Abstract
Wellbore instabilities frequently occur in mudstone formation with weak plane bedding because of strong anisotropies. The mechanics parameters of weak plane bedding are of vital significance to the wellbore stability analysis for mudstone formations. The conventional method for determining the mechanics parameters is to fit lots of triaxial test data due to the blindness of coring. In this paper, an evaluation method of the mechanics parameters of weak plane bedding is proposed to improve the accuracy of weak plane bedding mechanical properties. The mechanics parameters of weak plane bedding are obtained by combing the single-weak plane failure criterion with the compressive strength of rock obtained by the triaxial test of cores with different coring angles. It is seen that the new evaluation method is simple and convenient. On the other hand, a validation method of the mechanics parameters of weak plane bedding is proposed to ensure their accuracy. The compressive strength obtained from the core with the special coring angle is compared with the theoretical compressive strength for verifying the accuracy of weak plane bedding mechanical properties. It is observed that the proposed evaluation and validation methods can be used to measure the value of weak plane bedding mechanical properties precisely. The proposed methods are general and can be used for measuring the mechanical properties of fracture weak-plane and joint weak-plane.
Highlights
The anisotropic rock formed by the weak plane can be weaker than the intact rock
If the value of the compressive strength from the triaxial test is close to that from the theoretic calculation of the weak plane failure criterion, the previous cohesion and friction angle of the weak plane got by the method of evaluating the weak plane mechanical properties will be accurate
We choose to conduct the triaxial test for the rock specimens with θ = 30°
Summary
The anisotropic rock (mudstone, shale, etc.) formed by the weak plane (bedding, joint, etc.) can be weaker than the intact rock. Since its first introduction by Jaeger [2] in extending the earlier work of Bott [1], the single-weak plane failure criterion has been used successfully in evaluating wellbore stability [5,6,7,8,9,10,11,12,13]. In this criterion, the failure of both the bedding planes and rock matrix is described by using the Mohr-Coulomb criterion by two different sets of intact rock and weak plan constants. Heng et al [17] analyzed the mechanical properties of the bedding plane based on the experimental data of shale with four different α, where α is the angle
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