Estimating the strength and elasticity of rocks in the Dagi formation on the Sakhalin shelf

Resource development on the Sakhalin shelf requires estimating the deformation– strength and elastic characteristics of rocks as these values are needed both in well construction and in hydrocarbon reservoir engineering. The test subjects in determining the elasticity parameters and strength in compression and tension were dry cylindrical sandstone specimens with porosity from 10 to 26.5%. The specimens were made of core sampled from the Permianage Dagi formation using wells drilled on the eastern shelf of the Sakhalin Island. The tensile/compressive strengths, Young’s modulus (elasticity), Poisson’s ratio and the failure envelopes of rocks were determined in compliance with the state standards. The direct linear dependence revealed between the ultimate compression strength and porosity of the specimens can be recommended for estimating the strength of rocks depending on their porosity. The relationship between the ultimate compression strength and P-wave velocity also appeared to be informative and allowed the strength evaluation as function of the compressional velocity. On the other hand, the correlation of the ultimate tension strength and porosity of the test specimens provided no admissible dependence between these parameters. The standard tests yielded average estimations of strength and elastic characteristics for dry specimens of the Dagi formation sandstone in atmospheric conditions: compressive strength 27.2 MPa; tensile strength 5.77 MPa; Young’s modulus (statics/dynamics) 6.13/6.37 GPa; Poisson’s ratio (statics/ dynamics) 0.225/0.237; cohesion 7.00 MPa; angle of rupture (internal friction) 36.3 deg.      A few Mohr’s circles are plotted by the minimal, maximal and average values of the ultimate strength in compression and tension. The Mohr’s envelopes allowed evaluating the normal and shear stresses in the transition from the elastic to plastic deformation and made it possible to calculation the cohesion and angle of rupture. For the range of the effective compressive stresses from 40 to 50 MPa, close to the formation conditions, it is shown that the internal friction angle is 6.8 deg and the cohesion is 17.77 MPa.

Zhukov V. S. Estimating the strength and elasticity of rocks in the Dagi formation on the Sakhalin shelf. MIAB. Mining Inf. Anal. Bull. 2020;(4):44-57. [In Russ]. DOI: 10.25018/02361493-2020-4-0-44-57.

V.S. Zhukov, Dr. Sci. (Eng.), Senior Researcher, Chief Researcher, «Gazprom VNIIGAZ» Ltd., Moscow, Russia, e-mail: VZhukov@vniigaz.gazprom.ru.

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