Abstract
The rock mechanical behavior and damage characteristic is of great importance for in situ stress evaluation, wellbore stability analysis and hydraulic fracturing design. The velocities of elastic waves are usually reduced in the presence of rock damage, it may be used for determining the progressive damage of the rock. Therefore, this paper aims to investigate the damage characteristics of transversely isotropic tight sand formation, the rock mechanical and damage parameters in the vicinity of the wellbore were calculated using acoustic logging data. The results indicated that the Poisson's ratio and damage parameters decrease with increasing in radial distance, while the elastic modulus and Thomsen’s coefficients increase. At the same radial position, the vertical elastic modulus is smaller than that of the horizontal, the degree of anisotropy for P-wave is greater than that of S-wave, and the horizontal damage parameter is greater than that of the vertical, which indicated that the micro-cracks near the wellbore mainly occur in the horizontal direction. The changes in mechanical parameters, Thomsen’s coefficients and damage parameters rapidly changed in the range of 1.0–1.8 times of borehole radius. The variations of Thomsen’s coefficients and damage parameters in mudstone are obviously greater than that of sandstone, which may be due to the induced damage between rocks and drilling fluid of mudstone is much higher than sandstone.
Highlights
Tight sand is a kind of very important unconventional oil and gas reservoir, it is the highest class of unconventional oil and gas resources that developed all of the world (Zou et al 2018)
It has obviously been noticed that both vertical and horizontal damage parameters decreased with the radial distance, and the induced damage of α11 increased to 1.59 × 10−4GPa, α33 increased to 2.69 × 1 0−3GPa, In the depth of 4284 m, the formation lithology is mainly mudstone, and the original rock mechanical parameters explained by acoustic logging data are given as follows: μv = 0.308, μh = 0.205, Ev = 33.67GPa, and Eh = 51.07GPa
Almost all of the changes in mechanical parameters, Thomsen’s coefficients and damage parameters mainly occurred in the scope of 1.0–2.8 times of hole radius, and all parameters rapidly changed in the range of 1.0–1.8 times of hole radius
Summary
Tight sand is a kind of very important unconventional oil and gas reservoir, it is the highest class of unconventional oil and gas resources that developed all of the world (Zou et al 2018). In order to evaluate rock damage characteristics, the most useful way is to direct measure rock mechanical behaviors (Sayers and Kachanov 1995; Eberhardt et al 1999; Sayers 1999; Chang and Lee 2004; Chaki et al 2008; Ma and Chen 2014; Xue et al 2014; Zhang et al 2015; Ma et al 2016; Yang et al 2018; Zhou et al 2019; Cheng 2019; Chu et al 2020; Zhu et al 2019; Liu et al 2020; Song et al 2020), the uniaxial compression tests, triaxial compression tests, ultrasonic wave tests, acoustic emission tests, CT scans, and NMR tests were utilized to characterize rock damage Most of these studies indicated that rock damage is related to micro-cracking, the higher the degree of rock damage, the much more micro-cracking occurred in rock specimen. The results can help us understand the distribution and evolution of rock mechanical properties, anisotropy and damage characteristics in the vicinity of the wellbore, and it can provide some theoretical basis for in situ stress evaluation, borehole stability analysis and hydraulic fracturing design. Due to the symmetry of stiffness matrix (Cijkl), the unknown elements in the stiffness matrix (Cijkl) can be reduced from 81 to 21, and Eq (1) can be expressed as:
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