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.

Highlights

  • All the bulk samples were detected by X-ray diffraction (XRD), RockEval VI pyrolysis, N2 adsorption, mercury injection porosimetry (MIP), and ethylene glycol monoethyl ether (EGME) adsorption methods

  • It is found from the X-Ray Diffraction (XRD) curves of typical bulk rocks (Figure 2A)

  • The results indicate that clay minerals mainly benefit the development of pores, the relative contributions of clay minerals to pores with different diameters for the shales from different sections differ significantly

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Summary

Introduction

Hydraulic fracturing is one of the most important technologies for the successful exploitation of unconventional oil and gas resources with ultra-low porosity and permeability, especially for shale resources (Osiptsov, 2017; Wang and Sheng, 2017; Lu et al, 2018; Aadnøy and Looyeh, 2019; Yang and Guo, 2019; Lu et al, 2020; Shi et al, 2020; Xie et al, 2020; Wang et al, 2022). Shale presents pores with a wide diameter range, and the capillary pressure that relates closely to fluid flow differs according to the pore size if the properties of pore walls are the same. The distribution of confined fluids differs greatly according to the shale pore structure and surface properties, which are closely related to the shale mineral composition and burial evolution. Shale pores and surfaces are complex, and their characteristics and properties change significantly according to the mineral composition and Frontiers in Earth Science | www.frontiersin.org burial evolution. These complexities control the interactions between pore walls and confined fluids, and they must affect the shale oil mobility and production performance

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