Surface and Subsurface Requirements for Successful Implementation of Offshore Chemical Enhanced Oil Recovery
Authors: Raney Kirk H. , Ayirala Subhash C. , Chin Robert W. , Verbeek Paul 
Published: May 2011

Chemical enhanced oil recovery (EOR), including polymer and surfactant-based processes, is a method that operators consider to maximize oil recovery from onshore and offshore reservoirs. Due to the logistical, operational and environmental differences and the footprint and required weight needed for additional injection and production equipment, offshore chemical EOR processes are challenged by greater complexity and costs as compared to onshore applications of the same technologies.

Chemical EOR commonly requires large volumes of injection chemicals, as well as demulsifiers to break produced water/oil emulsions and inhibitors to control scale, resulting in high shipment and storage costs. The use of seawater and/or produced water for injection of the chemicals into offshore fields mandates stringent processing of both streams to allow optimal injectivity, sweep efficiency, and chemical effectiveness in the reservoir. Offshore production of saleable oil and clean water requires space- and weight-efficient oil-water separation equipment. Currently, conventional methods for processing produced fluids fall short in both efficiency and compactness. High offshore drilling costs lead to relatively large well spacing and more difficulty monitoring the EOR subsurface process as well as to restrictions on the number of disposal wells. Finally, environmental restrictions limit the overboarding of toxic or poorly biodegradable EOR chemicals.

Industry is currently investigating the limiting factors pertinent to offshore chemical EOR. As a result of these efforts, new enabling chemistries and technologies are being examined for improving surface operations to allow cost-effective offshore chemical EOR to be performed in an environmentally-sound and safe manner. Some of these recent chemical and fluids processing developments are described in this paper.

1. Introduction

The primary depletion and secondary water flooding of oil reservoirs typically recover only 20-50% of original oil in place, and hence the majority of oil still remains trapped after the application of these conventional processes. The low oil recoveries from secondary water floods are the result of inefficient macroscopic sweep efficiencies due to lack of mobility control and poor microscopic displacement efficiencies caused by the capillary trapping of oil, attributed mainly to interfacial forces. By overcoming these inhibiting factors, chemical-based enhanced oil recovery (EOR) processes are presently considered as promising tertiary technologies for increasing oil recovery from depleted oil reservoirs. (Thomas 2006), (Manrique 2010)

The chemical EOR processes, widely practiced in field applications, can be broadly categorized into two types; (1) polymer injection processes and (2) surfactant-based processes, in particular the alkaline-surfactant-polymer (ASP) injection process. Typical polymer injection processes utilize polymeric additives to flood water at concentrations ranging from about 500 to 2500 ppm. The addition of polymers to the injected water increases the aqueous phase viscosity thereby lowering the water-oil mobility ratio. This favorable mobility ratio aids in flood conformance control and hence improves the vertical (Figure 1) as well as areal sweep efficiencies. Polymeric additives to injected water also reduce porous media permeability and affect fractional oil flow for more efficient oil recovery. An added benefit is that much lower total volumes of water are produced for a given level of oil recovery.