Theoretical analysis of the influence of drilling compaction on compressional wave velocity in hydrate-bearing sediments

Borehole wall would be disturbed during sampling when drilling into gas hydrate-bearing sediments, thereby changing the reservoir physical characteristics obtained by well logging. It would cause significant errors using such logging data to estimate the reservoir parameters such as hydrate saturation and mechanical properties. In this paper, the theoretical method of combination cavity expansion theory with the time-average equation to predict the compressional velocity (V-p) of hydrate-bearing sediment after drilling disturbance were proposed based on the hypothesis that the compaction of hydrate-bearing sediments only causes the porosity change in sediments. The theoretical derivation process of calculation the porosity change caused by drilling compaction based on both Tresca's yield criteria and Mohr-Coulomb yield criteria were presented in detail. And the influence of drilling compaction on V-p were discussed by theory method developed in this paper and experimental method. Both theory-based results and experimental results show that the V-p difference caused by drilling compaction is considerable. Drilling compaction would cause the increase in V-p, and the increment increase with the increase of bit weight, and samples with low hydrate concentrations would be more sensitive to bit weight variations. The proposed method is feasible to simplify the influences of complex processes involved in the disturbance of hydrate-bearing sediments during drilling.