基于位移不连续法的超临界CO2压裂射孔井裂缝扩展规律研究

Study on Fracture Propagation Patterns of Supercritical CO2 Fracturing in Perforated Wells Based on Displacement Discontinuity Method

  • 摘要: 综合考虑CO2物性参数的温压响应特征和拉伸-剪切复合裂缝扩展准则,建立了热-流-固耦合超临界CO2压裂射孔井裂缝扩展模型,基于位移不连续法进行离散求解,并与传统二维Khristianovic-Geertsma-Deklerk(KGD)解析模型和常规水力压裂模型进行对比,研究了射孔角度、地应力差、注入流量和地层温度对超临界CO2压裂射孔井裂缝扩展规律的影响。结果表明:建立的模型与KGD解析结果均较好吻合;相较于传统水力压裂,超临界CO2压裂产生的裂缝长度更长,而裂缝宽度更小;射孔方位角增大时近井带裂缝挤压显著,裂缝宽度减小,且伴随着显著的裂缝转向,射孔角度为15°和90°时,裂缝偏转角度分别为12.71°和71.34°;随着水平地应力差的增加,裂缝偏转角度增大,最大水平主应力为30 MPa,最小水平主应力为10 MPa和25 MPa时,裂缝偏转角度分别为90.00°和77.74°,且伴随着更小的裂缝开度;随着注入流量增加,裂缝内净压力增大,增强了超临界CO2对裂缝的挤压程度,有利于形成缝长较长和开度较大的优质裂缝网络;随着温度升高,裂缝入口宽度减小,缝长增加。该研究有助于促进超临界CO2压裂技术的发展。

     

    Abstract: A thermo-hydro-mechanical coupled fracture propagation model was developed by incorporating the temperature- and pressure-dependent physical properties of CO2 and a tensile-shear mixed-mode fracture propagation criterion. The model equations were solved numerically using the displacement discontinuity method. Comparisons with the traditional two-dimensional Khristianovic-Geertsma-Deklerk(KGD) analytical model and conventional hydraulic fracturing were conducted to investigate the effects of perforation angle, in-situ stress difference, injection flow rate, and formation temperature on fracture propagation behavior. The results demonstrate good agreement between the proposed model and KGD analytical solutions. Compared to conventional hydraulic fracturing, supercritical CO2 fracturing generates fractures with greater length but narrower width. Increasing the perforation angle results in significant compression of fractures near the wellbore, reduced fracture width, and notable fracture deflection; fracture deflection angles of 12.71° and 71.34° were observed at perforation angles of 15° and 90°, respectively. As the horizontal stress difference increases, fracture deflection angles also increase; with a maximum horizontal principal stress of 30 MPa and minimum horizontal principal stresses of 10 MPa and 25 MPa, fracture deflection angles reached 90.00° and 77.74°, respectively, accompanied by reduced fracture apertures. Higher injection flow rates increase net fracture pressure, enhancing the compressive effect of supercritical CO2 on fractures and promoting the formation of high-quality fracture networks with longer lengths and larger apertures. Elevated formation temperatures result in decreased fracture inlet widths and increased fracture lengths. These findings contribute to advancing supercritical CO2 fracturing technology development.

     

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