Characterization of Multi-Scale Discrete Fracture Network Systems in Unconventional Reservoirs Using Dynamic Data

近年来，非常规致密或页岩油气藏的开发受到了广泛关注，因为全球范围内的资源估计量很大。采用多级水力压裂的水平威尔斯井可以增加储层接触面积，达到经济的产量。如果水力压裂由于存在现有裂缝而可以连接到最佳点，则这种增产技术特别有效。致密/页岩储层可以被概念化为具有两个不同子系统的双重介质：基质和裂缝。由于渗透率的高对比度，两个系统中的流动时间尺度显著不同。基质具有非常低的原位渗透性，而裂缝是具有非常高的渗透性的局部不连续性。裂缝可以是大的并且局限于井筒周围（例如，水力裂缝）或小的且远离刺激区域（例如，微裂缝）。应当指出，本提案侧重于页岩/致密油/气储层的原地开发，而不考虑油页岩的地面开采。* 总体目标是开发一套数值模型，适用于（1）使用基于有限元的技术模拟离散裂缝中的多相流;（2）捕获不同尺度上发生的裂缝的影响;以及（3）整合动态数据（例如，流量和压力测量），用于在鲁棒的历史匹配工作流中表征裂缝网络。* 这项研究提出了一种新的综合方法，用于分析低渗透多孔介质中的多相流动和流体分布，其中裂缝网络，即，水力裂缝和微裂缝同时存在于多个尺度上。这项研究将彻底改变目前的做法，主要集中在分析模型，通常忽略多相效应，有限体积/有限差分模拟，不处理有关致密/页岩储层的裂缝配置的复杂性。这种模型质量的损失使得数值模拟中的裂缝系统描述不完整，并且从动态数据中推断裂缝特征具有挑战性。许多熟悉非常规油藏开发最新研究的行业从业者表示需要更复杂的方法来分析和模拟这些油藏。预计随着计算技术的快速发展，有利于当前实践的计算负载正在消失，并且在不久的将来，可以在商业代码中实现更多的计算密集型技术，例如本提案中开发的那些技术。这些结果将影响我们优化压裂设计的能力，这也是地热能开发的一个重要考虑因素。

Development of unconventional tight or shale reservoirs has received much attention in recent years because of the large resource estimates worldwide. Horizontal wells with multi-stage hydraulic fracturing are drilled to enhance reservoir contact area and to achieve economic production rates. This stimulation technique is particularly efficient if hydraulic fractures can connect to sweet spots due to the presence of existing fractures. A tight/shale reservoir can be conceptualized as a dual-medium with two distinct sub-systems: matrix and fracture. The time scales for flow in the two systems differ significantly due to the high contrast in their permeability. The matrix has a very low in-situ permeability, while the fracture is a local discontinuity with a very high permeability. Fractures can be large and localized around the wellbore (e.g., hydraulic fractures) or small and located away from the stimulated zone (e.g., micro fractures). It should be noted that this proposal focuses on in-situ development of shale/tight oil/gas reservoirs, while surface mining of oil shale is not considered. ***The overall objective is to develop a suite of numerical models suitable for (1) simulating multi-phase flow in discrete fractures using finite-element based techniques; (2) capturing the impacts of fractures occurring over different scales; and (3) integrating dynamic data (e.g., flow and pressure measurements) for characterizing fracture networks in a robust history-matching workflow. ***This research proposes a novel integrated approach for analyzing multi-phase flow and fluid distribution in low-permeability porous media, where fracture networks, i.e., hydraulic fractures and micro fractures, are present over multiple scales simultaneously. This research would revolutionize the current practice that focuses predominantly on analytical models, which typically ignore multi-phase effects, and finite-volume/finite-difference simulations, which do not handle the complexities in fracture configuration pertinent to tight/shale reservoirs. This loss of model quality renders the description of fracture systems in numerical simulation incomplete and the inference of fracture characteristics from dynamic data challenging. Many industrial practitioners, who are familiar with the state-of-the-art research in unconventional reservoir development, have expressed the need for more sophisticated approaches to analyze and model these reservoirs. It is expected that computational loads that favor current practice are disappearing with the rapid advances in computing technology, and more computationally-intensive techniques, such as those developed in this proposal, could be implemented in commercial codes in the near future. The outcomes would impact our capability to optimize fracturing design, which is also an important consideration in geothermal energy development.