Flow Assurance: Validation of Wax Deposition Models Using Field Data from a Subsea Pipeline

Abstract

Abstract This paper describes an offshore production system with a subsea pipeline (12-inch diameter 23-km long) where wax deposition has been the primary flow assurance challenge. This subsea pipeline has been transporting nearly 55,000 BOPD from a central processing platform (CPP) to a Floating Production Storage Offloading (FPSO). The crude oil has been characterized for wax potential by performing several laboratory analyses which have been entered into available wax deposition models to predict the wax deposition rate inside the pipeline under various operating conditions. The transient flow rate and pressure drop data from the field have been analyzed to estimate the average wax deposit growth rate. This paper will share the findings from the field validation and application of the available wax deposition models. The results will help pipeline design engineers to accurately estimate the required insulation level for wax deposition control and will also assist in operating the pipelines with the optimum pigging frequency. This study has confirmed that Film Mass Transfer (FMT) model gives higher wax deposition rate than Equilibrium model (EM). The predicted deposition rates from both of the models have been much lower than the field data. Using default viscosity, both EM and FMT models gave reasonable predictions of the wax deposition rate as compared to deposition rate obtained from the field data. The field deposition rate is higher than the predictions by the EM model without any shear removal. By incorporating aging process to the FMT model, it was found that the value of Ka parameter (as defined in the aging model by Singh et al., 2000) of 1 matches the deposit wax content observed in the field. Matzain shear removal model (2001) over-predicts the impact of shear on the rate of wax deposition; however, Venkatesan's shear removal model (2003) shows a very small impact of the shear. The deepwater project development and engineering design needs an accurate prediction of the wax precipitation and deposition in subsea pipelines. A number of wax deposition simulators have been developed to predict the wax deposition rate. Although there have been several attempts made to validate the wax deposition prediction models with laboratory data, no reliable field verification study could be found prior to this study. Introduction Paraffin deposition takes whenever paraffinic oil gets in contact with a cold pipe wall or ambience below the Wax Appearance Temperature (WAT) of the oil, solid paraffin crystals can precipitate and deposit on the pipe surface. Deposition of wax from oil leads to a gradual decrement in production due to plugging of wells and/or pipelines and, in an extreme case, can render a pipeline or production facility abandoned. Various measures including chemical and mechanical have routinely been applied. Mechanical methods include pigging and wireline cutting to remove the deposited wax. Chemical methods include paraffin inhibitors/dispersants to inhibit the formation of deposit and hot solvents to remediate the deposit already formed. These methods to prevent and remediate wax deposit add significant operating cost to the production operation. Successful development of the offshore deep-water projects requires engineering designs of the facilities that can handle the flow assurance challenges such as wax deposit problems. A robust design can only be obtained with an accurate prediction of the wax precipitation and deposition. Therefore, an accurate modeling of wax precipitation and deposition is imperative to facilitate a successful engineering design, development, and operation of deep-water offshore projects.