煤层不同类型裂缝应力敏感性实验--以阜新盆地煤样为例

Stress sensitivity of coal seam with different fractures:Case study of Fuxin Basin

煤层气井排水降压阶段普遍诱发裂缝应力敏感性,导致井周裂缝快速闭合、渗透性大幅下降,严重影响煤层气开采。为探索能够降低煤层裂缝应力敏感性、提高有效渗透率的增产改造措施,采集阜新盆地含方解石脉煤样,以原煤样、巴西劈裂煤样以及酸化处理煤样为实验对象,研究了天然裂缝、人工裂缝、酸化裂缝的渗透率及其应力敏感性。结果表明:煤样中方解石脉酸溶后,酸化裂缝与人工裂缝处于同一渗透性级别;有效应力为3 MPa时,3类裂缝平均渗透率分别为2.94×10^(-3)μm^(2)、75.08×^(-3)μm^(2)、71.12×^(-3)μm^(2),而有效应力增加至15 MPa时,其渗透率分别下降至0.71×^(-3)μm^(2)、2.03×^(-3)μm^(2)、26.55×^(-3)μm^(2),应力敏感系数分别为0.14 MPa^(-1)、0.31 MPa^(-1)、0.084 MPa^(-1);应力敏感导致3类裂缝渗透性均产生不可逆损害,其中人工裂缝损害程度达80%,而天然裂缝和酸化裂缝分别为46.7%和27.7%。与原煤样和巴西劈裂煤样相比,酸化处理煤样应力敏感性最弱,应力加载过程能够保持更高渗透性,主要原因是酸化裂缝面粗糙度更大、裂缝产状更加复杂,相同应力条件下,酸化裂缝的应力敏感系数分别为天然裂缝和人工裂缝的60%和27%。研究认为:酸化处理能够有效弱化煤层裂缝应力敏感性,对于含方解石脉煤层,可以考虑酸化增产改造,或采用“水力压裂+酸化”的复合改造措施,使煤层在降压生产过程中长期保持较高渗流能力。

The decompression stage of drainage and depressurization is accompanied by stress sensitivity.This leaded to rapid closure of the cracks around the well and a significant drop in permeability,which seriously affects the production of coalbed methane.In order to explore improvement measures that can reduce the stress sensitivity of coal seams and increase the effective permeability,coal samples containing calcite veins collected from Fuxin Basin were employed in this study.The permeability stress sensitivity of natural cracks,artificial cracks,and acidified cracks were tested experimentally by using raw coal samples,Brazilian split coal samples,and acidified coal samples.Results show that the average permeability of the three types of fractures are 2.94×^(-3)μm^(2),75.08×^(-3)μm^(2),and 71.12×^(-3)μm^(2) at the effective stress of 3 MPa,respectively.Then the permeability drops to 0.71×^(-3)μm^(2),2.03×^(-3)μm^(2),and 26.55×^(-3)μm^(2) with the effective stress improving to 15 MPa,and their stress sensitivity coefficients are 0.14 MPa^(-1,)0.32 MPa^(-1),and 0.083 MPa^(-1),respectively.The damage degree of artificial cracks reached 80%,while that of natural cracks and acidified cracks were 46.7%and 27.7%,respectively.It shows a significant irreversible damage of permeability for the three types of cracks.Compared to the two other coal samples,the coal sample with acid treatment has the weakest stress sensitivity and can maintain higher permeability during the stress loading process for its great crack surface roughness and complex crack occurrence,and the stress sensitivity coefficient of acidized fracture is only 60%and 27%,respectively compared with natural fracture and artificial fracture with the same stress conditions.It is concluded that acid treatment can effectively weaken the stress sensitivity of coal seams.For coal seams containing calcite veins,acidification stimulation reform can be considered,or the combined reform measures of“hydraulic fracturing by acidification”can be adopted.In this way,high seepage capacity can be maintained for a long time during the coal seam depressurization production process.