The impact of thermal fracturing on the nearwellbore region during CO2 injection in depleted gasfields
本研究探讨了在碳封存项目中，热裂缝对枯竭气田近井筒（NWB）区域的影响。由于二氧化碳通常以超临界相注入，其注入流体具有高压和低温的特点，而枯竭气田的储层压力较低。这种压力的增加和温度的降低会导致热弹性响应，使储层内部的应力减小。一旦达到裂缝起始应力（即所谓的裂缝应力），热裂缝就会形成。热裂缝的形成仅是由于储层的大范围冷却。裂缝的形成会影响NWB区域，因为裂缝的开启会导致井底压力（BHP）下降，从而提高储层的注入能力。
本研究使用CMG GEM进行建模。模拟采用了一个均匀的双重渗透率盒子模型，模型被初始化为北海的一个典型枯竭气田储层。为了模拟裂缝，使用了Barton Bandis模型。该模型在满足裂缝条件时会改变裂缝网格的渗透率。通过该模型，通过关键参数的敏感性分析，研究了起裂时间、裂缝半长和储层注入能力。
研究发现，热裂缝的传播与热前缘最冷部分的传播一致。当由于流体通道或储层压力上升较慢而使裂缝条件更早满足时，热前缘会传播得更远。对地质力学参数的敏感性分析显示，只有储层的应力条件发生了变化，导致注入常数变化，从而产生不同的裂缝时间。储层对裂缝起始的反应方式相同，注入能力的提高也相似。有效渗透率（厚度和渗透率）与注入速度一起决定了储层压力上升的方式，从而略微改变了由于裂缝导致的注入能力增加。增加储层体积会导致储层内压力上升较慢，从而使热前缘传播得更远，因此裂缝长度更长。最后，对热效应的敏感性分析显示，储层与注入温度之间的温差越大，裂缝对压力上升的依赖性越小，导致更早的裂缝形成和更长的裂缝半长。压力上升没有变化，因此注入能力与基础方案相似。
总之，本论文提供了对关键参数如何影响热裂缝行为的认识。它还显示了可以预期的参数范围。将这两者结合起来，可以了解应重点关注参数，以便更好地描述热裂缝的行为，通过排除或包含对关键参数的广泛数据收集来优化操作。这有助于改进枯竭气田中的二氧化碳注入项目的注入策略。
CMG软件应用情况
本研究使用了CMG GEM软件来模拟CO2注入过程中热压裂的行为。GEM是一个用于组成、化学和非常规油气藏模拟的方程状态（EOS）模拟器。GEM具有强大的热模块和地质力学功能，适用于CCUS项目。模拟采用双介质模型，将储层划分为基质和裂缝两个部分。通过Barton Bandis模型，当满足压裂条件时，裂缝单元的渗透率会发生变化，从而模拟压裂对流体流动的影响。模拟还考虑了热孔隙弹性效应和热弹性调整因子，以准确描述储层在CO2注入过程中的应力变化。
结论
本研究通过数值模拟和敏感性分析，深入探讨了热压裂在枯竭气田CO2注入中的影响。主要结论如下：
- 热压裂的形成主要受储层冷却和压力变化的共同影响，热前沿的传播速度和起裂时间与储层的热力学和地质力学参数密切相关。
- 储层参数（如渗透率、厚度和有效渗透率）对压裂过程有显著影响。较低的有效渗透率会导致更快的压力积聚，从而导致更早的压裂和更长的压裂半长。
- 操作参数（如注入率和注入温度）对压裂行为也有重要影响。较高的注入率会导致更高的BHP值，从而延迟压裂时刻，而较低的注入温度会导致更早的压裂和更长的压裂半长。
- 地质力学参数的变化主要影响压裂时刻，但对注入性的影响较小。热压裂导致的注入性增加在不同参数下基本一致，最大注入性比为1.85。
- 通过敏感性分析，可以确定哪些参数对热压裂行为影响最大，从而在实际操作中优先考虑这些参数，以优化注入策略并减少不确定性。
Abstract
This research describes how thermal fractures impact the near-wellbore (NWB) region of a depleted gasfield in a carbon sequestration project. As CO2 is usually injected in its supercritical phase, the injection fluid is injected on high pressures and low temperatures. This is in high contrast with the depleted gasfields, which have a low reservoir pressure. The increase in pressure and decrease in temperature causes a thermoporoelastic response, resulting in a reduction of stress inside the reservoir. Once the fracture initiation stress, the so-called fracture stress, is reached, thermal fractures form. Thermal fractures form only due to extensive cooling of the reservoir. The fractures impact the NWB region; due to opening of fractures there is a drop in pressure in the bottomhole pressure (BHP). This increases the reservoir’s injectivity. This research uses CMG GEM to model this. The simulation uses a homogeneous box dual permeability model with the model being initialized as a generalized depleted gas reservoir in the North Sea. To model the fractures, the Barton Bandis model is used. This model changes the permeability in a fracture cell once fracture conditions are met. From this model, the moment of fracturing (fracture time), the fracture halflength and the injectivity of the reservoir is researched by performing a sensitivity analysis on key parameters. It is found that the thermal fractures propagate conform to the propagation of the coldest part of the thermal front. The thermal front propagates further once the fracture conditions are met sooner due to fluid highways or when the pressure build up in the reservoir is slower. The sensitivity on the geomechanical parameters showed that only the stress conditions in the reservoir changed, causing the injection constant to change and thus a different fracture time. The way the reservoir reacted to the initiation of fractures was the same; the injectivity was improved similarly for each parameter. The effective permeability (thickness and permeability) determines, together with the injection rate, the way the pressure builds up in the reservoir changes the increase of injectivity due to fracturing slightly. Increasing the reservoir volume causes a slower pressure build-up inside of the reservoir, allowing the thermal front to propagate further and thus longer fracture lengths. Lastly, the sensitivity on the thermal effects showed that a higher difference between the reservoir and injection temperature causes the fracture to be less dependent on the increase of pressure to fracture, resulting in earlier fracturing and longer fracture halflength. The pressure build-up is not changed, so the injectivity remains similar to the basecase scenario. All in all, this thesis gives an insight on how key parameters impact thermal fracture behavior. It also shows what range of parameters can be expected. Combining these two gives an insight on what parameters the focus should be on to better describe the behavior of thermal fractures, to economize the operation by leaving out or including extensive data collection on key parameters. This helps to improve the injection strategy with CO2 injection projects in depleted gasfields.
作者单位
荷兰代尔夫特理工大学
