Effect of Water-Induced Rock Softening on Rock Anisotropy During Drilling Process

Rocks have a softening effect under the long-term physical and chemical action of water, resulting in a reduction in the strength of rock and a weakening of the safety performance of project, which may have destructive effects for the wellbore instability, borehole contraction, leakage, and reservoir protection at a certain time. To investigate the effect of water-induced rock softening on its anisotropy for reservoir protection, the digital experiment was conducted on three types of rocks with four different saturation levels in five different directions. A digital drilling response model was employed to determine the drilling strength and specific energy at the critical point. The drilling parameters of the three types of rocks were recorded to calculate the softening coefficient and the anisotropy of the softening coefficient. The reliability of the proposed anisotropy of the softening coefficient was verified by introducing strength parameters based on the water-induced rock softening. The results show that the softening coefficient decreased with increasing saturation levels. The highest decrease with 43% was observed in mudstone. The anisotropy of the softening coefficient increased with an increasing saturation levels. The highest increase with 12% was observed in mudstone. Both fine-grained sandstone and mudstone exhibited the phenomenon of strength rebound with increasing saturation levels. A comparison with the previous studies reveals that this method can provide a reliable basis for determining the degree of rock softening using drilling strength.