沁水盆地郑庄区块北部煤层气直井低产原因及高效开发技术

Causes for low CBM production of vertical wells and efficient development technology in northern Zhengzhuang Block in Qinshui Basin

  • 摘要: 为了实现沁水盆地郑庄区块北部深部煤层气高效开发,通过深入分析煤储层地质条件和开发资料,并对比浅部煤层气开发数据,以含气量评价资源为基础,综合储层渗透性、地解压差,煤体结构和地应力状态等评价煤层气采出难易程度,明确郑庄区块北部煤层气井低产原因,并提出高效开发的适应技术。结果表明,郑庄北部煤层含气量整体较高,在20 m3/t以上;郑庄北部绝大部分地区裂隙发育指数均小于140,渗透率极低;绝大部分地区地解压差大于6 MPa,导致煤层气井有效解吸范围小,宏观解吸效率低;绝大部分地区煤层中碎煤比例在0.7以上,导致水力压裂裂缝较短;垂向应力大于水平应力,为大地静力场形地应力,水力压裂易形成垂直裂缝,裂缝延伸较短。极低的渗透率、相对较高的地解压差、高碎煤占比和大地静力场形地应力状态耦合导致郑庄北部煤层气开发效果较差。仿树形水平井采用人工井眼实现煤层密切割,缩短了煤层气、水渗流距离,利于实现协同降压增产,有力克服了高地解压差的不利影响;同时采用人工井眼代替压裂裂缝,解决研究区水力压裂造缝短的难题,取得了产量突破,但存在不利于排水降压和井眼易垮塌的风险,且其成本高、收效低。L形水平井密切割分段压裂技术克服了仿树形水平井不利因素。与直井相比,L形水平井成本增加2倍,但煤层气产量增加近100倍;且L形水平井与仿树形水平井产量相当,但其成本仅是后者的 40%。由此可知,L形水平井可以实现郑庄北部深部煤层气的高效开发。

     

    Abstract: In order to realize the efficient development of deep CBM in the north of Zhengzhuang block in Qinshui Basin, firstly, the geological conditions and development data of coal reservoir are deeply analyzed, and the development data of shallow CBM are compared. Secondly, based on the evaluation of gas content, the difficulty of coalbed methane recovery is evaluated by comprehensive evaluation of reservoir permeability, reservoir-desorption pressure difference, coal structure and in-situ stress state-Finally, the reasons for low production of CBM wells in the northern Zhengzhuang Block are clarified, and the adaptive technology for efficient development is put forward. The results show that the gas content in northern Zhengzhuang Block is generally higher than 20 m3/t. While the permeability is extremely low, with the fracture development index below 140. And the difference between reservoir pressure and desorption pressure(reservoir-desorption pressure difference) of CBM formation is greater than 6 MPa, resulting in small desorption range of CBM wells. In most areas, the ratio of the thickness of broken coal to that of whole coal seam is above 0.7, leading to shorter hydraulic fracturing fractures. The vertical stress, a geostatic-field-type stress, is greater than horizontal stress and the hydraulic fracturing is prone to form vertical fractures with shorter fracture extension. So the coupling of the extremely low permeability, relatively high reservoir-desorption pressure difference, high broken coal ratio and geostatic-field-type stress, all these factors result in poor production in northern Zhengzhuang Block. Artificial borehole is established by tree type horizontal well to realize high efficient stimulation of coal seam, and the coordinated depressurization is realized and the negative effects are overcome by shortening the seepage distance of CBM and water. At the same time, the artificial borehole is used instead of the relative shorter hydraulic fractures. The production of the tree type horizontal well is over 10 000 m3 per day. But the disadvantages of tree type horizontal well are risks such as unfavorable drainage and depressurization, easy hole collapse and high cost. Multi-stage hydraulic fracturing of L shaped horizontal well can overcome the disadvantages perfectly of tree type horizontal well. Compared with vertical well, the cost of L shaped horizontal well increases by 2 times while the production increases nearly by 100 times. The production of L-shaped horizontal well is as high as that of tree type horizontal well, while the cost of the L shaped horizontal well is only 40% of tree type horizontal well. In conclution, the L shaped horizontal well realize the efficient development of Northern Zhengzhuang Block.

     

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