Optimization of Pour Point Depressant Treating Rates for Crude and Condensate Blends Using an Onsite Densitometer
Authors: Wu Yaqin , Thomason William H. , Thomas Sally A. , Sigit Andika 
Published: May 2013

Production and transportation of high paraffinic crudes in offshore fields is a major flow assurance challenge for the oil and gas industry. The challenge is particularly great when the sea water temperature is lower than the pour point of the crude being transported. This paper describes flow assurance issues that have been addressed for handling subsea transportation of paraffinic crudes in Indonesia. Pour point depressant (PPD) has been continuously injected into the oil production manifold handling high pour point crude to cause the formation of a sufficiently weak gel in the subsea pipeline to enable restart after a long shut-in. Currently, production of a condensate has started that blends with the waxy crude. The PPT (pour point temperature) and live gel strength of the condensate and crude oil blends are significantly lowered and require less or no PPD injection. Since the PPD injection involves significant operating costs, this paper describes the joint effort by operations and technology staffs to develop a reliable method to optimize the PPD treating rate on a daily basis. PPTs and live gel strength were measured in the in-house laboratory using a densitometer (identical to the one used at the field lab) and a rheometer respectively. An equation was generated by fitting a smooth curve to correlate PPT with live gel strength. This equation provides a convenient method to estimate live gel strength based on onsite PPT measurements. Considering that the crude oil and condensate blend are changing over time, routine monitoring of blend PPT using a reliable and simple onsite method and estimating gel strength based on PPT results, enable identification of the optimum PPD dosage by the operations staff to ensure flow assurance and minimize treating costs in a timely fashion.

Introduction

Production and transportation by subsea pipeline of high paraffinic crudes such as the Kerisi crude (offshore Indonesia) present challenging operational issues due to wax deposition in the subsea pipeline and gel formation if the pipeline flow is stopped for a long period.1–3 Common methods to control crude gelling are to treat the crude with a pour point depressant chemical; to keep the crude hot; to add a diluent to the crude to generate a blend of acceptably low pour point; to displace the crude with a diluent when the pipeline is allowed to cool below gel temperature; or to design the pipeline such that breaking the gel with pressure is a an option. The gel break approach requires the proper combination of applied pressure, pipeline rating, and crude gel strength. Internal lab studies have shown that dissolved gas in the crude, blending with higher pour point crudes (or condensate), and addition (if needed) of PPD at low dosages can produce a significant decrease in the gel strength of crude blends, which greatly enhances the possibility of using the gel break approach.

This paper discusses the use of this combined approach (dissolved gas, diluent, low dosage PPD treatment, and gel break with pressure) for gel formation management in a 23 km subsea pipeline associated with the Kerisi Development located offshore Indonesia. The maximum allowable gel strength to enable gel break with pressure was identified. The dissolved gas in the crude was set by the high pressure separator operating pressure at the offshore platform. The ratio of the diluent (produced condensate) and crude production was variable. The PPD dosage was then added at a rate to generate a gel strength below the maximum allowable. In order to select the minimum acceptable PPD treating rate considering these changing parameters, an onsite measurement is needed in order to reliably identify the proper PPD rate. The onsite method needs to be suitable for a field lab and have high repeatability.