Mechanical study on the wellbore stability of horizontal wells in natural gas hydrate reservoirs

Currently, the exploration of natural gas hydrates (NGHs) is regarded as a worldwide research focus. Due to multiple characteristics of NGH reservoirs (such as weak bonding, low strength and high degradability), wellbore instability is extremely likely to occur during drilling operations. Based on the thermodynamic characteristics of NGHs, a heat-fluid-solid coupling numerical model of horizontal wells in NGH reservoirs that considers the decomposition of hydrates was established. Additionally, the evolution of stress fields and plastic zones induced by changes in the temperature-pressure field and the mechanical properties of NGHs before and after their decomposition were analyzed. The results showed that stress was reduced in the zone where NGHs were completely decomposed, due to the weakening of elastic parameters, and that the strata close to the wellbore were in a state of plastic flow because of stress concentration there. Moreover, the stress was dramatically increased in the transition zone for the decomposition of NGHs due to the change in the mechanical properties of the strata. Hence, the secondary stress concentration was formed in the strata where NGHs did not undergo decomposition, and part of this strata would be subjected to plastic deformation. With the further decomposition of NGHS, the extent of the strata with completely decomposed NGHs in the plastic stage was also increased, and the secondary stress concentration effect formed in the zone with NGHs that did not undergo decomposition, in the strata far from the wellbore. If NGHs were decomposed under the selected drilling conditions, the small wellbore was more affected by the decomposition of hydrates, with a greater risk of collapse. Moreover, the effect of the wellbore size became more significant with increased drilling time; thus, selecting a large wellbore for drilling was more favorable for the stability of the wellbore.