低渗透裂缝性油藏时变形状因子及非线性渗流模型研究

Most of the remaining oil resources in China reside in low permeability reservoirs with high development degree of fractures. Multi-porosity model is usually employed to depict fractured reservoirs. The shape factors and transfer functions are the cores of multi-porosity model. Current shape factors are obtained based on regular rectangular shape and the transfer functions are derived by assuming linear pressure relationship between matrix block center and surrounding fracture. Apparently such assumptions are not accurate and some complex phenomena may not be well explained, such as nonlinear pressure in numerical simulation and nonlinear response curve at early stage in well testing. In this paper, the theory model for time-dependent shape factors and transfer functions are developed by considering different matrix block shape, fracture scale and multiscale combination method. The combined diffusion kinetics and seepage flow are adopted to develop a new nonlinear seepage model in low permeability fractured reservoirs. A standard chart is proposed to delineate the effect of different time- dependent shape factors and transfer functions on dimensionless constant. The results provide theory foundation and scientific basis for numerical simulator development, productivity prediction, well test analysis, seepage law research and application. The study will help improve the seepage theory progress and effective development in low permeability fractured reservoirs, with significant social benefit and economic benefit.

中国大部分剩余石油资源量为低渗透油藏，大多裂缝发育。裂缝性油藏一般利用多重介质模型描述，多重介质模型的核心是形状因子和窜流函数。现有形状因子是基于规则矩形形状、窜流函数是假设基质岩块中心和岩块周围裂缝之间的压力成线性关系，显然这样的假设是错误的，得到的结果也不能解释在数值模拟中压力的非线性现象，以及试井解释中早期非线性现象等复杂情况。本课题通过考虑不同的基质岩块模型、裂缝规模及尺度组合的实验方法，获得时变形状因子及窜流函数的理论计算模型，采取扩散动力学与渗流力学相结合的方法推导新的低渗透裂缝性油藏非线性渗流模型，并建立不同时变形状因子和窜流函数对无因次常量的标准图版。这为非线性渗流的数值模拟软件、产能预测、试井解释、渗流规律研究及应用等方面提供理论基础和科学依据。本课题的研究有助于推动裂缝性低渗透油藏的渗流理论发展和有效开发，具有十分重要的社会和经济效益。

低渗透裂缝性油藏是我国剩余石油资源的重要组成部分，深入揭示这类油藏的渗流机理是提高其开发效果的基础。通常，裂缝性油藏的渗流采用多重介质模型进行表征，多重介质模型的核心是形状因子和窜流函数。然而，前人对形状因子的研究大都将其作为常数在拟稳态条件下进行推导，由于常量形状因子推导得到的窜流函数不能合理解释机质岩块中压力的非线性现象，导致其在窜流初期产生较大误差。因此，本项目基于裂缝性油藏的各向异性和启动压力梯度特征，并考虑裂缝的迂曲度，建立新的低渗透裂缝性油藏的时变形状因子和窜流函数，对裂缝性油藏的渗流机理从理论上进行拓展和创新。.首先，本次研究采用室内实验对低渗透裂缝性油藏的应力敏感性特征进行研究，弄清这类油藏在非稳态流动阶段应力敏感变化规律，从而为时变形状因子和窜流函数的研究奠定基础。其次，项目基于低渗透裂缝性油藏的各向异性和启动压力梯度，并考虑裂缝的迂曲度，推导了新的窜流函数和形状因子，对低渗透裂缝性油藏的窜流函数和形状因子从理论上进行了拓展和创新。接着，项目进一步考虑低渗透裂缝性油藏的应力敏感特征，通过引入与地层压力相关的变渗透率系数，得到了基于时变的形状因子及窜流函数，从而建立了低渗透裂缝型油藏的渗流模型，为这类油藏的产能评价、数值模拟和试井分析提供理论支撑。基于上述成果，建立了时变形状因子和窜流函数对无因次产量的标准图版，解决了实际应用中计算繁琐和应用不便的难题。最后，考虑到裂缝壁面粗糙程度造成的润湿滞后差异以及储层的部分润湿会影响形状因子和窜流函数，又采用实验对不同润湿性液体在粗糙表面的润湿滞后现象和部分润湿对多孔介质渗流规律进行研究，从而为形状因子和窜流函数的研究提供支撑。本项目的研究有助于推动裂缝性低渗透油藏的渗流理论发展和有效开发，具有重要的社会和经济效益。.