Abstract

A hybrid system of wind, solar, and diesel generators could provide an efficient alternative for powering water desalination projects in remote oilfield locations in Texas. Disposal of produced water from oil and gas wells is a costly procedure for production companies, but water-to-oil production ratios exceed 10:1 (by volume) at many wellsites. Much of the petroleum produced in the United States and elsewhere is found in arid regions that could benefit greatly if the produced water could be purified sufficiently for agricultural, industrial, or potable use. Our previous research identified and validated treatment options capable of recovering a high proportion of fresh water from oilfield brine. In this paper we further the earlier research by examining the possibility of using renewable energy to power the units in "off the power grid" situations. A macro-driven spreadsheet was created to allow for quick and easy cost comparisons of renewable energy sources for a variety of scenarios. Using this tool, wind and solar costs were compared for cities in regions throughout Texas. The renewable energy resource showing the greatest potential was wind power, with the analysis showing that in windy regions such as the Texas Panhandle, wind-generated power costs are lower than those generated with diesel fuel.

Introduction

Freshwater resources on Earth are limited; about 97% of the world's water is sea water with an additional 2% locked up in remote ice caps and glaciers. Saline groundwater further reduces the amount of potable water, leaving less than 0.5% of the Earth's water resource on land available for direct human consumption, agriculture, or industrial use. The world's population growth, improved living standards, increased demands from agriculture and industry, and declining quality of existing resources stress this limited resource. With a population expected to double in the next 50 years Texas also faces a growing demand for fresh water (Patel et al. 2004).

We developed a process for desalinating oilfield brines in a two-fold effort to provide fresh water for arid regions and to reduce the volume of waste water that oil and gas producers must dispose of. In the process, organo-clay adsorbants remove entrained oil from the water, which is then passed through a reverse-osmosis (RO) unit, producing a freshwater stream (or permeate) and a concentrated brine (or reject). Various pretreatments may be necessary, depending on the properties of the brine from a particular wellsite. An aspect of the process yet to be fully addressed was identifying the most economical electricity source with which to power pumps and other necessary electrical equipment. Since oil and gas wells are frequently sited in remote locations, access to a power grid is seldom available. Possible power sources include combustion, wind turbine, and photovoltaic generators. In this project we found a combination of these sources to be the most efficient solution.

Hybrid and Off Grid Desalination Systems

Portable combustion generators have existed for decades and therefore are a well-established technology, readily available for purchase in a number of sizes and with various features. Power outputs range from 2 kW to over 50 kW for portable models. Documents covering the technology, emissions, and costs of commercial available diesel and natural-gas-fired generators are widely available (USDE 2009).

A strong point of combustion generators is large power outputs relative to their size. As an example, a portable 3,600 W diesel generator has no dimension larger than 3 ft and weighs just over 200 lb. A generic wind turbine, capable of generating 3,200 W in 27 mph winds, weighs about 175 lb (not including the pole mount) and has a turbine diameter of 15 ft (Mareth 2006). Achieving 3,600 watts with conventional solar panel technology would require about 300 ft2 of panels and weigh nearly 800 lb (SECO 2005).

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