Insight into Water Occurrence and Pore Size Distribution by Nuclear Magnetic Resonance in Marine Shale Reservoirs, Southern China

Abstract

The exploitation of shale gas has been a hot topic, and the understanding of water occurrence and pore size distribution in shale reservoirs plays a crucial role in efficiently developing this type of resource. In this study, we target moderate-shallow marine shales from the Lower Cambrian Niutitang Formation and deep marine shales from the Lower Silurian Longmaxi Formation. Low-field nuclear magnetic resonance is applied to investigate the water occurrence and pore size distribution together with complementary experiments including mineral component analysis, scanning electron microscopy (SEM), contact angle measurement, and high-speed centrifugation. The SEM analysis shows that the moderate-shallow shales have some inorganic nanopores and cracks but almost no organic pores, while the deep shales have abundant organic nanopores. The average movable water saturation of moderate-shallow and deep shales is only 15.5% and 15.2%, respectively. It implies that the high irreducible water saturation is a typical characteristic for both marine shales, which is ascribed to the strong capillary force of the nanopore inhibiting water flowing out of the pore space. By means of high-speed centrifugation and contact angle measurement, the converting factor of the T 2 value into pore size was obtained. The average pore size in moderate-shallow shales is 19.4, 21.8, and 26.8 nm, respectively, while the average pore size in deep shales is 109, 57, and 68.4 nm, respectively, for the tested samples. This indicates that organic pores in deep shales contribute to the development of abundant nanopores. This study brings insight into characterizing the water occurrence and pore size distribution of moderate-shallow and deep marine shales.

Imbibition Mechanisms of Fracturing Fluid in Shale Oil Formation: A Review from the Multiscale Perspective

“…Due to the complexity of the pore structure of the actual shale reservoir, it is difficult for the nanoscale simulations to accurately characterize the imbibition law in a shale nanopore-fracture system. 63 Further study of the imbibition patterns within the pore network on the basis of nanoscale achievements provides a clearer understanding of the oil− water transport characteristics at the multiscale level.…”

Minireview of Microscopic CO2 Interactions with Fluids and Minerals in Shale: Advances and Outlook

“…For conventional reservoirs, the minimum miscible pressure (MMP) between CO 2 and oil can be measured by slim-tube, rapid pressure increment, vanishing IFT, rising-bubble apparatus, sonic response, NMR, and computed tomography (CT) methods . For example, NMR is applied to study multiphase fluid transport in porous media by measuring the transverse relaxation time T 2 of hydrogen nuclei in a magnetic field. , Li et al measured the transverse relaxation time of hydrocarbons at different CO 2 injection pressures using NMR. They found that the transverse relaxation time of hydrogen nuclei in hydrocarbons decreases with the CO 2 injection pressure increasing.…”

“…67 For example, NMR is applied to study multiphase fluid transport in porous media by measuring the transverse relaxation time T 2 of hydrogen nuclei in a magnetic field. 68,69 Li et al 70 measured the transverse relaxation time of hydrocarbons at different CO 2 injection pressures using NMR. They found that the transverse relaxation time of hydrogen nuclei in hydrocarbons decreases with the CO 2 injection pressure increasing.…”

Rock Fabric of Tight Sandstone and Its Influence on Irreducible Water Saturation in Eastern Ordos Basin

“…NA can characterize the specific area, average pore diameter, and pore volume of mesopores, while MIP can get the pore-throat size distribution from the nano to micro scale, based on which, the tortuosity can be calculated. − The pore parameters extracted from NA and MIP can be used to analyze the controlling factors of irreducible water saturation. Nuclear magnetic resonance (NMR) is a significant technique to investigate water occurrence, which when associated with a high-speed centrifugation machine can distinguish the pore size distribution of movable and irreducible liquid. − The principle of NMR is based on the fact that liquid in large pores has a longer relaxation time, and liquid in small pores has a short relaxation time. Thus, the whole pore size distribution can be obtained. − NMR T 2 relaxation time can only reflect the pore size distribution, but the real pore size can be obtained by a converting factor, which can convert the T 2 value into the real pore size .…”