Fresh-water resources in many of the world's oil producing region are scarce, while produced water from oil wells is plentiful, though high salinity and other contaminants make it unfit for most applications. Disposing of this water is a great expense to oil producers. This paper advances a technology developed to treat produced water by reverse osmosis and oil adsorption to render it suitable for agricultural or industrial use, while simultaneously reducing disposal costs. Pilot testing of the process thus far has demonstrated the technology's capability to produce good-quality water, but process optimization and control yet to be fully addressed are focuses of this work.
We developed a computer model of the process using a dynamic simulator, Aspen Dynamics, to determine the energy consumption of various process design alternatives, and to test control strategies. We proposed process control schemes using basic feedback control methods with PI controllers. By preserving the mechanical energy of the concentrate stream of the reverse osmosis membrane, we found that process energy requirements can be reduced several-fold from that of the previously developed configuration.
As is this case with much of the southwestern United States, Texas persistently faces water shortage issues across much of the state. As rapid population growth continues, the demand for water will continue to expand, exacerbating existing shortages.
In other parts of the world that have experienced similar shortages, desalination has successfully met the need in many cases. Several Middle Eastern nations such as Israel and Saudi Arabia obtain freshwater by desalination of brackish or sea water (Habali and Saleh, 1994). Energy-intensive thermal desalination methods have given way in recent years to reverse osmosis (RO) technologies which offer fresh water at around half the energy cost (Hensel and Uhl, 2004, Thomson et al. 2002).
In arid southern and western Texas, brackish ground water and oilfield produced water suitable for RO desalination are widely available and could potentially meet the region's water needs for the foreseeable future. The Permian Basin in Western Texas yields about 400 million gallons of saline water per day as a byproduct of petroleum production, which is larger than the daily water usage of Houston, Texas (Barrufet and Burnet, 2004, Barrufet et al. 2005).
Desalination of oilfield produced water is desirable for another reason. Disposal of produced water from oil and gas wells is a costly procedure for production companies. Water-to-oil production ratios generally increase as wells' production lives progress, exceeding 10:1 (by volume) in many cases. This increase, with its associated costs, creates the bottleneck that prematurely ends the production life of many wells (Barrufet et al. 2005). The water contains a number of contaminants, notably high salinity, hardness minerals, hydrocarbons, surfactants, and other chemicals used in the production process, and sometimes heavy metals. Currently, common practice at onshore wells is to dispose of the brine by injecting it into disposal wells. Since the disposal wells are usually off site, the production company incurs transportation costs in addition to injection costs. Clearly, reduction in the volume of this wastewater would benefit oil producers.