Yu. F. Kovalenko

Fracture of Sedimentary Rocks under a Complex Triaxial Stress State

Most sedimentary rocks have layered structure, and their strength properties are therefore anisotropic; as a consequence, the rock strength depends on the direction of the applied stresses. In this case, various fracture mechanisms are possible. The following two possible fracture mechanisms are considered: actions along the bedding planes, which are weakening surfaces, and along the planes where stresses exceeding the total rock strength are attained. A triaxial independent loading test bench was used to study the fracture conditions for layered rocks composed of productive oil-and-gas strata in complex true triaxial loading tests. The study shows a good qualitative agreement between experimental results and theoretical estimates.

Mechanical-mathematical and experimental modeling of well stability in anisotropic media

Virtually allmodern gas and oil production technologies are based on directional drilling, and hence well stability issues are of primary importance. It was established that the stability of slanted wells significantly depends on the deformation and strength characteristics of rocks, on the presence and degree of rock anisotropy, and also on the well geometry and the pressure at the well bottom. We suggest a new approach to this problem by modeling the hole making process with the use of a triaxial independent loading test system (TAILTS) based on the mechanical-mathematical model developed here.

Experimental study of the influence of a triaxial stress state with unequal components on rock permeability

We present the experimental results of true triaxial independent loading test bench studies of the influence of a triaxial stress state with unequal components on the filtration properties of rock in oil and gas gathering mains. We show that the permeability of rock subjected to stresses can irreversibly decrease or increase. The discovered effects are of great importance when designing optimal oil and gas well drilling and operation regimes.

Fracture Model of Anisotropic Rocks under Complex Loading

The paper proposes a deformation and fracture model for anisotropic stratified rocks and presents theoretical and experimental data on how the rock strength and fracture geometry are influenced by principal stresses and their orientation to bedding planes. Two possible mechanisms are considered for rock fracture under true triaxial load: along bedding planes of weakness and along planes in which Mohr-Coulomb stresses reach a critical combination with cohesion coefficients and internal friction angles typical of the rock. The transition of rocks to inelastic deformation is described in the context of two criteria of which one accounts for the above fracture mechanisms and the other, being a semi-empirical analogue of the Hill yield criterion, accounts for the effect of normal stress. The experimental data presented are for the strain and strength properties of rocks sampled from the Fedorovskoye and Talakanskoye oil and gas fields and tested on an original loading system for true triaxial compression with lateral pressure (similar to the Karman scheme) and for generalized shear (three unequal and nonmonotonic principal stresses). The experimental and theoretical results, including total stress-strain curves, are in good qualitative agreement and demonstrate the possibility to evaluate the parameters entered in the model from tests of particular rocks.

Modeling Deformation and Failure of Anisotropic Rocks nearby a Horizontal Well

The article describes modeling mechanical behavior of rocks, including selection of models of inelastic deformation process in anisotropic rocks, experimental determination of elastoplastic properties of rocks and calculation of stress states for specific design well bottoms. The research is carried out for the conditions of Fedorov oil reservoir. The scope of modeling embraces the situations of uncased well shaft and shaft with slotted holing.

Modeling Geomechanical Processes in Oil and Gas Reservoirs at the True Triaxial Loading Apparatus

The paper presents the results of the investigation of the strain-strength properties of rocks (dolomites) raised from a depth of more than 6 km of the exploration well of the Kainsayskaya Field. The experiments were carried out on a unique experimental setup – the Triaxial Independent Load Test System of IPMech RAS on cubic specimens with an edge of 40 mm. Two triaxial tests were performed on two specimens showing the presence of strong strength anisotropy of the rock. A physical simulation of the pressure reduction process in deep wells near an open borehole and the tip of a perforation hole was performed on three specimens. The conducted studies have revealed a rather low strength of the studied rocks, despite the great depths of their lying under the conditions of high rock pressure. They have shown that the beginning of the rock destruction depends vastly on the type of stress state arising in the formation. Carrying out physical modeling of geomechanical processes in oil and gas reservoirs using true triaxial loading is of great importance from the point of view of justifying methods of influence on deep-lying strata in order to increase the productivity of wells, as well as reduce risks of well destruction during their drilling and operation.

Modeling of Deformation and Filtration Processes Near Wells with Emphasis of their Coupling and Effects Caused by Anisotropy

The approach to modeling geomechanical processes in the well vicinity including mathematical modelling of deformation, fracture and filtration as well as experimental determining the parameters involved, under conditions, corresponding to the real in situ ones is presented. The approach involves three stages: (i) choosing the mechanical model and its adopting to the considered problem; (ii) determining the model parameters by using the direct experiments; (iii) mathematical modeling of deformation, fracture and filtration processes in question.

The geomechanics of gas recovery from coal seams

A new approach for the creation of scientific foundations for effective and environmentally safe recovery of methane from coal seams is proposed. A virgin coal seam possesses very low permeability. Free gas is contained in isolated microscopic pores and cracks of the coal seam under a pressure close to the rock pressure. An oriented system of cracks, which forms a coupled system of filtration channels, can be formed by means of directed unloading of the rock pressure from the seam due to expanding gas energy. The parameters of the manufacturing effect on the seam are determined based on physical modeling of actual mechanical and filtration processes using the experimental installation of truly three-axial loading and mathematical modeling.

Deformation of a Coal Seam with a System of Isolated Gas-Filled Fissures

The paper deals with the problems on mechanics of material with gas-filled fissure-like discontinuities, considered in terms of coal, rocks and gas outbursts in coal mines.

Dynamics of Gas Release from a Coal Seam in Driving a Working

Previously developed conceptions of evolution and origination mechanism of filtration process in a coal seam are used in mathematical modeling of driving a working. It is shown that the rate of change in gas release from the surface of the face is the parameter that gives the most reliable information about the degree of outburst hazard. Uninterrupted rate measurement can be the foundation of a new method for operative control of outburst hazard.