Flowline Deposition Assessment Using Chemical Tracers
Authors: Nisbet William J.R. , Cooper Kevin , Wollenberg Philip , Shell Moye Wicks , Balkanyi Szabolcs 
Published: October 2008

This paper discusses work completed by Shell Exploration and Production Company (SEPCo) and Shell Global Solutions (US) Inc. to develop and execute chemical tracer testing procedures on its deepwater dual flowline subsea systems. An overview will be given on the theory and evolution of the tracer technique as well as the results of testing programs conducted to date.

Tracer testing entails the determination of flowline system volumes in order to assess solids deposition over the field life. The flowline volume is determined through the evaluation of the residence time of injected tracer pumped at near constant flow rates. The tracer is injected at the loop inlet and circulated through the flowline with dry oil where it is detected at the outlet by various means (dependant upon tracer properties). By determining the in-service flowline volume, the deposition of paraffin and asphaltene's can be quantified and, thus, inhibitor chemical usage and optimization strategies can be efficiently managed.

A major drawback with the use of traditional radioactive tracers is the cost, safety and scheduling issues associated with conducting tests offshore. Using readily available methanol as a marker fluid enables tracer tests to be conducted essentially on an opportunity basis with relative ease. This in turn has enabled optimization of chemicals used to inhibit deposition, resulting in significant savings in operating expenses.

The tracer technique has been applied successfully at 3 subsea systems in the Gulf of Mexico. The results of these tests have been used to:validate reductions in asphaltene inhibitor usage andconfirm the effectiveness of hot oiling as a method for wax removal. The relative ease and accuracy of chemical (methanol) tracer testing has established it as the most effective method for quantifying deposition in subsea dual flowline systems.

Introduction

A major flow assurance concern associated with the operation of subsea fields is the potential for blocking or restricting flow as a result of either wax or asphaltene deposition. When unacceptable deposition is predicted the general approach is to adopt an inhibition (for asphaltenes) or inhibition and/or pigging (for wax) program.

Laboratory tests and models are continually being improved for selecting appropriate inhibitors and predicting deposited volumes. However, this still leaves significant uncertainty regarding the actual field results, which may result in:Excessive use of inhibitor resulting in increased operating expenseIneffective inhibition resulting in unacceptable depositionExcessive downtime from either pigging too frequently, or sticking a pig Therefore, accurate quantification of deposited volumes (in combination with laboratory testing) is required to optimize the performance of at-risk subsea systems.

Several deposit measurement concepts were evaluated prior to development of the chemical tracer test. The measurement of flow rate and pressure loss, both under production conditions with multiphase flow, and during dry oil circulation with single-phase liquid flow was particularly attractive due to the relative ease of conducting such a test and the minimization of downtime. In theory, such a simple measurement provides direct evidence of deposits, but in practice, significant uncertainty attends the important variables on which pressure drop depends: pipe roughness, flow rate, and fluid property variations along the line, especially with multiphase flow.