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Petroleum Exploration and Development, Volume 41, Issue 2, 2014, Pages 249-256.
Tianshou MA, Ping CHEN.WooCommerce
State Key Laboratory of Oil & Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chendu 610500, China.
Abstract
In view of the shale hydration problem in wellbore stability analysis during drilling in shale formation, a quantitative evaluation method based on CT scanning technology was proposed to research the meso-damage characteristics of shale hydration, and the CT scanning tests for shale samples in stages of shale hydration were conducted. The analysis results of CT images and their gray-level histograms show that the early period of soaking is the main period of meso-damage in shale samples, and as the soaking time increases, the meso-damage propagates slowly; the mark of meso-damage increasing dramatically is that the gray-level histogram of the CT image changes from unimodal style to bimodal style. The visual resolutions of CT images can be improved by the pseudocolor enhancement technique, and the integrity and damage degrees of CT images provide a convenient way for quantitative analysis. The relationship between damage variable and soaking time was obtained based on the segmentations of CT images, which shows that the meso-damage of shale hydration mainly occurs in the early stage of soaking, which is the initial stage and rapid evolution stage of meso-damage, from then on, as the damage variable increases continuously and slowly, the macro-damage of shale sample occurs.
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Significance Statement
In view of the shale hydration problem in wellbore stability analysis during drilling in shale formation, a quantitative evaluation method based on CT scanning technology, damage mechanics theory and digital image processing technology was proposed to research the meso-damage characteristics of shale hydration, and the CT scanning tests for shale samples in stages of shale hydration were conducted, on this basis proposed a novel damage constitutive model of shale rock after hydration, this model can simulate the complete stress-strain curve after shale hydration, which formed the theoretical method to quantitative descripe the mechanical effects caused by chemistry reaction for wellbore stability analysis. The analysis results of CT images and their gray-level histograms show that the early period of soaking is the main period of meso-damage in shale samples, and as the soaking time increases, the meso-damage propagates slowly; the mark of meso-damage increasing dramatically is that the gray-level histogram of the CT image changes from unimodal style to bimodal style. The visual resolutions of CT images can be improved by the pseudocolor enhancement technique, and the integrity and damage degrees of CT images provide a convenient way for quantitative analysis. The relationship between damage variable and soaking time was obtained based on the segmentations of CT images, which shows that the meso-damage of shale hydration mainly occurs in the early stage of soaking, which is the initial stage and rapid evolution stage of meso-damage, from then on, as the damage variable increases continuously and slowly, the macro-damage of shale sample occurs. The simulated complete stress-strain curve by damage constitutive models of shale rock coincided basically with the experimental results, this model could accurate describe the mechanical effects produced by shale hydration. This method provides a new method and thought for quantitative analysis of shale hydration and mechanics-chemistry coupling of wellbore stability analysis.
Figure Legend
Schematic drawing of damage characteristics analysis for shale hydration.
