Abstract:

Reservoir fracturing stimulation is the key to the successful development of tight oil and gas fields. To effectively improve the performance of fracturing stimulation, it is urgent to clarify the propagation laws of hydraulic fractures under the tectonically heterogeneous in-situ stress field of different structural types. A series of large-scale north-dipping faults and fold structures have developed in the Bozhi-Dabei area of the Kuqa Depression under the north-to-south thrust nappe action. The complex structural characteristics have caused significant disturbance to the in-situ stress field, which further restricts the propagation and extension of the fracture network formed by fracturing stimulation. In this paper, the typical structural characteristics of the study area were extracted, and geological models of fault structure, fold structure, and fault-fold composite structure were established respectively. Heterogeneous rock mechanical parameters were assigned to the models, followed by stress loading, to clarify the distribution characteristics of the in-situ stress field corresponding to different structures. On this basis, the three-dimensional hydraulic fracturing process in the heterogeneous in-situ stress field was simulated, and the differences in fracturing effects among different structural types were analyzed. The results show that the in-situ stress near the fault zone decreases significantly, and the closer to the fault zone, the lower the in-situ stress. For the fold structure, the in-situ stress is reduced in the core and crest of the anticline due to the tensile stress derived from tectonic deformation, while it is increased in the anticline bottom affected by the compressive stress derived from tectonic deformation. The in-situ stress field characteristics of the fault-fold composite structure present a superposition of those of the individual fault and fold structures. Fracture propagation is dominated by the heterogeneous in-situ stress field. Overall, the lower the in-situ stress, the easier the fracture opening. Within the same duration, the fracture propagation length is the largest in the fault-fold composite structure model and the smallest in the fold structure model. With the in-situ stress release near the fault zone, the growth rate of fracture opening area in the fault model and fault-fold composite structure model accelerates as the fracture propagates closer to the fault over time. For the fold model, the in-situ stress along the Z-axis remains unchanged; the fracture opens rapidly near the fluid injection point with sufficient energy, while the opening rate slows down as the fracture propagates outward. Within the same fracturing time, compared with the single fault and fold structures, the fault-fold composite structure features a larger fracture opening area and lower fluid pressure, thus being the most favorable for fracture propagation. It is recommended that well placement in the study area should be prioritized in the fault-fold composite structural belt. The optimal well location is on the hanging wall of the fault close to the fault plane, and the preferred drilling depth is above the neutral surface of the fold.

Key words: complex geological structures, heterogeneous in-situ stress field, propagation of hydraulic fractures, numerical simulation, Cohesive Zone method

摘要：

储层压裂改造是致密油气田成功开发的关键，为有效提升压裂改造效果，亟需明确不同构造非均质地应力场下水力压裂裂缝的扩展规律。库车凹陷博孜—大北地区在由北向南的逆冲推覆作用下发育了一系列大规模北倾断层与褶皱构造，复杂构造特征对地应力场产生了显著扰动，进而影响了压裂改造缝网的扩展与延伸。本文提取研究区典型构造特征，分别构建了断裂、褶皱以及断褶复合构造的地质模型，并进行非均质岩石力学参数赋值，后进行应力加载，明确不同构造的地应力场分布特征。在此基础上，模拟了非均质地应力场中三维水力压裂过程，分析了不同构造中压裂效果的差异。结果表明，断裂带附近地应力明显减小，越靠近断裂带，地应力越小。褶皱构造背斜核部和顶部受构造变形派生拉张应力影响，地应力减小；底部受构造变形派生的挤压应力影响，地应力增大。断褶复合构造的地应力场特征为断裂与褶皱地应力场特征的叠加。裂缝扩展受非均质地应力场控制，整体上地应力越小，裂缝越易于张开。相同时间内，裂缝扩展长度在断褶复合构造模型中最大，在褶皱模型中最小。断裂带附近地应力释放，随时间增加靠近断裂，断裂模型和断褶复合构造模型中裂缝开启面积，增长速度加快；褶皱模型在Z轴方向地应力无变化，靠近注液点能量充足时裂缝开启较快，随裂缝延伸，裂缝开启变慢。相同时间，断褶复合构造较断裂和褶皱构造，压裂裂缝开启面积更大、流体压力更低，因此最易于裂缝扩展。建议研究区优先在断褶复合构造带上布井，井位优选裂缝上盘且靠近断裂面处，钻深最好在褶皱中性面之上。

关键词: 复杂构造, 非均质地应力场, 裂缝扩展, 数值模拟, Cohesive单元法

CLC Number: 

• TE357

• P553