Changes of Operating Procedures and Chemical Application of A Mature Deepwater Tie-back -Aspen Field Case Study
Authors: Meng Weihong , Ogea Paul , King Mike 
Published: May 2011

The Aspen field is located in about 3050 ft water in the Gulf of Mexico on GC243 block and tied back to Bullwinkle production facilities on GC65 block via dual 16-mile wet-insulated pipelines. Its first production began in December 2002. The project was a fast-track, marginal field with an assumption of quick reservoir pressure decline. Hydrate is managed primarily by MeOH injection during startup and shutdowns. The reservoir fluids are complex. Downhole sample tests showed asphaltene onset pressure was 6500 psig (~500 psi above bubble point). An asphaltene inhibitor was continuously injected downhole as well as scale inhibitor since the field was suspected with scale and asphaltene issues in the earlier production life. The produced water has a total dissolved salt of 230,000 mg/l. The oil production has declined significantly from initial production while the water cut reaches up to 80%.

The characteristics of field production have been changed significantly since the initial production date. A review was conducted by flow assurance team working with its chemical supplier. During the study period a large amount of fluid tests were conducted and chemical application program were reviewed by chemical supplier's experts along with the operator's engineering team. Planning to field execution took more than one year, resulting in termination of downhole injection of asphaltene inhibitor and scale inhibitor. Additionally there was a reduction in methanol usage and other topside chemicals, and the operating guidelines were revised. The financial benefits include cost reduction, as well as quicker startup and shutdown.

This paper will present the methodologies, organization, tests, recommendations, field execution plan and results after implementation of the recommendations. The case study also demonstrates how the operation team and engineering support team of an operator can work with suppliers, utilizing the industry experience, to achieve best results.

Introduction

In deepwater water asset operations, understanding the multiphase flow system and associated chemical applications are two major components to properly formulate operating procedures. During the exploration and development phase of a project, design criteria usually tend to be conservative. After a field starts production, periodic reviews will confirm the validity of design promises and improve operational efficiency.

In the petroleum industry, wax, asphaltene, hydrates, scale, internal corrosion, sand production were always the major problems in production operation, great deal efforts were made to understand the issues and develop chemicals to ease these issues, especially for deepwater developments. Significant progress has been made on the chemistry aspects through university and industry consortia [1-5]. Petroleum production operation should be based on and treated as a system[6] operation. Systematic approach is more important to the success of the deepwater project due to the long subsea pipelines and cold environment as well as high costs associated with subsea intervention. Phase-equilibrium (chemistry, including inhibitors) and multiphase flow behaviors (modeling) are the two key subjects (components) to understand a production system, and are related to each other. A systematic approach with a focus on fluid characterization was presented by Jamaludin et al.[7] As more reserves are discovered in deeper waters; more attention is paid to the flow assurance issues.