Characteristics of Shale Pores and Surfaces and Their Potential Effects on the Fluid Flow From Shale Formation to Fractures

Abstract

Fluid flow is critical for the efficient exploitation of shale resources and can be split into two stages: the flow in the artificial fractures and, more importantly, the flow from shale formations to the artificial fractures. In this study, X-ray diffraction, N2 adsorption, mercury intrusion, and ethylene glycol monoethyl ether adsorption were conducted on the shales collected from Es3middle, Es3lower, and Es4upper sub-members in the Dongying Sag to reveal the potential effects of the characteristics and properties of pores and surfaces on the fluid flow in shale formations. The results are indicated as follows: 1) The shales from Es3middle and Es3lower contain more I/S and detrital minerals but less illite and carbonate minerals than those of Es4upper. 2) The shales from Es3middle and Es3lower are mainly composed of smaller pores present in larger surface areas and lead to the steeper slope between the BrunauerEmmettTeller specific surface area (BET-SSA) and pore volumes. 3) Clay minerals mainly contribute to pore development, and carbonate minerals inhibit pore development. All kinds of surface areas (especially the inner surface area) are sourced by clay minerals, while I/S and illite present opposite effects. 4) Pore size and surface properties affect significantly the fluid flow in shale formations. The shales from Es4upper are the favorable interval for shale oil accumulation and flow, especially for the shales with depth ranges of 3360∼3410 m, which possess high carbonates, illite and total organic carbon content, low clay mineral content, large pore volume, high large pore content, and small surface areas. Additionally, fluid composition needs to be paid more concern in the future.

A New Classification Method of Shale Reservoirs Based on the Relationship between Pore Volume and Surface Area: Shahejie Formation, Jiyang Depression, and Bohai Bay Basin

“…The particle size (<3 mm) has been proven to be useful for obtaining the PSD by the combination of HPMI and N2GA by previous researchers. 15,51 Additionally, previous researchers also confirmed that the maximum extraction particle is limited to a millimeter level. 52 Therefore, we believe that the particle size of <3 mm can be applied to N2GA, HPMI, and Soxhlet extraction at the same time and may be more appropriate for the aim of this study.…”

“…Based on the discussions above, all samples were crushed to less than 3 mm , and extracted with dichloromethane solvent for at least 7 days to remove soluble organic matter by Soxhlet methods. Then, the sample was split into two aliquots.…”

“…58 The wetting angle of mercury was 140°, the interfacial tension of mercury was 485 mN/ m, and the density was 13.5335 g/mL. 15 2.4. Determination of Pore Characteristics.…”

“…A similar phenomenon occurs according to much data recorded by many previous studies, although the pore classification in their papers is that of the IUPAC. 15,81,82 For sample 17, the Es3 lower is the lower section of the third member in Shahejie Formation. Es4 upper is the upper section of the fourth member in the Shahejie Formation.…”

“…The pore volume (PV) becomes larger as the specific surface area (SSA) increases. , It has been revealed that the contribution of micropores to the SSA is the largest followed by that of mesopores and macropores, and micropores have the advantage of markedly preserving methane. − It has been confirmed that the SSA decreases as the nanopore size decreases, and in the case of low SSA, the SSA is mainly related to the pores larger than 10 nm . In addition, many studies have shown that not only the PV but also the SSA changes after Soxhlet extraction. ,− Approximately 80% of soluble organic matter is distributed in the pore and on the surface . The above-mentioned findings indicate that the pore is closely related to the surface, which distinctly affects the occurrence of shale oil.…”