Numerical insights into factors affecting collapse behavior of horizontal wellbore in clayey silt hydrate-bearing sediments and the accompanying control strategy

Gas hydrates are a promising unconventional gas resource that can alleviate energy tension. Unfortunately, borehole collapse is one fatal geomechanical issue restricting its effective development. However, effective engineering measures have not yet been proposed in previous investigations to mitigate or avoid it. In the present work, a multiphysics coupled mathematical model for investigating borehole stability in hydrate-bearing sediments was developed and numerically solved. With the help of the Galerkin algorithm, the weak integral forms of the control equations for various physical fields were obtained. Meanwhile, applicability of the established model is verified by comparison with previous studies. The comparison results show that the difference between the two is small, and it can be almost ignored. This means that the mathematical model and investigation methodology presented in this study are feasible for simulation of wellbore stability in hydrate-bearing sediments. Besides, factors affecting wellbore stability in hydrate deposits were numerically investigated. It was found that increase in mud density or hydrate saturation is beneficial to maintaining wellbore stability in hydrate-bearing sediments. However, to achieve the similar results, it is necessary to lower both the mud temperature and salinity, shorten the drilling cycle, and reduce stress difference. Based on the simulation results of borehole collapse, the recommended design values of mud temperature and density for different acceptable borehole enlargement rates are obtained. The results reveal that the safe mud weight and temperature window both widen with the increase of mud salinity, and hydrate saturation for the same acceptable borehole enlargement rate. However, the safe mud weight and temperature window both narrow with the increasing soaking time and difference between in-situ stresses. Investigation herein provides technical support for implementation of safe and efficient drilling operation in hydrate-containing sediments.