直井射孔相位对水力压裂中破裂压力的影响规律研究

Study on the Influence of Straight Well Perforation Phase on the Fracture Pressure Pattern in Hydraulic Fracturing

摘要: 致密砂岩储层水力压裂时破裂压力高，导致压裂作业的难度与风险较大。射孔直井的射孔孔道是传递水力压力的通道，合理的射孔相位设计能够降低水力压裂时的破裂压力。本文根据最低起裂原则，建立了影响破裂压力的数学简化模型，分析了射孔方向和主应力夹角对破裂压力的影响规律，再以鄂尔多斯盆地杭锦旗地区砂岩储层数据为参考，建立基于内聚力单元的射孔井水力裂缝扩展有限元模型，分析了射孔相位角对破裂压力的影响规律。研究结果表明：沿水平最大主应力方向采用定向射孔时，岩石最易破裂，破裂压力约等于最小水平主应力加岩石抗张强度；采用螺旋射孔的情况下，某一孔眼方向与最大水平主应力方向重合时，螺旋射孔相位越大，破裂压力越高；当所有孔眼方向与最大水平主应力有夹角时，破裂压力随射孔相位角增大呈余弦变化，其中射孔相位角为45˚时破裂压力最低。

Abstract: Tight sandstone reservoirs exhibit high fracture pressures during hydraulic fracturing, increasing the difficulty and risk of fracturing operations. The perforation tunnels in perforated vertical wells serve as conduits for transmitting hydraulic pressure. A rational design of the perforation phase can reduce fracture pressure during hydraulic fracturing. Based on the principle of minimum initiation pressure, this paper establishes a simplified mathematical model to analyze the influence of perforation direction and the angle between the perforation and the principal stress on fracture pressure. Using data from the sandstone reservoirs in the Hangjin Banner region of the Ordos Basin, a finite element model incorporating cohesive elements is developed to simulate hydraulic fracture propagation in perforated wells and analyze the influence of perforation phase angle on fracture pressure. The results indicate that directed perforation along the direction of maximum horizontal principal stress leads to the easiest fracturing of the rock, with the fracture pressure approximating the sum of the minimum horizontal principal stress and the tensile strength of the rock. In the case of spiral perforation, the fracture pressure increases as the perforation phase angle increases when one perforation direction coincides with the maximum horizontal principal stress. When all perforation directions form an angle with the maximum horizontal principal stress, the fracture pressure varies as a cosine function of the perforation phase angle, with the lowest fracture pressure occurring at a perforation phase angle of 45˚.