沁水盆地南部郑庄区块中北部煤层气直井增产新技术研究与应用

Research and application of a new stimulation technology for deep coalbed methane vertical wells in central and Northern Zhengzhuang block,southern Qinshui Basin

为提高沁水盆地郑庄区块中北部煤层气直井产量,基于研究区评价井地应力测试资料、压裂裂缝监测资料,分析了研究区直井增产效果差的原因,提出了针对性增产技术,开展了现场对比试验,并对结果进行了分析,结果表明郑庄中北部直井产量低、措施增产效果差的主因为:①研究区以垂直裂缝为主、压裂缝长较短,且随着埋深增加,相同压裂规模形成的裂缝尺寸减小;②随着埋深增加,支撑剂嵌入深度增加,裂缝闭合加快,导致稳产时间短,产气曲线主体形态为“单峰型”;③经初次压裂后煤体结构更加破碎,新裂缝容易进入初次压裂裂缝,造新缝难度增加。针对上述原因,创新提出的充填预堵+大规模压裂+远端支撑增产技术,“充填预堵”即先采用相对较低的排量、砂比、规模充填初次压裂裂缝,然后再进行大规模重复压裂,实现堵老缝、造新缝。“大规模”压裂即大排量、大液量、高砂比压裂,将压裂液量由600~800 m^(3)提高至1300~2000 m^(3)以上,增加改造体积;将排量由6~8 m^(3)/min提高至10~14 m^(3)/min以上,增加裂缝长度和携砂性能;采用低黏压裂液体系配合低密度支撑剂,将砂比由7%~8%提高至10%~15%以上,提高铺砂强度,降低裂缝闭合程度。“远端支撑”即采用自悬浮支撑剂与大排量相结合,增长支撑剂运移距离,提高支撑裂缝比例。充填预堵+大规模压裂+远端支撑增产技术实施后平均单井日产量达到1380 m^(3),比措施前增产1190 m^(3),比邻井稳产气量增加近1000 m^(3),实现了郑庄中北部中深储层连片低产区直井产量突破。现场对比试验表明:实施“充填预堵”后再进行大规模压裂,平均净施工压力比初次压裂增加了3.3 MPa,形成了新裂缝,比直接进行大规模压裂增量提高1000 m^(3)。总体上,充填预堵+大规模压裂+远端支撑增产技术关键参数数值越大,增产效果越好。

In order to improve the production of coalbed methane vertical wells in the central and northern Zhengzhuang block,southern Qinshui Basin,Shanxi,China,this paper analyzes the reasons for the poor stimulation effect of mediumdeep coalbed methane vertical wells,based on the ground stress test data and fracturing monitoring data of these evaluation wells in the study area,proposes targeted stimulation techniques,designs and carries out field comparative pilot studies and analyzes their results.The results show that there are three main reasons for the low production of these vertical wells in the study area.Firstly,the vertical fracture is dominant,resulting in the shorter length of hydraulic fractures.With the increase of burial depth,the length and width of fractures decreases at the same fracturing scale.Secondly,with the increase of burial depth,the embedment depth of proppants into coal rock increases and the closure of hydraulic fractures speeds up.As a result,the stable production time is short,and the main form of gas production curve is“single peak”type.Thirdly,after the initial fracturing,the coal body structure is much more broken,and new fractures are easy to enter the initial fractures,which increases the difficulty of creating new fractures,resulting in poor stimulation effect.In view of the above reasons,the paper proposes an innovative technology of pre-plug,large-scale and remote-support re-fracturing.Preplug means that relatively low displacement,sand ratio and scale are first used to fill initial fractures,and then large-scale re-fracturing is carried out to create new fractures.Large-scale fracturing refers to the fracturing with large displacement,large fluid volume and high sand ratio.In order to improve the stimulation effect of middle-deep reservoirs,the fracturing fluid volume is increased from 600−800 m^(3) to more than 1300−2000 m^(3),the displacement is increased from 6−8 m^(3)/min to more than 10−14 m^(3)/min,and a low viscosity fracturing fluid system combined with low density proppants is used to increase the sand ratio from 7%−8%to more than 10%−15%.Remote-support is a combination of self-suspended proppants and large displacement to increase proppants migration distance and increase the proportion of the propped fractures.The average daily production of a single well reached 1380 m^(3),which was 1190 m^(3) higher than that before,and nearly 1000 m^(3) higher than the stable gas production of adjacent wells,realizing the production breakthrough of vertical wells in the low-production area of the middle-deep reservoir in the north of Zhengzhuang block.Results of the comparative pilot studies showed that the average net construction pressure increased by 3.3 MPa after pre-plug,compared with the initial fracturing,which means that new fractures were formed and resulted in the production increase by 1000 m^(3) compared with that of the wells without the Pre-plug.The pilot results show that,in general,the larger the technical parameters of preplug,large-scale and remote-support re-fracturing,the better the production performance.