Abstract

When reverse osmosis (RO) is used to desalinate brackish water feed streams, a small but significant amount of the brine is discharged as a "reject" stream from the RO unit. This brine contains concentrated dissolved salts and other materials. Disposing of this brine concentrate for traditional RO processes can represent a significant fraction of the cost of operating the unit to recover fresh water. Coincidently, in the oil and gas industry, high salinity brines are routinely injected into formations for pressure maintenance and secondary recovery by water flooding. If water from desalination operations could be injected into these oil- and gas-containing formations, the estimated cost savings could be as much as 30% of the cost of operating the desalination unit. This represents a significant cost savings for RO technology that would make fresh water available to communities in need of this valuable resource.

To provide a comprehensive assessment of the perceived benefits compared to the possible hazards of this practice, we use risk analysis theory to define this process in more detail. The potential for formation damage, reduced injectivity, produced water scaling, and environmental impact is evaluated through comparison with traditional waterflood compatibility studies. We also provide an analysis of how state and federal Underground Injection Control (UIC) rules may be used to regulate injection of RO reject brines. The risk analysis study goes beyond classical decision analysis theory to address the "triple bottom line" economic, environmental, and societal benefits afforded by the process and provides a roadmap to gather quantifiable information for regulators, businesses, and community leaders who might consider this technology.

Introduction

Environmentalists, regulators, industry personnel, and concerned citizens have a basic interest in how to set or negotiate environmental priorities given limited and possibly changing resources. When a new technology or process is being introduced into society, setting these priorities is a problem, especially if the technology has the potential to impact a significant part of the local community. Desalination of brackish ground water, oil field produced brine, or even seawater is one of those technologies. Those who study history have seen that water resources dictate the development of civilizations.1,2

Historically, one of the major impacts of the desalination process to create fresh water resources has been the problem of the disposal of the salts (RO "concentrate") and other materials removed from the source water. Assessing the impact of RO concentrate disposal requires knowledge of the physical, biological, or social conditions associated with various risks. Placing this relatively new process among a host of other environmental priorities of our society requires not only ranking risks but also finding solutions to risk problems. Priority-setting entails trade-offs among competing values when resources are inadequate to do everything; resource consumption demands prudence; or additional resources require negotiation.

Our Texas A&M group is working in fresh water resources research.3 One of the processes the group has been testing is the desalination of oil field produced brine to make it available for beneficial use. The technology is based on waterflood process designs routinely used by the industry for decades (Figure 1). We need to answer the following questions: Is this process viable? Can fresh water resources be recovered from oil field brine? What is the impact of this new technology?

Engineers are accustomed to evaluating technical options when considering the development of a new project.4 Assessing the uncertainty and comparative economic risk of a drilling prospect is also common. What is not common, however, is an effort to quantify the qualitative aspects of a project. It is uncommon for a proposed engineering program to address public and other stakeholder issues that might be important in considering the impact of the project on society and the environment.

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