Stress Inversion of Coal with a Gas Drilling Borehole and the Law of Crack Propagation

Abstract

Abstract:For studying the law of crack propagation around a gas drilling borehole, an experimental study about coal with a cavity under uniaxial compression was carried out, with the digital speckle correlation method capturing the images of coal failure. A sequence of coal failure images and the full-field strain of failure were obtained. The strain softening characteristic was shown by the curve. A method of curve dividing-named fitting-damaging-was proposed, combining the least square fitting residual norm and damage fraction. By this method, the five stages and four key points of a stress-strain curve were defined. Then, the full-field stress was inverted by means of the theory of elasticity and the adjacent element weight sharing model. The results show that σ ci was 30.28-41.71 percent of σ f and σ cd was 83.08-87.34 percent of σ f , calculated by the fitting-damaging method, agreeing with former research. The results of stress inversion showed that under a low stress level (0.15 σ f < σ < 0.5 σ f ), microdamage evolving into plastic failure later was formed around the cavity. Under a high stress level (0.5 σ f < σ < 0.85 σ f ), the region of stress concentration suddenly crazed and formed a brittle crack. When σ ≥ 0.85 σ f , the crack was developing, crack lines were connecting with each other, and the coal finally failed. The outcome of the stress inversion was completely concomitant with the images of crack propagation. Additionally, the stress around the cavity was able to be calculated accurately.

Wave Velocity and Power Spectral Density of Hole-Containing Specimens with Different Moisture Content under Uniaxial Compression

“…The wave velocity change has three stages during loading, which are stability (I), slight decrease (II), and rapid decrease (III), as shown in Figure 6. This result is in good agreement with stages of surface deformation by Zhang [1]. The surface deformation was divided into 4 stages during the failure of specimens, which are elasticity, crack initiation, crack penetration and specimen failure.…”

“…The wave velocity change has three stages during loading, which are stability (I), slight decrease (II), and rapid decrease (III), as shown in Figure 6. This result is in good agreement with stages of surface deformation by Zhang [1]. The surface deformation was divided into 4 stages during the As shown in Figure 5, the shapes of the waveform from 0 to 12 kN are same, and there is only a delay of 1 to 2 µs, indicating that the waveform collects well, and the damage of the specimens at this stage is little, and the influence on the ultrasonic propagation is slight.…”

“…While the low moisture content specimens are mainly concentrated among 1 × 10 3 Hz and The power spectral density shifts downward, which means the ultrasonic waves are absorbed inside the specimens. At this time, the specimen is below 80%σ p , which corresponds to the elastic phase and the crack initiation stages described in Zhang [1]. In these two stages, the tiny crack inside of specimen is emerging.…”

“…By studying the failure process of the hole-containing specimens, the damage area around the hole can be obtained, and the failure mechanism of the hole-containing specimens can be revealed, which provides theoretical guidance for underground gas drainage drilling engineering. During the destruction of the hole-containing specimens, many cracks form around the hole [1].…”

Back Analysis of the Initial Geo-Stress Field of Rock Masses in High Geo-Temperature and High Geo-Stress

“…Moreover, because of difficult downhole conditions and high costs [14], measured in situ stresses are usually insufficient, such that it is difficult to use the measured in situ stresses to express the macroscopic distribution of the initial geo-stress field of rock masses. As a result, in order to obtain the initial geo-stress field with macroscopic distribution, it is of great theoretical significance to invert the initial geo-stress field of rock masses [15][16][17]. In summary, the initial geo-stress field of rock masses is very important to underground engineering projects.…”