通过建立新的模型，科研人员能够准确计算天然气开采状况！
编译 | 惊蛰
通过更好地预测难以察觉的断裂力学，新的计算模型可以潜在地提高天然气生产效率和利润。 除此之外，新的计算模型还准确地计算了在开采过程中释放的气体量。
该模型由几所大学的多名教授共同推出，美国西北大学的Zdeněk Ba?ant教授说：“我们的模型比现有的行业模型和软件更加真实，结果也更加准确。这种模式可以帮助油气行业提高效率，降低成本，并提高盈利能力。”
尽管油气行业在快速发展，油气产量也在不断增长，但对开发商来说，大部分的压裂过程仍然是神秘的。 由于裂缝在地下深处，研究人员无法观察到页岩中气体释放的断裂机制，这在一定程度上限制了采收率的提高。
美国Los Alamos国家实验室的计算地球科学家Hari Viswanathan表示：“这项工作提供了更好的天然气开采预测能力，可以帮助开发商更好地控制生产，同时通过减少压裂液的使用来减少对环境的影响。该计算模型应该可以优化各种参数，如泵送速率和循环，压裂液性质的变化（如粘度等）。计算结果的提高会帮助开发商从深层页岩地层中提取更多的天然气，而目前天然气采收率仅约为5 ％，很少超过15％，开采效率很低。”
通过考虑近期构造事件引起的早期裂缝封闭，并结合此前从未考虑过的渗水力数据，研究人员开发了一种新的数学和计算模型，显示了压裂过程中分支裂缝是如何形成的，能够提高底层中天然气的释放量。截至目前，该模型是第一个能够预测压裂裂缝分支的模型，同时其计算结果与实际开采中页岩释放的已知气体量一致。整体来看，新的模型可能会提高油气行业的开采效率。
该研究结果已经发表在1月11日的“美国国家科学院院刊”上，题为“页岩中封闭天然裂缝的水力裂缝分支：如何掌握渗透率”。
了解页岩裂缝的形成方式也可以改善封存管理，将来自油气开采的废水泵回地下。目前压裂是一种提取页岩中天然气的主要手段，通过在页岩层下钻一个洞（井筒），通常在地表下几公里，然后水平段延伸数英里。当含有添加剂的压裂液在高压下被泵入地层中时，页岩中就会产生纳米尺寸的裂缝，天然气从孔隙中释。
根据经典的断裂力学研究预测，那些从水平孔垂直延伸的裂缝应该没有分支，但这些裂缝本身并不能解释在此过程中释放的气体量。实际上，气体产生率比实验室中提取的页岩岩心测得的渗透率计算高约10000倍，这也证明当前计算模型亟需改进。
在此前的研究中，研究人员曾假设水力裂缝与页岩中预先存在的裂缝有关，使页岩渗透性更高。但是Ba?ant和他的同事发现，这些构造产生的裂缝大约有1亿年的历史，必然会被应力下页岩中的粘性流动所封闭。
在此基础上，Ba?ant和他的同事在相反方向展开了研究，假设页岩层沿着现存的封闭裂缝具有微弱的微裂纹层，并且这些裂缝分支也会促进主裂缝形成。与以前的研究不同，它们还考虑了水扩散到多孔页岩中时的渗透力。
当他们使用这种弱层的新概念开发模拟过程以及所有渗流力计算时，他们发现结果与现实中的结果相匹配。Ba?ant说：“我们第一次证明，裂缝可以横向分支，如果页岩不是多孔的，这是不可能的”。在确立了这些基本原则之后，研究人员希望能够在更大范围内对这一过程进
页岩中分支出高密集的水力裂缝对有效的油气开采是必不可少的。研究人员认为在压裂中地层会发生这种变化，但现有的数学模型和商业软件无法预测它。研究人员提出了一种预测分支何时发生的方法，以及如何控制它，将为页岩油气的开采提高最大助力。
A new computational model could potentially boost efficiencies and profits in natural gas production by better predicting previously hidden fracture mechanics. It also accurately accounts for the known amounts of gas released during the process.?
“Our model is far more realistic than current models and software used in the industry,” said Zdeněk Ba?ant, McCormick Institute professor and Walter P. Murphy professor of civil and environmental engineering, mechanical engineering, and materials science and engineering at Northwestern’s McCormick School of Engineering. “This model could help the industry increase efficiency, decrease cost, and become more profitable.”
Despite the industry’s growth, much of the fracing process remains mysterious. Because fracing happens deep underground, researchers cannot observe the fracture mechanism of how the gas is released from the shale.
“This work offers improved predictive capability that enables better control of production while reducing the environmental footprint by using less fracturing fluid,” said Hari Viswanathan, computational geoscientist at Los Alamos National Laboratory. “It should make it possible to optimize various parameters such as pumping rates and cycles, changes of fracturing fluid properties such as viscosity, etc. This could lead to a greater percentage of gas extraction from the deep shale strata, which currently stands at about 5% and rarely exceeds 15%.
By considering the closure of preexisting fractures caused by tectonic events in the distant past and taking into account water seepage forces not previously considered, researchers from Northwestern Engineering and Los Alamos have developed a new mathematical and computational model that shows how branches form off vertical cracks during the fracing process, allowing more natural gas to be released. The model is the first to predict this branching while being consistent with the known amount of gas released from the shale during this process. The new model could potentially increase the industry’s efficiency.
The results were published in the Proceedings of the National Academy of Sciences on Jan. 11, in a paper titled Branching of Hydraulic Cracks in Gas or Oil Shale with Closed Natural Fractures: How to Master Permeability.
Understanding just how the shale fractures form could also improve management of sequestration, where wastewater from the process is pumped back underground.
To extract natural gas through fracing, a hole is drilled down to the shale layer — often several kilometers beneath the surface — then the drill is extended horizontally, for miles. When water with additives is pumped down into the layer under high pressure, it creates cracks in the shale, releasing natural gas from its pores of nanometer dimensions.
Classic fracture mechanics research predicts that those cracks, which run vertically from the horizontal bore, should have no branches. But these cracks alone cannot account for the quantity of gas released during the process. In fact, the gas production rate is about 10,000 times higher than calculated from the permeability measured on extracted shale cores in the laboratory.
Other researchers previously hypothesized the hydraulic cracks connected with pre-existing cracks in the shale, making it more permeable.
But Ba?ant and his fellow researchers found that these tectonically produced cracks, which are about 100 million years old, must have been closed by the viscous flow of shale under stress.
Instead, Ba?ant and his colleagues hypothesized that the shale layer had weak layers of microcracks along the now-closed cracks, and it must have been these layers that caused branches to form off the main crack. Unlike previous studies, they also took into account the seepage forces during diffusion of water into porous shale.
When they developed a simulation of the process using this new idea of a weak layers, along with the calculation of all the seepage forces, they found the results matched those found in reality.
“We show, for the first time, that cracks can branch out laterally, which would not be possible if the shale were not porous,” Ba?ant said.
After establishing these basic principles, researchers hope to model this process on a larger scale.
Branching into densely spaced hydraulic cracks is essential for effective gas or oil extraction from shale. It is suspected to occur, but the existing mathematical models and commercial software fail to predict it. A new paper from Northwestern University and Los Alamos National Laboratory presents a method to predict when the branching occurs, and how to control it. Photo: Los Alamos National Laboratory.