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海拉尔盆地低煤阶煤储层压裂地质特征与改造效果评价

Fracturing geological characteristics and evaluation of fracturing effects in low-rank coal reservoirs in Hailar Basin

  • 摘要: 海拉尔盆地低煤阶煤层气资源量初步估算在1.59×1012 m3以上,资源勘探潜力巨大。近年来,区内陆续开展了煤层气资源评价、勘探试气等工作,总体上效果不理想,制约下一步勘探开发。结合本区2口煤层气试验井生产资料,对常规压裂和缝网压裂改造工艺下储层改造效果进行评价,提出本区低煤阶煤储层压裂技术建议,结果表明:煤岩学方面,本区煤中镜质组质量分数在53.60%~75.00%,宏观煤岩组分以亮煤、镜煤为主导,煤脆性强,适合开展储层体积压裂;本区煤层天然裂缝发育,主要发育内生裂隙及气胀节理群,裂隙线密度在0.50~0.88条/cm,裂缝开度大,内部无矿物充填,具有优异的导流能力,且本区煤岩层理缝极度发育,对于开展体积缝网压裂十分有利;本区低煤阶煤力学强硬,纳米压痕试验结果显示煤岩硬度为0.09 GPa,断裂韧度为0.11 MPa·m0.5,整体高于国内多数煤层气开发区块煤岩,煤体结构完整,煤粉源集合体少见,利于实施储层高导流立体压裂;本区地应力状态为垂向挤压−水平拉张型,煤储层裂缝面闭合应力适中,利于裂缝保持持久导流能力,压裂过程中施工能耗低,压缩压裂工程成本;压裂物模试验表明,层面方位对压裂裂缝扩展特征具有重要约束,井筒垂直层理方向上压裂流体更能激活大量天然裂隙,裂缝成网效率更高;现场压裂施工曲线表明,相对常规压裂,缝网压裂曲线破裂事件更多,表明煤储层次级破裂频繁,天然裂缝激活效率高,压裂裂缝形式更为复杂,后期气井产能数据也证实了这一点(常规压裂直井峰值日产气量仅为257.00 m3/d,而缝网压裂直井峰值日产气量达519.00 m3/d,是前者的2倍),表明缝网压裂在本区具有技术适用性;缝网压裂能够均衡改造储层,缩短煤层甲烷气渗流路径,降低煤层气启动压差,提高压裂液渗吸驱替效率。通过提升低煤阶煤层气储层整体疏导能力,取得了较好的试气成果。后期应坚定缝网压裂改造技术,采取控近扩远造缝策略及组合加砂全域支撑模式,打造海拉尔盆地低煤阶煤层气高效压裂改造技术模式,突破低煤阶煤层气勘探开发技术瓶颈。

     

    Abstract: The preliminary estimate of the low-rank coalbed methane resources in the Hailar Basin is over 1.59×1012 m3, indicating huge potential for resource exploration. In recent years, the area has successively carried out the evaluation of coalbed methane, exploration and gas testing, etc. However, the effect has not been satisfactory. Based on the production data of two coalbed methane wells in this area, the reservoir modification effects under the conventional fracturing and volumetric fracturing were evaluated. Meanwhile, technical suggestions for fracturing of low-rank coal reservoirs in this area were put forward. The research shows that: the content of vitreous groups in the coal ranges from 53.6% to 75.0%. The macroscopic components of the coal are dominated by bright coal and vitreous coal. The coal are highly brittle and suitable for volumetric fracturing of reservoirs. Natural fractures in the coal are developed, mainly endogenic fractures and gas expansion joint groups. The fracture linear density is 0.50 to 0.88 per cm. The fracture opening is large, and there are no filling minerals inside. It has excellent flow conductivity. Moreover, the fractures in the coal are developed, which has a guiding effect on volume fracturing and is conducive to the fracturing transformation. The results of the nanoindentation test show that the hardness of the coal is 0.09 GPa, and the fracture toughness is 0.11 MPa·m0.5. The coal structure is complete, and coal fines are rare, which is conducive to the implementation of high-flow fracturing in reservoirs. The in-situ stress in this area is of the vertical extrusion - horizontal tension type. The closed stress at the fracture of the coal reservoir is moderate, which is conducive to maintaining the persistent conductivity of the fractures, reducing the construction energy consumption during the fracturing process, and lowering the cost of the fracturing project. Fracturing tests show that the orientation of the plane has an important constraint on the propagation of fractures. The fracturing fluid in the vertical bedding direction of the wellbore can activate a large number of natural fractures more effectively, and the fracture network formation efficiency is higher. The fracturing curve indicates that compared with conventional fracturing, there are more fracture events under volumetric fracturing, suggesting frequent hierarchical fractures in coal, more activation of natural fractures, and more complex fracture forms. The gas well production capacity data also confirm (the peak daily gas production of conventional fracturing vertical wells is only 257.00 m3/d, The peak gas production of the vertical well under volumetric fracturing can reach 519.00 m3/d, which is twice that of the former. In the later stage, volumetric fracturing can evenly transform the reservoir, shorten the seepage path of gas, and improve the permeation efficiency of fracturing fluid. The strategy of controlling near-range and covering far-range fracture creation and the full-domain proppant mode should be adopted to create a technical model for the efficient utilization of low-rank coalbed methane in the Hailar Basin.

     

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