of compressive strength and elastic modulus of the coal rock,and the decrease degree decreased with the increase of soaking time.So the re-fracturing of CBM wells was more easily form short and wide fractures which resulted in poor re-fracturing effect.So releasing the fracturing fluid quickly after fracturing will improve the re-fracturing effect.With the increase of confining pressure,the elastic modulus increased when confining pressure was below 6 MPa and then decreased after confining pressure was above 6 MPa.So when the buried depth is between 700 m and 800 m,the re-fracturing can form long fractures.The compressive strength of the coal rock decreased with the decrease of confining pressure,and the decrease amplitude decreased after immersion,which resulted in the increase of fracture closure and decrease of reservoir permeability.So fracturing fluid should be released quickly just after fracturing.