Nanostructural control of methane release in kerogen and its implications to wellbore production decline.
Nanostructural control of methane release in kerogen and its implications to wellbore production decline.
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DOI：
10.1038/srep28053
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发表时间：
2016-06-16
影响因子：
4.6
通讯作者：
Wang Y
中科院分区：
文献类型：
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作者：
Ho TA;Criscenti LJ;Wang Y
尽管过去几十年美国页岩气生产取得了巨大成功，但人们仍然对井筒产量的急剧下降以及递减曲线长期预测的巨大不确定性深感担忧。可靠的预测必须基于对页岩基质中甲烷释放的机理理解——这是页岩气开采的一个限制步骤。通过分子模拟，我们在此表明，纳米孔隙干酪根基质中的甲烷释放具有以下特征：随着气体压力降低，首先是加压游离气的快速释放（约占采收率的30% - 47%），然后是吸附气的缓慢释放。第一阶段由气体压力梯度驱动，而第二阶段由气体解吸和扩散控制。我们进一步表明，纳米孔隙干酪根中所有甲烷的扩散行为与体相不同，扩散系数要小得多。分子动力学模拟还表明，沉积在干酪根中的甲烷有相当大一部分（3% - 35%）可能会被困在孤立的纳米孔隙中，从而无法开采。我们的研究结果为理解非常规储层中气体释放和产量递减的机理提供了新的视角。长期产量递减似乎受气体释放的第二阶段控制。
Despite massive success of shale gas production in the US in the last few decades there are still major concerns with the steep decline in wellbore production and the large uncertainty in a long-term projection of decline curves. A reliable projection must rely on a mechanistic understanding of methane release in shale matrix–a limiting step in shale gas extraction. Using molecular simulations, we here show that methane release in nanoporous kerogen matrix is characterized by fast release of pressurized free gas (accounting for ~30–47% recovery) followed by slow release of adsorbed gas as the gas pressure decreases. The first stage is driven by the gas pressure gradient while the second stage is controlled by gas desorption and diffusion. We further show that diffusion of all methane in nanoporous kerogen behaves differently from the bulk phase, with much smaller diffusion coefficients. The MD simulations also indicate that a significant fraction (3–35%) of methane deposited in kerogen can potentially become trapped in isolated nanopores and thus not recoverable. Our results shed a new light on mechanistic understanding gas release and production decline in unconventional reservoirs. The long-term production decline appears controlled by the second stage of gas release.