Characterization of irregular complex fractures in unconventional oil and gas reservoirs
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摘要: 非常规油气资源储量大，开发难度高。储层压裂改造是非常规油气资源开发的关键技术手段。天然裂缝和压裂裂缝具有不规则性和复杂性。针对现有裂缝表征方法难以准确刻画裂缝真实形状和宽度变化等不规则特性这一问题，提出了基于非结构PEBI网格的不规则复杂裂缝表征方法。首先，建立基于PEBI网格的天然裂缝表征流程，实现对任意区域或限定区域天然裂缝的准确表征；其次，建立基于Delaunay三角形网格和PEBI网格的压裂裂缝表征及优化方法，分析网格尺寸及优化次数对裂缝表征精度的影响；第三，建立基于非结构网格的非平面裂缝表征方法，实现对弯曲裂缝的刻画和表征，裂缝形态和分布与实际情况更相符；第四，提出非均匀裂缝宽度表征方法，实现对变宽度即同一条裂缝宽度和导流能力不均匀分布裂缝的精细表征；第五，在全区域和限定区域内，实现耦合不规则压裂裂缝和不规则天然裂缝的复杂缝网表征。针对大规模天然裂缝与压裂裂缝相交、裂缝宽度非均匀分布、非平面裂缝等复杂条件下的裂缝网络表征，通过调整网格优化次数，能够提高缝网表征质量。利用PEBI网格能够灵活准确逼近裂缝复杂边界条件的优势，实现快速、准确地显示处理大量不规则天然裂缝和压裂裂缝。形成的不规则复杂裂缝表征方法，有助于提高非常规油气藏裂缝网络的表征精度和数值模拟计算准确性。Abstract: Unconventional oil and gas resources have large reserves and are difficult to develop. Reservoir fracturing is a key technical means for the development of unconventional oil and gas resources. Natural fractures and induced fractures are irregular and complex. To address the issue that existing fracture characterization methods cannot accurately depict the true shapes and width variations of fractures, a method based on unstructured PEBI grids for characterizing irregular and complex fractures was proposed. First, a natural fracture characterization process based on PEBI grids was established, allowing for accurate characterization of natural fractures in any region or a specified area. Second, a characterization and optimization method for induced fractures based on Delaunay triangulation and PEBI grids was developed, analyzing the impact of grid size and optimization iterations on fracture characterization accuracy. Third, a method for characterizing non-planar fractures using unstructured grids was established, enabling the depiction of curved fractures, making the fracture morphology and distribution more consistent with actual conditions. Fourth, a method for characterizing non-uniform fracture width was proposed, achieving fine characterization of fractures with non-uniform distribution of width and conductivity along the same fracture. Fifth, a complex fracture network characterization method coupling irregular induced fractures and irregular natural fractures in the whole area and specified regions was realized. For fracture network characterization under complex conditions such as large-scale intersections of natural and induced fractures, non-uniform fracture width distribution, and non-planar frac- tures, adjusting the number of grid optimization iterations could improve the quality of the fracture network characterization. Utilizing the advantage of PEBI grids to flexibly and accurately approximate complex fracture boundary conditions, this method enabled the rapid and accurate handling of a large number of irregular natural and induced fractures. The developed method for characterizing irregular and complex fractures helps improve the accuracy of fracture network characterization and numerical simulation calculations in unconventional oil and gas reservoirs.
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图 1 基于PEBI网格的天然裂缝表征流程
Figure 1. Characterization workflow of natural fractures based on PEBI grids
图 2 天然裂缝随机分布表征结果
Figure 2. Characterization of randomly distributed natural fractures in arbitrary and designated areas
图 3 优化次数对压裂水平井Delaunay三角形网格(左)和PEBI网格(右)生产质量的影响
Figure 3. Effect of iteration steps on Delaunay triangular grid (left) and PEBI grid (right)
图 4 网格尺寸对压裂水平井Delaunay三角形网格(左)和PEBI网格(右)的影响
Figure 4. Effect of initial grid size on Delaunay triangular grid (left) and PEBI grid (right)
图 5 不同网格尺寸下的裂缝附近PEBI网格放大图
Figure 5. Enlarged views of PEBI grids around fractures under different initial grid sizes
图 6 多条非平面裂缝的非结构网格剖分及局部放大图
Figure 6. Characterization of non-planar fractures based on Delaunay triangular grid and PEBI grid and enlarged views of PEBI grids in x-y plane
图 8 裂缝非均匀宽度值和裂缝宽度均匀分布、非均匀分布下的非结构网格划分
Figure 8. Value of non-uniform fracture aperture and characterization of fractures with uniform and non-uniform fracture apertures using unstructured grids
图 9 裂缝宽度非均匀变化的非结构网格划分及PEBI网格局部放大图
Figure 9. Characterization of fractures with non-uniform fracture aperture based on unstructured grid and zoom-in view in x-y plane
图 10 非平面变宽度裂缝表征(PEBI网格局部放大图)
Figure 10. Characterization of non-planar fractures with non-uniform fracture aperture based on PEBI grids
图 11 基于PEBI网格的裂缝宽度均匀变化和等差递减分布的裂缝表征
Figure 11. Characterization of fractures with uniform fracture aperture and arithmetic decreasing fracture aperture based on PEBI grids
图 12 压裂水平井(平面裂缝)和随机分布的天然裂缝网络表征结果
Figure 12. Characterization of fracture networks with MFHW (planar fractures) and randomly distributed natural fractures based on PEBI grids
图 13 压裂水平井非平面压裂裂缝和限定区域内随机分布的天然裂缝表征
Figure 13. Characterization of fracture networks with MFHW (non-planar fractures) and randomly distributed natural fractures in designated areas
表 1 储层、井筒、裂缝(均匀宽度和非均匀宽度)的基础参数
Table 1. Basic parameters of reservoirs, wellbores, and fractures with uniform and non-uniform apertures
参数类型 数值 参数类型 数值 储层长度/m 2 500 裂缝数量/条 6 储层宽度/m 1 000 井筒半径/m 0.10 储层高度/m 50 水平井长度/m 1 550 裂缝均匀宽度/m 0.03 压裂裂缝非均匀宽度参数 μ=-1.0 σ=0.531 7 
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表 2 储层、井筒、裂缝(非均匀宽度)的基础参数
Table 2. Basic parameters of reservoirs, wellbores, and fractures with non-uniform apertures
参数类型 数值 参数类型 数值 储层长度/m 1 000 裂缝数量/条 3 储层宽度/m 1 000 裂缝半长/m 200 储层高度/m 50 压裂裂缝非均匀宽度参数 μ=-0.5
σ=0.4井筒半径/m 0.10 水平井长度/m 600
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