Abstract

The Pembina Cardium reservoir is the largest conventional oil reservoir discovered thus far in Canada, and has been extensively waterflooded since the late fifties. Operating waterfloods are now reaching their economic limits and improved oil recovery techniques are being considered to extend the life of the reservoir. This report investigates the technical feasibility of applying a chemical flood to the Pembina Bear Lake reservoir. The investigation has two major components. First a chemical system was designed that lowered the brine/oil interfacial tension sufficiently to mobilize waterflood residual oil. Secondly, the effectiveness of the chemical system was tested using corefloods at reservoir conditions.

Through phase behavior studies an optimum chemical composition was determined: 5 wt% Petrostep B100 + 2 wt% n-butanol + 0.5 wt% NaCl in Bear Lake Brine. Injection of a slug of this chemical composition led to an incremental recovery of up to 48.5% OOIP Bear Lake Crude in homogenous rock material. Using real reservoir core the average incremental oil recovery was only 15% OOIP. The low oil recovery during the chemical floods in reservoir cores is a direct result of the extreme heterogeneity in the conglomerate rock.

Introduction

The Bear Lake reservoir consists of several deposited sandstone layers overlain by a high permeability conglomerate layer. During waterflooding, the conglomerate acts as a thief zone through which most of the water channels. The waterflood residual oil saturation has been estimated to be approximately 50%, reducing the oil recovery during waterflooding even further. As a result large quantities of oil are left behind due to poor sweep and poor displacement.

This kind of problem can be addressed in several ways. One option would be to place a blocking agent into the conglomerate zone to force more water into the sandstone layers. The sandstone layers would be better swept, improving the oil recovery. However, a large amount of waterflood residual oil would be locked in the conglomerate layer and lost from: future production. Therefore, it was decided to look at a chemical flooding option to lower the waterflood residual oil and improve the sweep efficiency at the same time.

Lowering the interfacial tension between reservoir brine and oil to the point of micro-emulsification has been proven to be tremendously effective in reducing residual oil saturations. The interfacial tension can be lowered by injecting a properly selected surfactant solution into the reservoir. Increasing the viscosity of the injection brine with polymer addition enhances the oil recovery by improving the sweep efficiency and displacing emulsified oil. In this work a micellar-polymer process was investigated for application in the Pembina Bear Lake Unit.

Theory

Micellar-polymer flooding relies on the injection of a micellar surfactant solution to lower interfacial tension (IFT) to ultralow levels, lower than 10−3 mN/m. Ideally, micro emulsions are formed during micellar-polymer flooding which help to solubilize and transport residual crude oil. This reduction in the interfacial tension reduces the capillary forces which trap residual oil in place in porous media, making displacement of the oil easier.

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