Novel Method for Determining the Safe Mud Temperature Window in Marine Hydrate-bearing Sediments
The hydrate deposits in the Shenhu area in the north of South Sea typically exhibit poor cementation as well as lowest compressive strength and such geomechanical issues as borehole instability easily occur during the development operations.This paper builds up a multi-physics coupled mathematical model to perform numerical analysis of wellbore stability during drilling in hydrate reservoir,discusses the impact of drilling fluid temperature on wellbore stability,proposes a method for determining the safe mud temperature window considering moderate borehole collapse,and obtains a recommended design chart for the safe mud temperature and density.It finds that the dissociation ranges of gas hydrate obtained by the mathematical model in this study closely aligns with the results in previous investigation though they are a bit different indicating the mathematical model is suitable for simulating wellbore stability in hydrate-bearing sediments,drilling fluid with higher temperature stimulates the dissociation of gas hydrates around the wellbore,thereby deteriorating the stability of wellbore,the borehole enlargement rate increases from 4.2％to 42.6％as the mud temperature rises from 14.79℃to 24.79℃,and the recommended area for drilling fluid density and temperature design values significantly increase with the increase of the allowable borehole enlargement rate(the decrease in requirements for wellbore stability).The research results not only provide technical support for the design of drilling fluid for hydrate reservoirs,but also a new idea for maintaining wellbore stability.
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