Hydrate Formation And Flow Behavior Of Hydrate Slurry After Gas Intrusion In Deepwater Wellbore

In the development of deepwater oil and gas resources,problems such as low temperature near the mudline,low temperature and high pressure of the shallow strata,shallow gas,shallow hydrates,and narrow safety density window in the deep strata may lead to the gas intrusion from formation to the drilling wellbore.Hydrate phase transition is induced in the wellbore at the same time,which makes it difficult to predict and control the wellbore pressure.Meanwhile,in recent years,the test exploration technology of natural gas hydrates in the sea area has continued to progress.But,low temperature and high pressure of hydrate layer,poor stability of hydrate layer,weak cementation of reservoir and other factors still trigger the reformation of hydrate in horizontal drilling and production wellbore,which increases the difficulty of wellbore pressure management and induces high risk of wellbore flow assurance.Therefore,in order to prevent the hydrate risk in deepwater drilling and offshore natural gas development,it is necessary to study the mechanism of hydrate formation in deepwater drilling wellbores,the influence of hydrate formation on drilling fluid rheology,and the wellbore multiphase flow behavior with hydrate phase transition,based on the existing researches.This paper provides the theoretical basis for the development and control of methane hydrate in deepwater drilling and the optimization of deepwater drilling construction plans.By combining experimental research and theoretical modeling,the methane hydrate formation experiments with different void fractions,flow rates and drilling fluid additive concentrations under bubbly flow conditions were carried out.It was found that gas-liquid mass transfer is the main controlling factor for hydrate formation.The hydrate formation under bubbly flow shows nonlinear relationship with the time.The effects of bubble dynamics,bubble-turbulent swirl collision,void fraction,flow rate,hydrate concentration,and drilling fluid additive concentration on gas-liquid mass transfer and gas-liquid interface distribution are also analyzed.Based on the mass transfer theory,a comprehensive hydrate formation mass transfer coefficient is proposed,and a mass-transfer limited hydrate formation model for bubbly flow is constructed to take into account the influence of multiple factors and the rheological model of coupled continuous-phase fluids,respectively.We carried out rheological experiments on hydrate slurry in pure water and water-based drilling fluid and investigated the influence of hydrate concentration,xanthan gum(XG)concentration,and carboxymethyl cellulose(CMC)concentration on the rheology of hydrate slurry.The relationship between hydrate slurry shear rate and strength and apparent viscosity is established.The hydrate slurry in this paper is a power-law fluid.In the bubbly system,as the shear rate of hydrate slurry grows from low to high,the hydrate slurry is gradually transformed from a shear-thinning fluid to a shear-thickening fluid due to the interaction between hydrate particles.At low shear rates,the hydrate slurry shows shear-thinning.At high shear rates,it shows shear thickening.The apparent viscosity of the hydrate slurry increases as the hydrate concentration increases.With the addition of xanthan gum and carboxymethyl cellulose,the hydrate slurry shows shear-thinning.The increase in hydrate and additive concentration leads to an increase in the shear stress and the apparent viscosity of the hydrate slurry.Based on the non-Newtonian HB model and experimental data,a hydrate slurry rheology model considering hydrate concentration,xanthan gum concentration,and carboxymethyl cellulose concentration was constructed,respectively.The hydrate formation experiments under bubbly flow and hydrate slurry rheology experiments were carried out to reveal the influence of the interaction behavior of fluid-pipe wall,hydrate particles,hydrate particles-pipe wall and hydrate particles-fluid on the pressure drop of hydrate slurry flow.A model for calculating the flow pressure drop of bubbly hydrate slurries under both laminar and turbulent flow modes is presented.It is found that under laminar flow conditions,resistance of the fluid-wall and liquid-solid recycling flow are the main factors contributing to the hydrate slurry flow pressure drop,accounting for about 40%-95% and 5%-60% of the total flow pressure drop,respectively.Under turbulent flow conditions,resistance of the fluid-wall and collision of hydrate particles are the main factors contributing to the pressure drop of hydrate slurry flow,accounting for about 49%-64% and 35-49% of the total flow pressure drop,respectively.When the flow rate is too low,the carrying capacity of fluid for hydrate particles is reduced.The hydrate slurry begins to show stratified flow,which gradually increases the absolute calculation error of the model.Thus,the calculation method of the critical flow rate of hydrate slurry is proposed for predicting the risk of hydrate flow assurance in deepwater wellbore,which provides a theoretical basis for establishing a hydrate risk management scheme in deepwater drilling wellbore.