Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
National Ground Water Association’s 2005 Ground Water Summit, San Antonio, April 17-20
Please Pass the Salt: Using Oil Fields for the Disposal of Concentrate from Desalination Plants
The demand for fresh water in Texas will increase in the future owing to population growth and because conventional sources such as surface water and groundwater will not be sufficient to cover needs. Desalination of brackish water is a viable alternative being actively pursued by the State, whose technology of choice includes reverse osmosis and electrodialysis. Feed water is processed to produce a permeate that typically augments other water sources. In the process, a concentrate is created that collects all salts rejected from the permeate. The concentrate’s salinity ranges from 2 to 5 times that of the feed water. Several options are available for disposing of desalination concentrates, such as discharge to the ocean in coastal communities or discharge to a sewer system. Another promising possibility is deep-well injection. In Texas, the oil and gas industry has been reinjecting saline produced waters into the subsurface for years. A combination of deep-well injection and reinjection is attractive for Texas, where oil and gas fields are plentiful. Formation pressures in oil and gas reservoirs have been greatly lowered because of past production, creating an opportunity for injecting foreign fluids at a lower cost. However, legal and technical issues can arise.
A high level study was undertaken to understand challenges and opportunities offered by well disposal. Potential injection of concentrate in depleted fields raises the following questions: (1) Is the maximum authorized injection pressure high enough to overcome low permeability in some reservoirs and adequate to take up the entire concentrate stream? (2) Are environmental downhole conditions, especially pressure and temperature, conducive to mineral precipitation within the concentrate stream? (3) Is the formation water compatible with the concentrate stream? (4) Is the salinity shock prone to mobilizing formation fines and clays as they contact water of smaller ionic strength and/or different ionic makeup? (5) Is the current legal environment adequate? For a more realistic analysis, we selected areas from six sedimentary basins across Texas: the Anadarko Basin in the Texas Panhandle, the Permian Basin, the East Texas Basin, the Fort Worth Basin, the Maverick Basin at the Mexico-US border, and the south part of the Gulf Coast Basin.
Despite some differences, the six analysis areas show a consistent picture. They all have a history of fresh-waterflooding, especially in their early production periods. We also analyzed the distribution of injection flow rates. Achievable injection rates are not on average historically high, which is confirmed by the low-permeability values of the Paleozoic formations. The East Texas and Gulf Coast reservoirs have higher permeability and subsequent maximum potential injection rates. Multiple wells will be needed to accommodate the desalination concentrate stream of a typical plant.
To analyze the impact of changes in environmental conditions, we used a batch geochemical code in combination with a Monte Carlo approach. We sampled the formation-water and brackish-water data sets multiple times and mixed the selected samples in different proportions. Lack of detailed chemical analyses did not allow for a thorough study of all scale-forming minerals, barite in particular. However, the scaling tendency by calcite and gypsum is not outside of that typically encountered and dealt with by the oil and gas industry. We also analyzed the impact on clay of fresher-water injection. Water sensitivity of the clayey material can be accommodated using operational solutions such as pretreatments with appropriate chemicals or buffer solutions.
Injection of desalination concentrates and produced waters from oil and gas fields naturally falls under the jurisdiction of Underground Injection Control regulations (UIC) under Class I (injection of hazardous and nonhazardous wastes beneath the base of usable-quality water), Class II (disposal of saltwater and other fluids co-produced with oil and gas), and Class V. Permitting the injection of concentrate could be made easier through general permitting of a special non-hazardous Class I injection well. Class II wells can be used only if the concentrate aids in hydrocarbon production. It may also be possible to create a special category of Class I or Class V injection well. There are also potential federal solutions, but changing rules at the federal level has been an onerous and thus far unsuccessful endeavor. In short, it is technically feasible to inject concentrate into oil and gas fields and there are several options for making the permitting of concentrate disposal wells easier and more affordable.