基于扩展有限元的现场尺度水力裂缝扩展机制模拟研究

Simulation research of field-scale hydraulic fracture extension mechanism based on XFEM

摘要:

干热岩储层水力压裂形成复杂缝网对于建设增强型地热系统（EGS）至关重要。为了深入认识干热岩储层水力裂缝扩展机制，揭示工程参数和地质参数对水力裂缝扩展的影响规律，本文以青海共和盆地某干热岩储层为研究对象，基于扩展有限元（XFEM）数值模拟软件ABAQUS展开研究，建立现场尺度的干热岩储层模型，实现热流固耦合分析水力裂缝的扩展机制。结果显示：正断型应力状态下，裂缝发生了很明显的朝向Z轴的转向，走滑型应力状态下，裂缝扩展主要沿水平方向，有轻微倾向于Z轴转向的趋势，逆冲型应力状态下，裂缝沿水平方向扩展；随着弹性模量的增大，裂缝的起裂压力减小，裂缝延伸的注水压力也减小，弹性模量越大裂缝起裂的时间越早，泊松比越大水力裂缝的破裂压力越大，破裂压力整体呈现出正断型>走滑型>逆冲型；随着温度的升高，水力裂缝破裂压力减小，大约降低2~3 MPa左右，3个应力状态下都呈现出相同的趋势。本文丰富了干热岩水力裂缝扩展的数值模拟手段，相关研究成果可为干热岩储层中水力压裂扩展预测与不同参数对其的影响分析提供技术支撑。

关键词:水力压裂;储层裂缝;干热岩;扩展有限元;现场尺度;热流固耦合

Abstract:

Hydraulic fracturing of hot dry rock（HDR） reservoirs to form a complex network of seams is essential for the construction of enhanced geothermal systems （EGS）. In order to deeply understand the mechanism of hydraulic fracture extension in HDR reservoirs and reveal the influence of engineering and geological parameters on hydraulic fracture extension， in this paper， a HDR reservoir in the Gonghe Basin of Qinghai Province is taken as the research object， and based on the extended finite element method （XFEM） numerical simulation software ABAQUS， a field-scale HDR reservoir model is established to realize the thermo-hydro-mechanical coupling and to analyze the extension mechanism of the hydraulic fracture. The results show that： （1） In the normal fracture-type stress state， the crack undergoes a very obvious turning toward the Z-axis， in the strike-slip-type stress state， the crack expands mainly along the horizontal direction with a slight tendency to turn toward the Z-axis， and in the thrust-type stress state， the crack expands along the horizontal direction. （2） With the increase of elastic modulus crack initiation pressure decreases， the injection pressure of crack extension also decreases； the larger the elastic modulus the earlier the crack initiation time； the larger the Poisson ratio the greater the rupture pressure of hydraulic cracks； the rupture pressure as a whole shows a normal fracture-type>strike-slip-type>thrust-type. （3） The hydraulic fracture rupture pressure decreases with the increasing temperature， by about 2~3MPa or so， showing the same trend for all three stress states. This paper inriches the numerical simulation methods of hydraulic fracturing extention for HDR， the relavant research results can also provides technical supports for the prediction of the hydraulic fracturing extention and the influences affected by different perameters for HDR reservoirs.

Key words:hydraulic fracturing;reservoir fracture;hot dry rock;extended finite element method;field scale;thermo-hydro-mechanical coupling