The Modeling Study On Wax Precipitation In Multiphase Systems

With the exploration of offshore reservoir,especially the mining of waxy crude oil,the flow assurance of offshore multiphase pipeline which is influenced by the cold subsea environment,has been seriously challenged by a new problem—wax deposition.The accurate prediction of wax precipitation of petroleum fluids is essential for the prevention and controlling of wax deposition.Therefore,the wax prediction models for liquid-solid and vapor-liquid-solid equilibria were developed,where the description methods for the non-ideality of vapor,liquid and solid phases were improved.Then,to realize the worth in industrial application of these two models,they were applied to crude oil and gas condensate systems,based on the introduction of characterization theories.The main work is as follows:First,a liquid-solid equilibria model for paraffinic systems was established.A new developed solution model,namely IRSW,was used to describe the non-ideality of wax phase.The contribution of excess enthalpy and excess entropy on excess gibbs free energy was considered in IRSW,where the enthalpy was calculated by regular solution model and the entropy is obtained by Wilson equation.For liquid phase,regular solution model and Flory free-volume equation are adopted.Additionally,in terms of molecule configuration,the end effects between paraffinic molecules and the polymorphism phenomenon during solidification were taken into account,which completed the model's physical significance and ensured the model's high accuracy for predicting wax appearance temperature?WAT?,wax precipitation amount and wax composition.Next,an improved EOS-G^E model was built for highly asymmetric paraffinic systems.t-PR EOS was selected to consider the volume translation effects.G^E was calculated by UNIFAC model.To modify the traditional assumption of UNIFAC that"all groups are isotropic in solution",the nonlinear calculation of the molecules'segment fractions in?^C?the combinatorial activity coefficient?is adopted,which enhanced the ability of EOS-G^E model for predicting the bubble points in asymmetric paraffinic systems.Then,considering the effects of pressure on solid non-ideality,a vapor-liquid-solid equilibria model was established,based on the above research of liquid-solid equilibria model.Sloan algorithm was used to solve the three phase equilibria model.Results showed that the vapor-liquid-solid equilibria model agreed well with the experimental WATs in high pressure paraffinic systems.In addition,it theoretically revealed the relationships between the feed composition and operating pressure and the WAT.At last,two wax prediction models were developed respectively for crude oil and gas condensate systems.Combined with characterization theories,above liquid-solid and vapor-liquid-solid equilibria model were successfully applied to crude oil and gas condensate systems.In the model for crude oil,the distribution of n-paraffin was estimated by the exponential decreasing method proposed by Coutinho.The solid activity coefficients were calculated by IRSW model with the ignorance of liquid non-ideality.In the model for gas condensate,the reasonable estimation of feed composition was achieved by the integrated application of Pedersen's,Riazi's and Coutinho's method.The improved EOS-G^E model was replaced by EOS-k_ij model,due to the inefficiency of EOS-G^E model in gas condensate systems.Kinds of crude oils and gas condensates were investigated to test the two developed wax prediction models.Results proved that these models kept good consistent with experimental WATs in petroleum fluids.The research findings of this dissertation will give some guidance on lowering the pipe blocking risks and enhancing the capacity of flow assurance in multiphase gathering pipeline.