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Pore accessibility and trapping of methane in Marcellus Shale
International Journal of Coal Geology ( IF 5.6 ) Pub Date : 2021-09-14 , DOI: 10.1016/j.coal.2021.103850
Andrzej P. Radlinski 1 , Tomasz Blach 1 , Phung Vu 1 , Yeping Ji 1 , Liliana de Campo 2 , Elliot P. Gilbert 2 , Klaus Regenauer-Lieb 1, 3 , Maria Mastalerz 4
Affiliation  

Accessibility of pore space in unconventional reservoirs is an important factor influencing both methane storage capacity and the kinetics of methane desorption. The determination of open (accessible) versus closed (inaccessible) porosity is therefore vital for the prediction of gas production potential. This study investigates accessibility of pores to methane in overmature middle Devonian Marcellus Shale samples (cut parallel and perpendicular to bedding) using small and ultra-small neutron scattering (SANS and USANS) with contrast matching (CM), supplemented by other complementary techniques, such as mercury injection capillary pressure (MICP) and low pressure gas (N2 and CO2) adsorption.

Our results demonstrate that for the samples studied, only about 6% of pores with diameter 25–500 nm are accessible to methane. The accessible fraction for pores larger than 500 nm is 35%. For nanopores smaller than 25 nm, pore accessibility could not be quantitatively determined due to increased methane density and condensation effects in confinement. Our observations indicate that methane penetrates the accessible small mesopores and micropores down to at least 1 nm in diameter, and the density of confined deuterated methane (CD4) is 0.68 g/cm3 for pores of diameter 25 nm and gradually increases with the decreased pore size. Moreover, elevated gas pressure causes formation of additional high-density methane nano-clusters. These clusters have a form of slightly anisotropic polydisperse discs oriented along bedding plane, about 1–12 nm in diameter and with average thickness of 3.6 nm. Utilizing samples cut parallel and perpendicular to the bedding, this study also briefly addresses anisotropy of pores. Based on the isosize intensity ratio ℛ, our SANS and USANS results demonstrate anisotropy in the out-of-bedding direction and suggest that the degree of anisotropy depends on the pore size. Specifically, for pore diameters ~2.5 to 250 nm, the degree of anisotropy is larger than for pores ~500 nm to 6 μm in diameter. Finally, comparison of pore size distribution results calculated from SANS/USANS to those obtained using MICP shows good agreement at low pressures, but large difference at pressures above 1000 bar. This discrepancy requires further testing; it is possible that the high mercury pressure used in MICP alters the mesopore and micropore structure of shales.



中文翻译:

马塞勒斯页岩中甲烷的孔隙可及性和捕集

非常规储层孔隙空间的可达性是影响甲烷储存能力和甲烷解吸动力学的重要因素。因此,确定开放(可接近)与封闭(不可接近)孔隙度对于预测天然气生产潜力至关重要。本研究使用具有对比匹配 (CM) 的小和超小中子散射(SANS 和 USANS),并辅以其他互补技术,研究过成熟的中泥盆纪 Marcellus 页岩样品(平行和垂直于层理切割)中的孔隙对甲烷的可及性如压汞毛细管压力(MICP)和低压气体(N 2和CO 2)吸附。

我们的结果表明,对于所研究的样品,只有约 6% 的直径为 25-500 nm 的孔隙可进入甲烷。大于 500 nm 的孔的可及部分为 35%。对于小于 25 nm 的纳米孔,由于甲烷密度增加和限制中的冷凝效应,无法定量确定孔的可及性。我们的观察表明,甲烷渗透到直径至少为 1 nm 的可接近的小介孔和微孔中,并且受限氘代甲烷 (CD 4 )的密度为 0.68 g/cm 3对于直径为 25 nm 的孔,随着孔径的减小而逐渐增加。此外,升高的气压会导致形成额外的高密度甲烷纳米团簇。这些簇具有沿层理面取向的略微各向异性的多分散盘形式,直径约 1-12 nm,平均厚度为 3.6 nm。利用平行和垂直于层理切割的样品,本研究还简要说明了孔隙的各向异性。基于等径强度比ℛ,我们的 SANS 和 USANS 结果证明了层外方向的各向异性,并表明各向异性的程度取决于孔径。具体而言,对于~2.5 至 250 nm 的孔径,各向异性程度大于直径 ~500 nm 至 6 μm 的孔径。最后,从 SANS/USANS 计算的孔径分布结果与使用 MICP 获得的孔径分布结果的比较表明,在低压下具有良好的一致性,但在 1000 bar 以上的压力下差异很大。这种差异需要进一步测试;MICP 中使用的高汞压可能改变了页岩的中孔和微孔结构。

更新日期:2021-11-11
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