Pore Structure Characterization of Eocene Low-Permeability Sandstones via Fractal Analysis and Machine Learning: An Example from the Dongying Depression, Bohai Bay Basin, China

Abstract

Poroperm analysis, mercury injection capillary pressure (MICP), and nuclear magnetic resonance (NMR) measurements were performed to delineate the pore structures and fractal behaviors of the Eocene low-permeability sandstones in the Dongying Depression, Bohai Bay Basin, China. Three types of pore structures (I, II, and III) have been classified by applying the self-organizing map (SOM) clustering model. Comparative analysis of three different fractal models indicates that the MICP tubular model and NMR model are quite effective for pore structure characterization. The results show that the reservoirs generally exhibit high fractal dimensions, indicative of complex pore structures. The presence of small pore throats is primarily responsible for the heterogeneities and complexities in the Eocene low-permeability sandstones. A modified Winland model was established for the permeability estimation using MICP data. Different from high-permeability reservoirs or unconventional (e.g., shale and tight formation) reservoirs, r 10 is the best parameter for permeability estimation, indicating that the permeability of the Eocene low-permeability sandstones is largely controlled by the large pore systems. Additionally, a porosity model derived from movable fluids using NMR data has been established and provided better prediction effect compared with the classic Coates and Schlumberger Doll Research (SDR) models. Fractal analysis and permeability estimation are shown to be quite effective for investigating microscopic behaviors and in predicting the reservoir quality of low-permeability sandstone reservoirs.

Influence of Acid Treatment on Pore Structure and Fractal Characterization of a Tight Sandstone: A Case Study from Wudun Sag, Dunhuang Basin

“…An MI test was conducted using an automatic PoreMaster-33 mercury porosimeter. The maximum pressure of PoreMaster-33 was 33000 psi (227.57 MPa), corresponding to a minimum test aperture of 6.45 nm (Lu and Liu, 2021). The samples (irregular blocks with a side length of approximately 10-15 mm) were desiccated in a vacuum drying oven for 48 h and then loaded into the penetrometer to obtain the volume and pressure of mercury intrusion/extrusion.…”

Pore Structure and Movable Fluid Characteristics of Typical Sedimentary Lithofacies in a Tight Conglomerate Reservoir, Mahu Depression, Northwest China

“…High-pressure mercury intrusion technology is extensively used to determine the total pore volume and pore size distribution of reservoir rocks. 47 As shown in Figure 6A−C, the capillary force curves of mercury injection samples in the study area mostly conform to the types of II, IV, and V of six typical capillary force curves, 48 and the curve shape of different lithofacies is obviously different. The capillary pressure curves of TFC and PSS are mostly between II and V, which are characterized by an uneven middle section, poor pore sorting, and heterogeneous distribution of rock pore and throat but low mercury injection threshold pressure.…”

“…High-pressure mercury intrusion technology is extensively used to determine the total pore volume and pore size distribution of reservoir rocks . As shown in Figure A–C, the capillary force curves of mercury injection samples in the study area mostly conform to the types of II, IV, and V of six typical capillary force curves, and the curve shape of different lithofacies is obviously different.…”

Reservoir characteristics and factors influencing shahejie marl in the shulu sag, bohai bay basin, eastern China

“…The physical properties of the reservoir are mainly affected by the original sedimentary and diagenetic processes (Liu et al, 2016;Zhu et al, 2017;Lu and Liu, 2021), among which, the sedimentary mechanism was the most important. Specifically, the original sedimentary environment and mechanism determine the material basis of the reservoir, while the subsequent geological processes only modify it.…”