基于裂缝前缘邻井光纤应变的压裂裂缝参数解释方法

Interpretation method of fracturing fracture parameters based on optical fiber strain at the front margin of fracture in adjacent wells

利用裂缝前缘光纤应变进行裂缝参数的准确高效解释,是目前邻井光纤应变技术实现大规模推广应用的关键。基于三维位移不连续边界元建立光纤应变计算方法,并构建了基于裂缝前缘光纤应变的裂缝参数反演模型;基于光纤应变模型,计算分析了裂缝前缘光纤应变分布随裂缝参数的变化特征,建立了峰值应变及零应变位置与裂缝参数的关系式;以裂缝前缘光纤应变的正演结果为人工合成监测数据,论证了裂缝前缘光纤应变反演裂缝参数的可行性;利用北美地区水力压裂试验场光纤应变监测结果,进行了基于裂缝前缘光纤应变的实际井裂缝参数解释。研究结果表明:①裂缝前缘的光纤应变与裂缝长度、高度和宽度分布均相关,对缝长最为敏感;②零应变位置对缝高较为敏感,而峰值应变位置对缝高不敏感,通过峰值应变位置可对缝长进行快速解释;③内点法可有效求解该反演问题,计算效率高,适合工程应用;④基于裂缝前缘的邻井光纤应变可有效解释缝长变化,而缝高、缝宽及缝宽分布具有较强多解性。

Accurate and efficient interpretation of fracture parameters using optical fiber strain measurements at the fracture front is crucial for the broad application of optical fiber strain technology in adjacent wells. For this purpose, a method for calculating optical fiber strain was established based on three-dimensional displacement discontinuous boundary elements, and a fracture parameter inversion model based on optical fiber strain at the front margin of fracture was built. Further, based on the optical fiber strain model, calculation and analysis were performed for the variation characteristics of the distribution of optical fiber strain at the front margin of fracture with fracture parameters, and the relational expressions between the peak strain and zero strain positions and the fracture parameters were established. Next, the feasibility of inverting fracture parameters by optical fiber strain at the front margin of fracture was demonstrated using the forward modeling results of optical fiber strain as the artificially synthesized monitoring data, Finally, using the optical fiber strain monitoring results from the North American hydraulic fracturing test site, the actual well fracture parameters were interpreted based on optical fiber strain at the front margin of fracture. The research shows as follows:(1)The optical fiber strain at the front margin of fracture is related to the distribution of fracture length, height, and width, and is most sensitive to fracture length.(2)The zero strain position is sensitive to fracture height, while the peak strain position is not sensitive to fracture height. The fracture length can be interpreted quickly by the peak strain position.(3)The interior point method can effectively solve the inversion problem, characterized with high computational efficiency, and is suitable for engineering application.(4)The optical fiber strain in adjacent wells based on the front margin of fracture can effectively interpret the variation of fracture length, while the distribution of fracture height, fracture width, and fracture width has high multiplicity of solutions.