Rock Physics of Unconventional Reservoirs, volume II

The exploration and comprehensive assessment of fractured-vuggy reservoir information have perennially constituted focal points and challenges within the domain of oil and gas reservoir evaluation. The verification of geological phenomena, identification of various fracture and hole types, and the quantitative characterization thereof currently present pressing challenges. This study meticulously examines the deep carbonate reservoirs within the Dengying Formation in the Penglai gas region of the Sichuan Basin. The Core Rolling Scan images reveal five discernible features: unfilled holes, filled holes, filled fractures, open fractures, and algae. The analysis pinpoints three primary challenges in semantic segmentation recognition: the amalgamation of feature scales, class imbalance, and the scarcity of datasets with substantial sample sizes. To address these challenges, this paper introduces a Multi-Scale Feature Aggregation Pyramid Network model (MFAPNet), achieving a pixel accuracy of 68.04% in recognizing the aforementioned five types. Lastly, the model is employed in calculating core porosity, exposing a scaling relationship between wellbore image porosity and core porosity ranging from 1.5 to 3 times. To a certain extent, it reveals the correlation between the wellbore image logging data and the actual formation of the Dengying Formation in the Penglai Gas Field of the Sichuan Basin, and also provides a basis for the subsequent logging evaluation of the formation. The partial code and CHA355 dataset are publicly available at https://github.com/zyng886/MFAPNet.

Shales are composed of minerals and organic matter, whose individual properties are essential to determining the rock’s macroscopical deformation and strength. Scanning electron microscopy combined with electron energy dispersive spectroscopy (EDS) has been extensively used to evaluate composition, while peak-force atomic force microscopy (AFM) has been used on the determination of elastic modulus with nanometric resolution. Still, there is a need for tools to conduct an in-depth study of the minerals’ tribomechanical properties. Atomic force microscopy is a tool that can contribute to these studies, as it can simultaneously measure the tribomechanical properties and identify the phases. In this work, we propose using atomic force microscopy and energy dispersive spectroscopy to identify the shale components and to measure the in situ tribomechanical properties from the different phases. Friction images between the atomic force microscopy tip and the surface were acquired as a function of load. Minerals and organic matter were later identified by colocalized energy dispersive spectroscopy mapping. Then, the frictional characteristics of the major shale constituents were obtained by adjusting the Derjaguin-Muller-Toporov model to the selected components. Moreover, the identification of the different phases was performed. The results show that friction at the nanometer scale was observed to be higher for organic matter than for any other shale constituent, while shear strength was observed to be higher for quartz and lower for organic matter. These characteristics were used to differentiate shale constituents. It is shown that a careful comparison of friction can be used to differentiate the sulfite pyrite, tectosilicates (quartz, andesine, and albite), phyllosilicate biotite, and organic matter. The presented methodology gives novel information on friction properties in the nanoscale that are comparable to available centimetric characterization techniques contributing to the understanding of rock strength.

The elastic moduli are a function of properties that could vary between samples and change during maturation. Consequently, the effects of organic matter maturation on the elastic wave velocities of organic-rich rocks are challenging to describe. This work analyzes the isolated maturation effects on the organic content, pore volume, microstructure, and propagation velocities of elastic waves. To avoid any initial rock heterogeneity, we prepared a series of homogeneous samples from a unique outcrop block collected on the Eagle Ford formation with mineral composition initially determined using X-ray diffraction. From the initial set, four samples were held in their original condition and four were artificially maturated by hydropyrolysis until transformation rates were up to 95%. Then, the evolution of the sample properties was examined using an association of LECO TOC, Rock-Eval pyrolysis, vitrinite reflectance, mercury intrusion porosimetry (MIP), and ultrasonic pulse propagation. In addition, scanning electron microscopy images registered the microstructure evolution. To evaluate the effects of maturation on pore geometry and the organic matter elastic moduli, we analyze the relationship between the measured quantities using a rock physics inclusion model with the unmeasured properties taken as fitting parameters. The hydropyrolysis maturation increases the vitrinite reflectance from the initial 0.55% to 1.34% on the most matured sample. A total organic carbon reduction from 4.2% to 2.1% and a porosity increase from 9.2% to 21% are associated with observed maturation. The geochemical characterization on cleaned samples reveals an initial increment of soluble organic matter followed by a monotonical reduction related to oil migration out of bulk volume. The measurement of wave propagation velocities as a function of confining pressure displays an increasing pressure sensitivity with a downward trend in both velocity moduli. The petrophysical analysis indicates that the porosity increases through organic matter consumption and pore creation. The rock physics diagnoses indicate a decrease in the pore aspect ratio with an increase in the elastic modulus of the organic matter with maturation.