Physical simulation aspects of structural changes in rock samples under thermobaric conditions at great depths
Authors: Ilyinov M. D. , Petrov D. N. , Karmanskiy D. A. , Selikhov A. A. 
Published: December 2023

When designing the parameters for the development of oil and gas field at significant depths, crucial to comprehend how certain factors affect the behavior of reservoir rocks and host rocks. These factors include the high level of rock stress, the ambient temperature field, and the hydro- and gas-dynamic processes within the mass. The impact of one or a combination of these factors can result in alterations to the construction, structure, composition, and properties of the rock mass and, ultimately leading to a mismatch between the design solutions and the actual conditions.

The purpose of the research is to establish a methodology for conducting laboratory studies that investigate the impact of the mode of occurrence of oil and gas field reservoirs at great depths on the properties of rock samples.

The research objectives encompass a theoretical analysis and the identification of the principal factors influencing rock behavior and changes in internal structure. Additionally, the objectives include developing laboratory research methods that comprehensively simulate these factors and conducting trial experiments to assess their effects.

As part of the project, tests were conducted on sandstone samples collected from depth ranging from 3.5 to 4 km within the hydrocarbon field. These studies were performed while simulating thermobaric reservoir conditions, which include temperature, rock pressure, and reservoir pressure.

The results of these experiments, aimed at examining the behavior of rock samples as closely as possible to their natural reservoir occurrence at depth of 3.5–4 km, are presented. It has been observed that rock samples of the same lithology, collected from nearly identical depths, can exhibit significant differences in deformation characteristics, both in the pre- and off-limit regions of loading. The findings from these studies provide the initial data for the development and refinement of geomechanical model behavior for materials that take into account not only fracture strength criteria but also dilatancy processes at various stages of rock deformation. Increasing lateral pressure within the range of 0 to to 55 MPa causes relatively minor change in ultrasonic vibration velocities, typically ranging from 1 to 10%. This makes it challenging to determine the necessity of utilizing these results for indirectly assessing changes in rock properties within the mass. Nevertheless, within the context of geophysical studies, considering variations in velocity values enhances the quality of result interpretation, especially given the substantial geometric dimensions of the rock masses under investigation.

Research into the acoustic emissions of rocks in a complex stressed state enables the monitoring of spatial micro- and macrofracturing processes throughout the entire loading phase of samples. This provides a more comprehensive understanding of changes in their internal structure. The article delves into the factors that impact structural changes in oil and gas field rocks, particularly as their development extends to greater depths. The study outlines methodological approaches that facilitate the investigation of physical and mechanical properties of rock samples, while accurately modeling complex thermobaric conditions. Additionally, the it describes the technical specifications of the testing equipment, ensuring the closest possible replication of the actual conditions of reservoir rock occurrences. Lastly, the study reveals key features related to the deformation and fracture of rock samples during testing under lateral pressures of 55 MPa and pore pressures of 30 MPa, along with the creation of temperature fields up to 100 °C.