Experimental Study of Rock Creep under True Triaxial Loading
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perimental Study of Rock Creep under True Triaxial Loading V. I. Karev1* , D. M. Kilmov1 , Yu. F. Kovalenko1 , and K. B. Ustinov1** 1
Ishlinsky Institute for Problems in Mechanics of the Russian Academy of Sciences, pr. Vernadskogo 101, str. 1, Moscow, 119526 Russia Received July 28, 2019; revised August 1, 2019; accepted August 30, 2019
Abstract—The aim of this sutdy is to research the time dependence of deformations under complex stress conditions arising in the ground formation during the exploitation of oil and gas fields. The experiments were carried out on the IPMech triaxial independent loading test system on the rocks of the Prirazlomnoye oil field using loading programs simulating the stress state in the near-borehole region with a decrease in pressure in the well. Experimental dependences of deformations on time during step loading are presented. The basic requirements for constructing a model of a stress-strain state taking into account the influence of time effects are discussed. DOI:
Keywords: rocks, inelastic deformation, creep, true triaxial tests.
1. INTRODUCTION Strength characteristics of materials significantly depend on the characteristic time spent under load with their increase, the strength decreases. This property is caused by softening processes that develop over time. These or other processes lead to the development in time of irreversible deformations, usually called creep deformations. Taking into account the phenomenon of creep of rocks is of great importance in predicting the longterm strength of boreholes, especially in the case of an open hole. If the rock in the vicinity of the uncased borehole is able to deform under constant pressure at the bottom of the borehole, then the rock will gradually extrude into the borehole, which may ultimately lead to the destruction of the borehole. There are many models for describing deformation that develops over time: hereditary elasticity, the creep theory, main varieties of which are theories of aging, flow, and hardening [1–4]. Fairly complete reviews are presented in [1–6]. Among the monographs devoted to the study of creep of rocks, [7, 8] should be noted. However, not all models seem suitable for describing creep deformation together with plastic deformation under conditions of a complex three-dimensional changing stress state; first of all, this applies to models such as Maxwell, Feucht and others, obtained by combining standard elastic and viscous elements. Many creep theories are constructed by analogy with the theory of plasticity, the functions used in them (plastic potential, plasticity modulus) become explicitly or implicitly timedependent [2, 3]. It is such theories that are most suitable for the set goals. Unfortunately, all creep theories, without exception, are built on the basis of speculative conclusions, are formal in nature and contain a large number of parameters or material functions to be determined. As noted in [2], in the absence of the necessary amount of experimental data, it is not possible to give p
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