Abstract

Host governments in developing countries are placing increased focus on gas conservation where gas cannot be easily exported. In Angola, a recent field development plan included the provision for Water Alternate Gas (WAG) injection to maximize ultimate oil recovery and minimize flaring.

In August 2006, the first phase of WAG injection was implemented successfully on an Angolan development project. This phase involved conversion of a well located on a Surface Wellhead Platform (SWHP) from gas injection to water injection. Upon completion of the conversion, this surface well became the first successful SWHP well conversion attempted in Angola and the first one within ExxonMobil's deepwater portfolio.

This paper describes the process, flow assurance, and wellbore evaluations performed to analyze the risk of hydrate formation in the tubing during WAG conversion, and evaluate potential integrity risks to the process facilities onboard the SWHP. This also paper includes a discussion on the challenges surrounding wellbore gas depressurization into facilities that were not designed for this purpose, and the results obtained from the steady-state and transient model analyses. This work focused on the fluid behavior during gas depressurization and water injection startup. Potential piping integrity risks caused by Joule Thompson (JT) cooling effects and the potential risks of hydrate plug formation which can cause expensive coil-tubing interventions were also evaluated. Further, this paper presents a comparison between the theoretical and the actual results obtained during the conversion. In conclusion, recommendations for future designs of deepwater surface WAG injection systems are provided from lessons learned during the design and execution phases of this groundbreaking project.

Introduction

Today's operators are challenged to develop deepwater fields in developing countrieswhere gas export facilities are not readily available. Water Alternating Gas (WAG) injection is an existing technology that helps manage gas handling constraints while improving voidage replacement and ultimate hydrocarbon recovery. WAG injection technology in a deepwater field is not straightforward and can carry safety, technical and business risks. WAG injection presents technical challenges not only from a reservoir point of view but also from facilities design and flow assurance perspectives. Some of these challenges, if not properly understood, could develop into unwanted risks with high financial consequences for the operating unit.

Esso Exploration Angola Limited Block-15 (EEAL), an affiliate of ExxonMobil Corporation, operates Block 15 with co-venture partners of BP, Statoil and ENI on behalf of Sonangol. Although there are no gas exporting facilities in Angola, an LNG plant is planned. EEAL focuses on minimizing flaring and is committed to recycle hydrocarbon gas in order to save important gas resources critical to sustain Angola's fast paced development. Thus a field development plan was implemented that included the early conversion of a surface well from gas to water injection.

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