Interwell EOR Pilot Proves Advantages of Polymer Flooding
Authors: Feder Judy 
Published: May 2019

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 190175, “Results of the UK Captain Field Interwell EOR Pilot,” by Annette Poulsen, SPE, Adam Jackson, SPE, Nicolas Ruby, SPE, and Karl Charvin, Chevron; Michael Shook, Mike Shook and Associates; Varadarajan Dwarakanath, SPE, and Sophany Thach, Chevron; and Mark Ellis, prepared for the 2018 SPE Hydraulic Fracturing Technology Conference, Tulsa, Oklahoma, USA, 14–18. The paper has not been peer reviewed.

This paper discusses an enhanced-oil-recovery (EOR) polymer-flood pilot at the Captain field in the UK North Sea during 2011–2013. The polymer flooding accelerated 6 years of potential waterflood recovery to 1 year and minimized potential water injection and handling by 25.2 million STB. A polymer incremental recovery of 16% original oil in place and improved sweep of 4.7 million STB demonstrated the advantage of using polymer EOR for viscous oils. Finally, the pilot data provided sufficient information to deliver a reliable subsurface assessment for the full-field project. These data also provide evidence that chemical-flood pilots not only can be economical but also can compete economically with waterflooding in some challenging locations when designed and operated efficiently.

Introduction

The Southern Upper Captain Sandstone (SUCS) reservoir is a complex channel in-fill of a localized canyon eroding the underlying Mid Captain Shale (MCS) and Lower Captain Sandstone (LCS). The canyon was created when two megaslump scars merged into a linear valley cut, giving the reservoir its characteristic shape. The slump scar process also triggered deposition of debrites (a mixture of MCS and LCS sediments) of poor reservoir quality, isolating the underlying LCS from the subsequent SUCS canyon fill.

Fluids from the Captain field are unlike those normally encountered in black-oil reservoirs. The fluid is a low-gravity oil with a relatively low dissolved-gas/-oil ratio. The oil is heavily biodegraded with some property gradation across the field. There is an apparent general increase in biodegradation from east to west. The oil in the SUCS is analogous to that in the main part of the Upper Captain Sandstone (UCS). Pressure/volume/temperature and special core-analysis laboratory data reveal a highly unfavorable calculated endpoint mobility ratio equal to 31. The following field observations support the adverse mobility ratio:

Strong coning tendency

Early water breakthrough

Modest oil-rate decline

Long oil-tail production in waterflood-recovery profiles

Sharp oil-saturation changes between unswept and swept regions

Injected water slumping to the base of the reservoir, advancing along the base of the sand, and bypassing the oil

Pilot Objectives, Mechanisms, and Implementation

The polymer flood for the field was originally envisioned in the field- development plan, and the concept was included in the original facilities design to enable a field trial. The major challenge in deploying a polymer flood is field implementation. Though onshore polymer flooding may be considered a mature technology, field experience is difficult to transfer and the offshore environment adds another level of complexity.