GEOPHYSICAL RESEARCH, 2021, vol. 22, no. 2, pp. 62-81. https://doi.org/10.21455/gr2021.2-4

UDC 550.8.05: 550.3

Abstract  References   Full text (in Russian)

CHANGES IN ELASTIC PROPERTIES OF OIL AND GAS RESERVOIRS AT TEMPERATURE INCREASE

V.S. Zhukov

Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia

Abstract. The paper presents the results of laboratory studies of rocks at temperature increase. Previously published data on the influence of temperature increase up to 600 °C on physical and mechanical properties of dry sandstones, carbonate, metamorphic and sedimentary rocks are considered. It is shown that the main reasons for the change in their elastic properties are differences in the expansion of mineral grains and the occurrence of cracks at a high temperature (about 400 °C), as well as the decomposition of complex minerals such as kaolinite.

Analysis of published data on changes in longitudinal wave velocities with an increase in temperature from 20 °C to 100 °C in sandstone samples saturated with crude oil, air and brine showed that the velocity of longitudinal waves in samples saturated with crude oil decreases faster than in dry or water-saturated samples. This is due to the rapid decrease in longitudinal wave velocity with increasing temperature in the crude oil, possibly due to the melting of individual oil components. Since, the influence of temperature increase with depth and with additional thermal effects on the physical properties of hydrocarbon reservoirs is poorly investigated, the estimation of velocity reduction in water-saturated reservoirs is an urgent task.

The results of experimental studies of the influence of temperature increase from 20 °C to 80 °C on the velocity of longitudinal and transverse waves of water-saturated sandstone samples at various effective pressures are presented. It is shown that for water-saturated samples, at an effective pressure of 2.0 MPa, the changes in the velocities of longitudinal and transverse waves from temperature are almost the same, but at an effective pressure of 37.0 MPa, the changes in the velocity of the longitudinal wave from temperature are almost two times less than the velocity of the transverse wave.

The dependences of elastic moduli on temperature obtained experimentally showed that only Poisson’s ratio increases with increasing temperature, and a higher effective pressure only enhances this tendency. All other elastic moduli decrease as the temperature increases, which makes it possible to expect a decrease in strength of rocks, in particular, shear strength. This may look like an increase in the plasticity of rocks with an increase in temperature, which is accompanied by an increase in Poisson's ratio. The dependences of the elastic characteristics of oil and gas reservoirs on temperature obtained in this paper can be used in the preparation of field development projects, in particular, in the planning of thermal impact methods.

Keywords: elastic properties, rock, oil and gas reservoir, temperature increase, water saturation, reservoir conditions, effective pressure.

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