Fundamental Study On The Reformation Of Hydrate Reservoirs By CO₂ Injection

A new idea "reservoir reformation" is proposed,i.e.,reforming natural gas hydrate reservoir by constructing an artificial impermeable overlying CO₂ hydrate cap.In this work,the slow hydrate formation and accumulation in sediments from dissolved gas was firstly studied to obtain the physical properties of hydrate reservoir with discontinuous hydrate distribution,such as hydrate occurrence,spatial distribution,electrical and acoustic properties.After that,depressurization production from the prepared hydrate bearing sample was carried out.The gas and water production behaviors were studied as well as the spatial water saturation in the sediments during hydrate dissociation.The key factors for the low efficiency of depressurization were analyzed.To optimize the CO₂ injection technology,kinetic study of hydrate formation in sediments was conducted.Finally,the reformation-production of hydrate reservoir was simulated to verify the feasibility of the proposed technology.The gas production efficiency and the stability of the CO₂ hydrate cap during depressurization were mainly concerned.The detail contents of this work were as below:Slow hydrate formation and accumulation process from dissolved gas in nature was simulated in large scale sandy sediments using a three dimensional hydrate simulator?TDHS?.Low hydrate formation rate?0.37%pore volume/day?was obtained by controlling the supersaturation of dissolved methane and migrating velocity of pore fluid to very low level.Electrical and acoustic measurements were applied to track hydrate formation in sediments.A dynamic hydrate evolution process was captured i.e.crystallization-migration-accumulation-recrystallization.This evolution process has a critical effect on the connectivity of pores and the strength of sediments frame.After experiencing the dynamic evolution process,stiff frame supporting hydrate and patchy hydrate were the final morphologies in pores,and variation of acoustic velocity indicates that frame supporting was the dominated morphology in pores.Based on this,hydrate saturation was inversed from resistivity and acoustic velocity by corresponding models for frame supporting hydrate.The calculated results show that value order of hydrate saturation at different sites from resistivity were very consistent with that from acoustic velocity though overestimated hydrate saturation was obtained from resistivity inversion.Analysis of spatial hydrate distribution shows that hydrate distribution in the sediments was discontinuous.Field of concentration,temperature and fluid velocity play key role on this discontinuity.The discontinuity in hydrate distribution is suggested to be considered in future experimental and numerical studies.Depressurization production from the prepared hydrate sample with high water saturation and discontinuous hydrate distribution was carried out.It was found that the resistivity of the hydrate bearing sediments was mainly controlled by the saturation of accumulated gas phase and the residual hydrate saturation in the pores.During depressurization,water and gas were stimulated to redistribute in the sediments.The efficiency of the whole hydrate dissociation process was restricted by the "water invasion "and "water lock "effects.The overlying water migrated to the hydrate dissociation zone,which caused the main product at the initial production stage was water.And part of the released gas during hydrate dissociation was trapped in the pores of sediments.Rapid gas production just proceeded intermittently in a short time.Based on the analysis of water distribution in the sediments during depressurization,the gas transported ways were found to be restricted to a narrow zone,which also decreased the production efficiency.The variation of resistivity in the sediments indicated that the sites with high hydrate saturation were affected weakly by the water invasion,but theses with low hydrate saturation were vulnerable.It was also observed that water lock effect was more remarkable at the sites far away from the production port,a large number of gas was being trapped at that sites during the whole depressurization process.Several factors were investigated to understand their contribution to the formation kinetics of CO₂ hydrate in sandy sediments.The rates of hydrate formation were increased with the decrease of temperature.But when the subcooling was high,rapid hydrate formation increased the resistance of mass transfer,which lowered the hydrate formation rates.Higher conversion ratios of water always were obtanbined with the lower temperature.Formation rates from the gaseous CO₂ with low pressure were far slower than that from the liquid phase with higher pressure.Lower initial water saturation always obtained faster initial rates of hydrate formation and higher water conversion ratios.Sands with smaller size were favorable to the hydrate formation,but water in the sands with higher size can be converted to hydrate completely.With low pressure,hydrate formation in the sand beds was remarkably inhibited by the low dosage of inhibitor,but with the high initial pressure,the inhibitor had weak effect on the hydrate formation with high driving force.Typical experiments were carried out in a vertical one-dimensional visual hydrate simulator?ODVHS?to verify the feasibility of this technology.Tests on hydrate dissociation by depressurization with and without artificial CO₂ hydrate cap were conducted.The results indicate that depressurization induces the invasion of overlying seawater into CH₄ hydrate bearing zone without CO₂ hydrate cap.After reformation by CO₂ injection to the permeable overburden,a confined environment with an impermeable CO₂ hydrate cap is constructed successfully for depressurization operation.The artificial cap can maintain mechanical stability for enough long time until the finish of the production process.With the protection of the artificial CO₂ hydrate cap,the production efficiency is greatly improved and the water yield is decreased remarkably.The CO₂ hydrate cap experiences a dynamic evolution process:thickening-degeneration-deformation.Downward migration of residual CO₂ in the cap thickens the initial cap,but propagation of CO₂-CH₄ double hydrate in the depleted CH₄HBZ and CO₂ breakthrough degenerate the cap gradually,long-term process of dissolution-diffusion deforms the cap eventually.Certain minimum thickness of initial CO₂ hydrate cap with certain CO₂ hydrate saturation is suggested to be achieved before depressurization by optimizing the injection condition.Fractal models were built to predict the relative water permeability of the hydrate bearing sediments.In the models,the hydrate distribution,pore habits,pore structure of the sediments and hydrate formation history were considered.In the fractal models for homogeneous hydrate distribution,the prediction results were just satisfying when hydrate saturation was low.And the fractal models for the heterogeneous distribution obtained precise predictions.For the hydrate bearing sediments cored in natural and made in laboratory from excess water,the large pore filling model gave precise predictions.Relative water permeability of hydrate bearing sediments formed in the presence of free gas was found to be consistent with the predictions of large pore coating models.The large pore coating model successfully interpreted the shift of permeability of hydrate bearing sediments formed from excess gas.The pore habit transition was concluded as the result of the large pore occupancy and the increase of heterogeneity in hydrate distribution.It was also found that the permeability of hydrate bearing sediments formed from excess water could be predicted by the large pore coating model at low hydrate saturation.