Effect of Annulus Phase Transition on Equivalent Circulating Density in Deepwater Hydrate Formation Drilling: Analysis of Hydrate Trial Production Well Drilling in the South China Sea

Summary Caused by the weak cementation of the reservoir rock, the safe pressure window for drilling in deepwater hydrate formations is narrow. However, during drilling in different layers, such as the hydrate-bearing layer (HBL) or three-phase layer (TPL), the migration and decomposition of hydrate cuttings and the generation of hydrates through gas phase transition result in complex multiphase flow in the annulus. This poses a challenge to the accurate prediction and control of equivalent circulating density (ECD). To study the effects of annular hydrate phase transition on ECD, we constructed a gas-liquid-solid transient flow model, taking into account factors such as drilling fluid salinity, interphase heat and mass transfer, hydrate cuttings migration and decomposition, and gas phase transition. We then compared the feasibility of the model against some drilling data from the second hydrate trial production well in the South China Sea, and used the model to analyze the influence of hydrate phase transition on ECD during the first and second hydrate trial drillings in different layers. The results indicate that the annular hydrate phase transition during riser drilling can reduce the ECD, and the maximum reduction in ECD at bottom under calculation conditions was 48.36 kg/m3. However, the effect of annulus phase transition on ECD is small in riserless drilling. Under the calculation conditions, the maximum increase in ECD at bottom was 3.09 kg/m3, caused by gas phase transition to hydrate when drilling in a TPL, and the maximum decrease in ECD at bottom was 6.72 kg/m3 because of hydrate cuttings decomposition when drilling in an HBL. ECD prediction is useful in the planning phase, and the proposed model is meant to provide a quantitative reference for understanding the magnitude of ECD variations under different conditions.