页岩气水平井缝内砂塞分段工艺的增产机理

川渝地区页岩气储层在压裂过程中频繁出现套管变形、机械分段工具无法下入等井下复杂情况,以及因尾追砂量受限导致近井地带导流能力低,制约了页岩气水平井的返排效果及投产产量。为此,针对以桥塞为主的机械分段工艺在川渝页岩气现场应用中的制约状况,提出了应用水平井缝内砂塞分段工艺来解决以上难题的方法。该工艺核心技术在于缝内砂塞的成功封堵转层并在返排生产过程中保持裂缝长期的高导流能力,而裂缝长期的高导流能力是决定该工艺增产效果的关键。将Hertz接触及分形理论引入到砂塞强度的分析中,结合室内工程模拟实验结果,建立了支撑剂缝内砂塞接触力学模型,从强度准则及摩擦等方面提出了缝内砂塞稳定性判据,完善了缝内砂塞渗透率分形模型。实验结果表明:(1)在返排初期砂塞的稳定性主要受到流体冲刷作用的影响,应严格控制排液速率;(2)在生产后期砂塞的稳定性主要受到裂缝闭合应力及流压的影响,适当提高支撑剂颗粒的屈服强度对保持裂缝高导流能力具有重要意义。结论认为,缝内砂塞分段工艺可以为页岩气水平井分段多簇体积压裂提供一种新的储层改造手段。

Mechanism of multi-stage sand filling stimulation in horizontal shale gas well development

Fracturing operations in shale gas reservoirs of the Sichuan–Chongqing area are frequented by casing deformation, failures in delivery of mechanical staging tools and other down-hole complexities. In addition, limitation in volumes of tail-in proppant in the matrix area significantly restricts the conductivity in the near zones of the wellbore. Eventually, flowback performance and productivity of shale gas horizontal wells are negatively affected. With consideration to the limitations in the implementation of the mechanical staging technique with bridge plug for shale gas development in the Sichuan–Chongqing area, the technique of multi-stage sand filling stimulation in horizontal wells was proposed to solve the abovementioned problems. By filling sands in fractures, it is possible to divert fluids to maintain long-term high conductivity of fractures, which is the key to satisfactory EOR performances. By introducing the Hertz contact and fractal theory in the analysis of sand plug strength, and in combination of lab engineering simulation test results, the mechanical model for sand plugs in fractures with proppant was constructed. In terms of strength criteria and friction, the stability criteria of sandplug were put forward. Thus, the permeability fractural model for sand plugs in fractures was perfected. Test results show that the stability of sand slug in the earlier stage of production is mainly affected by fluid washing during flowback, so it is necessary to control the flowback rate strictly. In the later stage of production, the stability is mainly affected by fracture closure stress and flow pressure, so it is necessary to enhance the yield strength of proppant to maintain high conductivity of fractures. In conclusion, the multi-stage sand filling stimulation provides a new technique for multi-stage clustering fracturing operations in shale gas horizontal well development.