Disposal of Produced Water from Oil Fields
Oil and natural gas exist, mixed with salt water in various proportions, in the porous underground rocks and shale. Water is also injected during the process of hydraulic fracturing in natural gas wells such as Barnett Shale. This water also comes out in different proportions when oil or natural gas is extracted. Most of the time, this extracted water is mixed with sand, polymers, oil, salt, clay and other dissolved materials. The disposal of this salt water, which is also called produced water, is a major concern during the production and processing of oil and natural gas.
There are various ways of disposing this extracted water. In many cases, efforts are made to recycle a major part of it. The process of recycling involves distillation or evaporation. This is feasible only when there is constant flow and supply of water to keep the treatment plant working. Hence this method is adopted in places where there are several producing wells. Even after recycling, a large amount of salt water disposal and other solids remain for other methods of disposal.
In cases where this water comes from conventional reservoirs or oil, this water is injected back into the reservoir. Another method of return of this salt water or oilfield brine is to inject it into under-ground porous rock formations which are not producing gas. It is to be ensured that these rock formations are having impermeable and unbroken strata above and below. These wells used for disposal has to adhere to the strict standards set by the Commission for disposal well construction, which includes casing with three layers. These disposal wells are also inspected by Railroad Commission on a yearly basis.
The main purpose of these regulations is to ensure that there is no ground water contamination due to the disposal of oilfield brine. In Texas about 50000 wells have been permitted for disposal of extracted water. Though Barnett Shale Play, which is the largest natural gas plant in the state, has been operating since 1997, there have not been any instances of groundwater contamination due to water disposal so far in the area.