Research on evaluation and prevention of hydrate formation and blockage risk in wellbore during deepwater gas wells drilling

The hydrates formation and blockage in wellbore is directly related to the safety of deepwater gas well drilling. However, the current research on hydrates risk prevention during deepwater drilling is still insufficient. In this work, a novel hydrate formation-blockage risk assessment method in wellbore is proposed. Firstly, a new hydrate formation region (HFR) prediction model consists of mass, momentum and energy balance equations was established and was verified on a deepwater gas well in South China Sea. Secondly, the wellbore blockage risk in different drilling stages is evaluated and the prediction results were in good agreement with field results. Finally, economical-effective hydrate inhibition measures are proposed to ensure the safety and efficiency of deepwater drilling. The results show that the increase of gas invasion rate, drilling fluid injection temperature and drilling fluid circulation rate are helpful to shorten HFR. In this study, the total length of HFR can be reduced by 59.26% when the gas invasion rate increases from 0 to 14 L/s, while the total HFR can be reduced to 74.93% when the drilling fluid circulation rate increases from 0.01 to 0.09 L/s. Increasing drilling fluid circulation temperature is the most direct and effective measure to reduce HFR and the HFR in annulus will disappear when drilling fluid circulation temperature rises above 36 degrees C. Due to the limitation of gas hydrate induction period and multi-phase flow characteristics, the wellbore plugging risk in drilling fluid circulation stage is very low, while the hydrate blockage risk in well shut-in stage is high relatively. Meantime, properly increasing drilling fluid circulation rate and injection temperature, choosing appropriate drilling balance mode during drilling fluid circulation stage, reasonable controlling the wellbore annulus pressure, optimizing shut-in time and injecting appropriate inhibitors during well shut-in stage can effectively ensure the completion of safe-economic-efficient deepwater gas wells drilling.