Abstract

Historically, treatment utilizing freeze-crystallization has been an economically proven method for purifying most waters. Water purification using naturally occurring freezing is an economic and attractive process in oil and gas producing regions where climatic conditions seasonally promote freezing. Research sponsored by Amoco Production Company, the US Department of Energy, and Gas Research Institute to develop a commercially-economic natural freeze-thaw/evaporation (FTE) purification process for produced waters has been conducted since 1992. Preliminary FTE field demonstration results from northwestern New Mexico during the winter of 1995-96 indicate significant and simultaneous removal of salts, metals, and organics from produced water. Despite the unusually warm winter, process yields demonstrate disposal volume reductions on the order of 80% and confirm the potential for economical production of water suitable for various beneficial uses. The total dissolved solids (TDS) concentrations of the FTE demonstration streams were 11,600 mg/L (feed), 56,900 mg/L (brine), and 940 mg/L (ice melt).

During the winter of 1996-97 and the summer of 1997, a new innovative and more economic evaporation pond design will be constructed and demonstrated in conjunction with the continued operation of the FTE demonstration plant. This paper provides the demonstration results and preliminary economics of the FTE process and the new evaporation pond design.

Introduction

Oil and gas produced water management costs significantly contribute to energy production costs. Water disposal costs are particularly significant in coal bed methane production. Furthermore, future domestic energy production will be increasingly dependent on production from unconventional sources of oil and gas, such as coal bed methane, and also from economically marginal formations. For these reasons, water management practices and subsequent water disposal costs are issues of growing importance.

Freeze-crystallization is a low-cost, energy-efficient method for purifying water. Freeze-crystallization has been shown to be effective in removing a wide variety of constituents from water. Water purification by using natural conditions to promote freezing is an attractive process for the treatment of produced water in areas where natural climatic conditions will seasonally promote freezing. The climates of New Mexico's San Juan Basin as well as the oil and gas producing regions of Colorado, Wyoming, Montana, North Dakota, and Canada appear to be well suited for application of a naturally induced freeze-crystallization process.

A more economic and effective method for disposing and treating produced waters is obtained by coupling natural freezing and thawing processes with evaporation, which allows produced water to be effectively managed throughout the year. The freeze-thaw/evaporation (FTE) process utilizes water treatment by freeze-thaw cycling during the winter months, when evaporation is ineffective, and water disposal by evaporation during the summer months. The coupling of freeze-thaw cycling with evaporation is easily achieved and allows natural conditions to be used to purify or dispose of produced water on a continuous basis. The FTE process is conducted using the same equipment required for evaporation, and switching from evaporation to freezing can be done simply by monitoring the ambient temperature.

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