Study of mud weight window of horizontal wells drilled into offshore natural gas hydrate sediments

Abstract

Drilling horizontal wells into natural gas hydrate (NGH) sediments may be one of the most promising methods to increase the cost effectiveness of the exploration of NGH. However, many challenges, such as wellbore collapse and wellbore fracturing, will be encountered. Therefore, an accurate prediction of the mud weight window (MWW) is very important. In this work, a model was built to calculate the MWW of horizontal wells drilled into offshore NGH sediments, considering the NGH saturation and the effects of inlet parameters of the drilling mud, and was combined with the transient heat transfer model to calculate the temperature distribution of the borehole. A case study was carried out, and the effects of several parameters were discussed: 1) When drilling horizontal wells in NGH sediments, the dissociation of NGH in the formation will not occur. 2) With increasing circulation time, the borehole will be cooled, and the cooling effect will lead to a decrease in both the fracture pressure and collapse pressure of the target zone. 3) The mud flow rate has a large influence on the mud temperature profile, especially in the horizontal section, and a higher mud flow rate leads to a lower fracture pressure and collapse pressure in the target zone. 4) The inlet temperature has limited influences on the mud temperature profile in the horizontal section and the MWW of the target zone. 5) The saturation of NGH in a formation has a significant influence on the collapse pressure but little influence on the fracture pressure, and higher saturation will lead to a wider MWW. This study will support the safe control of well stability and application of horizontal wells drilled into offshore NGH sediments.