Innovative Treating Processes Allow Steamflooding With Poor Quality Oilfield Water
Authors: Garbutt C.F. 
Published: December 2007

Field testing has confirmed the newly-discovered Modified Hot Lime Process (patent pending) as a significant improvement over existing precipitation softening options. A Permian Basin produced oilfield water containing 2000 ppm hardness, 500 ppm sulfides, 10,000 TDS and 200 ppm oil is successfully being converted to steam generator quality feedwater. Demonstrated success of this new process may open-up steam injection projects in other areas where available water supplies are not economically treatable using existing technologies. Alkali consumption and sludge production have been reduced by 50% with the Modified Process when compared to conventional Hot Lime. In addition, alkali consumption by entrained CO2 is eliminated. Many hot lime softeners currently in service can be inexpensively converted to this more efficient process.

This paper also discusses use of two alkalis to improve performance of the precipitation process and effectiveness of CO2 as the stripping gas for sulfide removal.

Introduction

Economic treating of water is one of the most critical obstacles to achieving a successful steam injection project. Because of strict environmental regulations and lack of available fresh water, produced oilfield water is typically used as source water for steam generation. For most steamfloods, quality of the produced water is fair - sufficient treating can be accomplished using typical oil removal and softening techniques. In many west Texas oil fields, however, produced water quality is much worse than that currently used for steam generation. Hardness and sulfide levels are ten times the average for California steamflood source waters. In fact, this pilot's water source may be the poorest ever used for conversion to steam generator feed. This, along with the general absence of typical heavy oil steam targets, accounts for the historic reluctance to pursue steam projects in the Permian Basin. However, Marathon has identified a unique thermal opportunity in a large west Texas oil field.

Realizing the significant reserve and economic potential of a thermal project in this area as well as potential pitfalls associated with treating poor quality water, Marathon Oil Company embarked on an operational steam field test in 1995. The test consisted of installing a 5,000 BWPD oilfield water purification facility, three single pass waste heat steam generators and drilling one steam injection well.

According to published guidelines, all oil, suspended solids, hardness and sulfides should be removed from the oilfield water in order to prevent damage to steam generator tubes. Since primary challenges associated with this water deal with sulfide and hardness removal, the majority of this paper will focus on removal of those two constituents.Hardness Removal - Many different commercial hardness removal systems were explored for the project: Strong Acid/Strong Acid Ion Exchange, Strong Acid/Weak Acid Ion Exchange, Weak Acid/Weak Acid Ion Exchange, Hot Lime Soda, Cold Lime Soda and Thermosoft. A detailed review of each of these processes is given by R.J. Jan and T.G. Reed, Jr. Economic analysis of the above processes identified hot lime softening as the most attractive of the existing technologies for the high hardness produced water to be used. Following additional lab and field testing, a new, more efficient process was discovered and commercially demonstrated. The process is a variation of the standard hot lime process and will be discussed in more detail herein.Sulfide Removal - To effectively remove sulfides, water's pH must be reduced to convert ionic sulfide to hydrogen sulfide (H2S) gas. A packed column is then used to strip the H2S. This paper discusses an uncommon technique of using CO2 for both pH reduction and stripping instead of the typical approach of using acid for pH reduction. The benefit to downstream softening is explained.

P. 491^