Study On CO₂ Flow Behavior In Wellhole Of Deepwater Drilling

With the increasing demand for oil and gas resources,countries all around the world are paying more attention to the exploitation of offshore oil and gas fields.Offshore oil and gas development will be the mainstream for future exploration.Offshore drilling has special working conditions,for example,the limitation of actual construction environment on offshore platform and low sea water temperature.In addition,offshore drilling needs to strictly control flowwback and pay attention to marine environmental protection.The carbon dioxide flow in the process of land drilling fluid have already gained certain research results.When carbon dioxide enters the stratum,it can reach the supercritical state in some certain situation.Supercritical carbon dioxide has some very special characteristics like low viscosity,high density and high diffusivity.It has high ability of mobility,solubility and transitivity.What’s more,it has small pollution to the reservoir.At present,using CO₂ as a drilling fluid has significant advantages in land drilling which can also fit offshore drilling.The geothermal gradient is lower than ground drilling because of the seawater part.The wellbore structure has differences between these two.The impact of these factors on wellbore is unknown,for instance,whether the fluid can become supercritical carbon dioxide before reaching the bottom.All the problem are needed to be discussed.Therefore,this paper will study on the flow characteristic changing under offshore drilling condition.The coupling calculation and analysis of pressure parameters,temperature parameters and carbon dioxide property parameters are realized by optimizing the temperature and heat transfer calculation model.For this purpose,in this paper,the calculation of convective heat transfer coefficient is firstly obtained.Then optimizing the temperature and heat transfer calculation model,in order to realize the coupling calculation and analysis of pressure,temperature and carbon dioxide property parameters.After that,a wellbore flow model is established to realize the coupling calculation,analysis of the pressure and temperature parameters,and carbon dioxide property parameters during the wellbore flow.Finally,the results of wellbore flow of carbon dioxide under the condition of deep water drilling compared with land drilling flow characteristics,and compared with conventional ocean drilling fluid.After the comparison,the advantages and disadvantages can be analyzed,and most but not the least,the results can provide technological support for carbon dioxide using in deepwater drilling.The results show that the property changing of carbon dioxide in offshore wellbore is similar to that in land wellbore.And carbon dioxide can stay supercritical state for a long distance in the annulus.Different mass flow has little influence on its characteristic changing but much on pressure drop.The bigger mass flow is,the higher pressure drop is.With the higher initial injection temperature of carbon dioxide,the pressure inside drillpipe drops more obviously.And the supercritical point has a small increase.Larger initial injection temperatures lead to lower carbon dioxide density.Moreover,the outlet pressure increasing influences the whole wellbore pressure,and rises the density of carbon dioxide.When the temperature of the seawater is low,the position of supercritical state where the carbon dioxide reaches is deeper.Therefore,attention should be paid to the insulation layer and riser.Compared with the performance of conventional offshore drilling fluids,carbon dioxide can maintain a supercritical state in the annulus having a better rock-carrying efficiency.