深层天然裂缝性页岩储层水力压裂光纤监测远场应变分析

Far-field strain analysis for fiber optic monitoring of hydraulic fracturing in a deep naturally fractured shale reservoir

四川盆地南部地区(下文简称川南地区)深层(埋深大于4000 m)页岩储层天然裂缝发育,地应力分布复杂,水力压裂过程中套管变形等问题严重制约了页岩气资源的高效开发。为了解决深层页岩储层天然裂缝压裂过程中远场应变监测问题,采用邻井光纤(DAS)应变监测技术对泸州区块A平台2口井拉链式压裂过程进行监测,并结合压裂施工设计和储层特征对监测结果进行分析和讨论。研究结果表明:①300~350 m监测井距和泵送式套内光纤布设方式可以对远场裂缝窜通演化评估提供大量有利数据,但对压裂井主体改造区域提供的相关信息较少;②同一平台不同井压裂时远场应变响应差异可能很大,高风险井的压裂施工参数设计对远场裂缝窜通起到了关键影响作用,低风险井主要表现为跟随作用,裂缝窜通区域一旦形成,极易演变成2口压裂井共同的裂缝窜通区;③远场应变响应区域与当前压裂段相对位置可能相差200~300 m,同井不同段间的应变沟通会提前形成,与当前压裂段可能相差400~500 m;④深层页岩储层天然裂缝压裂形成的远场裂缝形态具有全新特征,包括普遍性的倾斜裂缝和挠曲段地层附近的水平层理缝2类情况。结论认为,低频DAS应变监测技术提供了深层页岩储层天然裂缝压裂过程中远场应变演化分析的新方法,可与地质工程一体化压裂设计方案、套变机理与防控研究等工作结合,助力该区深层页岩气的规模效益开发。

Deep shale reservoirs(burial depth greater than 4000 m)in the southern Sichuan Basin(hereafter referred to as southern Sichuan)are characterized by developed natural fractures and complex in-situ stress distribution.Casing deformation and other problems in the process of hydraulic fracturing restrict the efficient development of shale gas resources severely.To monitor the far-field strain during the fracturing of natural fractures in deep shale reservoirs,this paper applies the offset-well DAS strain monitoring technology to monitor the zipper fracturing process of two wells on the Platform A in the Luzhou Block.And then,the monitoring results are analyzed and discussed on the basis of the fracturing design and reservoir characteristics.And the following results are obtained.First,the 300–350 m monitoring well spacing and the pumping-type inside-casing optical fiber layout can provide a lot of favorable data for the assessment of far-field fracture communication and evolution,but less information for the main stimulated regions of a fractured well.Second,during the fracturing of different wells on the same platform,the response of far-field strain may be quite large.The fracturing parameter design of high-risk wells usually play a key role in the communication of far-field fractures,while low-risk wells mainly show a following role.Once a fracture communication region is formed,it is very likely to evolve into the common fracture communication region of two fractured wells.Third,the response region of far-field strain and the current fracturing stage may differ in location by 200–300 m.The strain communication region between different stages of the same well may be formed in advance,and it may be 400–500 m from the current fracturing stage.Fourth,the far-field fractures formed by fracturing the natural fractures in deep shale reservoirs present completely new morphological characteristics,including universal inclined fractures and horizontal bedding-parallel fractures near the flexural section.In conclusion,the DAS strain monitoring technology provides a new method for analyzing the evolution of far-field strain during the fracturing of natural fractures in deep shale reservoirs.And combined with geology-engineering integrated fracturing design scheme and casing deformation mechanism and prevention research,it is expected to assist the large-scale benefit development of deep shale gas in the southern Sichuan.