Alkaline/Surfactant/Polymer (ASP) Flood Potential in Southwest Saskatchewan Oil Reservoirs
Authors: Huang S. , Dong M. 
Published: March 2010

Most of the medium oil reservoirs in southwest Saskatchewan are in thin pay zones of less than 8 m. Primary and waterflood methods have reached more than 80% of their estimated oil recovery potential. These medium oil reservoirs are basically untouched by enhanced oil recovery techniques.

An initial study was conducted to assess the suitability of Alkaline/Surfactant/Polymer (ASP) flooding for southwest Saskatchewan reservoirs. On the basis of screening criteria in the literature, the region's reservoir conditions, except for some of the formation types, are favourable for ASP flooding. If only sandstone formations are considered for this process, about 49% of the pools in the region are good candidates for ASP flooding. Extensive oil/water interfacial tension and viscosity measurements were taken using screening criteria for surfactant, alkaline, and polymers. A series of sandpack flood tests were conducted in sandpacks to evaluate ASP flooding for the oil. A tertiary oil recovery of 39% IOIP (72% ROIP) was obtained for the test using all three chemicals. These sandpack flood test results showed that a synergistic enhancement among the chemicals did occur in the ASP system for the medium oil with a very low acid number. The results also indicated that, for a medium oil, mobility control was essential and selection of the right surfactant was important.

Introduction

Oil reservoirs located in southwest Saskatchewan, with 3.1 billion barrels of proven oil, are characterized by thin pay (2 to 8 m in thickness) and shaley sand. The estimated oil recovery by primary and secondary methods from these reservoirs is about 25% initial oil in place (IOIP)(1). Waterfloods in most of these reservoirs have experienced early water breakthrough and high water cut due to the high oil-to-water viscosity ratio. For some of these reservoirs, waterfloods have nearly reached their economic limit. For achieving additional oil recovery and subsequent financial benefits, the development of an enhanced oil recovery technique is essential.

Chemical-enhanced oil recovery methods mainly include micellar/polymer flooding, alkaline flooding, polymer flooding, and combinations of two or all of the three methods. Extensive studies of these processes have found that successful chemical flooding needs to be able to efficiently lower the oil/water interfacial tension and to have very good mobility control.

The displacement mechanism of an ASP flood is similar to that of micellar/polymer flooding except that much of the surfactant is replaced by low-cost alkali. Therefore, the overall cost is lower even though the chemical slugs can be larger. Polymer is usually incorporated in the larger, dilute slug. It has been recognized that oil recovery can be greatly improved by the synergism of chemicals used in the ASP formulations.

ASP processes are also being tested in the field. A field-wide project in Wyoming reports costs of US$10 to $22/tonne (US$1.6 – 3.5/bbl) of incremental oil produced(2). The economic analysis of two ASP pilot tests in the Daqing oil field, China, showed that the chemical cost was US$21 to $28/tonne (US$3 – 4/bbl) of incremental oil produced(3).