Process Analyses And Numerical Models For Multiphase Flow And Phase Change In CO₂-ECBM Engineering

Injection of CO₂ into deep unminable coal seam（CO₂-ECBM）,achieving the goals of carbon dioxide geological sequestration and increasing coalbed methane recovery attracts more and more people’s attention.Previous studies mostly focused on the competitive adsorption of gases,relative permeability,adsorption induced permeability change,etc.As a fact,the depth of coal seam is usually relatively shallow,mostly within 1 km,likely lead to phase change of multi-components（CO₂/CH4 etc.）.Wellbore acts as an indispensable part for any geological engineering,where the phase change could occur.There is not so much research on it.In this study,we discuss the multiphase flow in coalbead and phase change behavior based on program development,field trial,results analysis.There are three difficulties to analyze multiphase flow and phase change behavior: 1)mutual dissolution of multi-component;2)phase change;3)wellbore-reservoir coupling.We solve them by program development.ECBM module is introduced to describe gas/aqueous balance of multi-components system and ECO₂ M module is used for phase change of CO₂.We incorporate ECO₂ M into ECBM to realize the simulation need.For the wellbore-reservoir coupling,different governing equations are applied for different sub-domains,whereas only one Jacobian matrix is setup to be solved.Appling the program developed into a field trial,to analyze the field-scale multiphase flow and phase change behavior.From 2011 to 2012,CSIRO performed a CO₂-ECBM field trial in Shanxi,China in collaboration with China United Coalbed Methane（CUCBM）.Only one vertical well was drilled for injection and production.The field trial includes “production-injection-production” three stages.A multi-lateral horizontal well with 2.3 km in seam length was used for CO₂ injection.Two pulse of non-adsorbing tracer gas was injected with CO₂ at the start and towards the end of the injection period.A U-tube sampling system was installed in a monitoring well 25 m from the main horizontal branch for observing the migration of CO₂ and tracer.To improve the efficiency of injection and transport,CO₂ was injected in liquid state,under the function of gravitational potential energy work and heat transfer from surrounding formation,it will cause phase change.What’s more,the injection is episodic,lots of injection-shut in operation make it more complex.Local phase changes may bring redistribution of pressure and temperature along the whole wellbore,which in turn caused phase change of other areas.Phase changes affected the temperature and pressure in both the wellbore and reservoir,thus affecting CO₂ injection and production efficiency.This investigation is on the basis of field trial,by adopting the combination of program development and numerical models.A wellbore-reservoir coupling model is built.By comparing simulation results to measured data,to deduce the physical properties of reservoir and to get a more deep understanding of multiphase flow and phase change behavior.At last,a three-phase drift flux model was derived to analyze CO₂ leakage rate and phase change process along the wellbore under different governing equations,initial/boundary conditions.Simulations results show that,for temperatures and pressures at the injection wellhead and bottom,they fit well at the early stage,then slightly lower than the measured data.For the temperature of injection well bottom,from the Joule-Thomson effect,temperature is proportional to pressure;also lower that of measured ones.The fitting error of wellhead pressure is larger,pressure measured goes down during the whole period of shut-in,while the simulated one keeps stable during the period.The reason for that are some CO₂ leaks during the shut-in period.For reservoir,pressures get slightly higher only near the horizontal well.No obvious change of temperature is observed,because the injection amount of CO₂ is relative small.The gas saturation,instead,gets lower near the horizontal wellbore area.The reason is during the injection process,the pressure in cleat and matrix goes up together,due to the higher permeability,pressure in cleat dissipates faster,and gets lower than matrix;water and methane backflow to cleat,making the gas saturation and CO₂ content in gas phase decrease.For the tracer,because the velocity in the wellbore is much higher than that in coal seam,after the first of CO₂ injection,the tracer mainly concentrates in the area about 250 m away from injection well.About the phase evolution in the wellbore,for a single injection event,bottom pressure keeps increasing during injection.After a rapid decline,bottom temperature also rises as pressure.While,the temperature of wellhead maintains relatively low during the whole process.After the injection,bottom pressure goes down slowly.However,for wellhead,due to the heat exchange of the wellbore with surrounding formation,fluid adsorbs heat and expands,leading to a steep rise of the wellhead pressure.That is also the reason for blowout phenomenon after shut-in in actual CO₂ injection field.Along the whole horizontal well,well bottom has quick response to injection – shut-in process.CO₂-liquid phase appears when injection starts,disappears immediately when injection stops.For the downstream of the horizontal well,there is a delay,the more close to the well end,the more significant the hysteresis phenomenon.According to the observing points along horizontal wellbore,pressure varies little from along the wellbore.The increase of pressure liquefy CO₂,temperature recovers after well shut-in.Because the downstream of the wellbore is shallower,CO₂ flows up under the buoyancy effect,making downstream recover more slowly（the hysteresis phenomenon）.CO₂ leakage from the subsurface is one of the major concerns in relation to CO₂ sequestration in geological formations.In this study we propose a transient,three-phase non-isothermal flow model to describe the relevant CO₂-water（brine）leakage process through a wellbore.The model is formulated on the basis of the T2 WELL program by incorporating the ECO₂ M equation of state and a three-phase drift-flux model.Three case studies are presented for verification,validation,and application of the proposed CO₂-leakage model.In case-1,an analytical solution is derived as a limiting case of the present model,and it allows one to obtain an analytical insight into the flow profile when the leakage scenario is a two-phase steady one without phase transition.Comparison of this analytical solution with the relevant numerical solution shows that both are fully consistent.In case-2,we compare our numerical model with a classic Darcy-law-based CO₂-leakage model,demonstrating that the latter one would generate significant deviations because of excluding the inertia effect.In case-3,we use the present model to simulate the potential CO₂-leakage profile in the Ordos CO₂-sequestration site（China）,and demonstrate that very sophisticated flow scenarios could be created,in which three-phase co-existence regime could be present,and highly oscillating flow profile would occur.The temperature of the leaking fluid with high-concentration of CO₂ may also drop dramatically because of the Joule-Thomson effect,even leading to formation of ice phase at the wellhead.Our model may be useful for evaluating the potential of CO₂ leakage from wellbores and for assessing the safety issues related to CO₂ geological storage.