Amid growing public concern and tightening regulation of water resources, exploration and production operators are seeking customized treatment solutions that can improve both environmental and bottom-line performance. An array of water treatment technologies is available to meet widely varied oil and gas field conditions and needs.
Deluge of Challenges
How clean is clean? When it comes to removing impurities from oil and gas field water, the answer may be in the eye of the beholder. The answer may also depend on the intentions of cost-conscious exploration and production companies looking for new ways to optimize use of available water resources.
Issues surrounding the management of water used in upstream oil and gas operations are attracting attention. As the public becomes more aware of freshwater scarcity and the potential environmental impact of oil and gas field operations, disposal by pumping into subsurface zones of fracturing flowback and produced water faces increasing regulatory restrictions. Hauling produced water to disposal wells also can be expensive, and this practice comes with liabilities for truck accidents as well as emissions from truck traffic. Recycling produced water for reuse has become more common, but volumes may exceed what can be recycled.
Geological formation or exploration methods—whether steam flooding, water flooding, slick water fracking, gel fracking or other approaches—cause specific constituents to be present in water, which may complicate treatment.
All of this generates demand for cost-effective, flexible treatment and disposal options to minimize total water life-cycle costs in the oil field. To meet this demand, a range of technology solutions is available to treat specific constituents for the planned use of the treated effluent. The right expertise—needed to determine the best approach and develop a customized solution—is as important as the right equipment.
One solution for produced water reuse that is popular with oil field operators in the Midwest and Western states is a ceramic membrane-based process that removes oil, solids and scale formers. Ideal for treatment of flowback and produced water for reuse in exploration and production operations, this system combines a high-rate chemical softening and thickening technology and ultrafiltration membranes to produce water suitable for reuse.
Feedwater is first treated using a high-rate chemical softening technology to remove scale formers. This process uses a proprietary draft tube mixing technology to assist in the formation and separation of crystalline solids. The softened water and crystalline solids are then processed through a ceramic membrane ultrafiltration technology. Operated in cross-flow mode, the membranes remove free oil, suspended solids and bacteria to very low concentrations in a single step.
The solids from the ultrafiltration are continuously recycled to the crystallization tank of the chemical softening unit, providing seed for crystal growth. The treated water from the ultrafiltration process is neutralized and reused in shale oil operations as frac water, reducing freshwater demand.
Surface Discharge or Reuse
In situations where produced water volumes exceed what can be reused, an optimized pretreatment and separation technology produces water that is clean enough for reuse or discharge in either shale oil or conventional oil applications. The first version of this system was introduced in 2007 at Chevron’s plant in the San Ardo Field in Monterey County, California (see Image 1).
This technology goes a step further, providing high-quality water suitable for reuse in steam boilers, irrigation or surface water discharge (low boron levels). The compact produced water treatment technology uses the previously discussed oil, solids and scale former removal system for pretreatment prior to ion exchange and reverse osmosis. The filtrate from the first system is further treated with ion exchange softening using weak acid cation (WAC) resin in sodium form for further removal of hardness and metals to lower concentrations, without pH correction.
The pretreated water is then pressurized through reverse osmosis, operated at an elevated pH in single or double pass mode, to reduce total dissolved solids (TDS), boron and organics. The treated water exceeds state and federal requirements for quality.
Zero Liquid Waste/ZLD
In Eastern states where TDS levels in produced water are very high, such as the Marcellus and Utica shale gas plays in Pennsylvania and Ohio, a thermal process such as an evaporator or crystallizer is needed. One process uses a patented crystallization technology at low temperatures and deep vacuum, removing sodium and calcium chlorides as well as heavy metals found in produced water. Offered as part of a zero liquid discharge (ZLD) solution that includes other proprietary technologies in a centralized treatment system approach, this technology is efficient and economical for produced water with high levels of TDS.
In Canadian oil sands operations, thermal processes are required to manage the highly soluble brine and reduce system fouling. A typical treatment train would consist of de-oiling pretreatment as well as evaporation and crystallization technology to recover high volumes of water for reuse.
Tailored Solutions for Every Situation
As the oil field industry continues to transition to a more sustainable approach to water use, determining strategies for cost-effectively managing produced water will be increasingly important. When it comes to the treatment of produced water and its intended use after treatment, one size does not fit all. Determining the best technology and most sustainable solution requires the analysis of each variable and the expertise and support of a water technology provider working in close partnership with the operator.
When that solution is combined with an operations contract, performance is guaranteed for the life of the contract, eliminating the risk and environmental liabilities that are typically associated with produced water management. A sustainable solution with guaranteed performance reaps benefits not only for the oil field producer but also for the surrounding communities.
Benefits of Water Treatment Solutions
Key benefits of water reuse technologies:
- Reduces fresh water demand, disposal costs, truck traffic and emissions
- Effectively removes scale formers (hardness, silica, iron, barium, strontium)
- Effectively removes particulates (oil, solids, bacteria)
- Creates an absolute barrier to particulates, minimizing downstream carryover
- Achieves high system recovery rates (greater than 98 percent), minimizing liquid waste generated
- Reduces costs with packaged, modular, shop fabricated technology
- Reuses and recycles produced water from shale oil production
Benefits of a solution for surface water discharge:
- High water recovery rate
- Low waste volume
- Effective control of scaling and fouling
- High salt rejection
- Continuous clean-in-place
- Production of effluents suitable for surface discharge or reuse
- Ideal for both flowback and produced water
- Efficient removal of sodium and calcium chlorides as well as the heavy metals in the effluent generated in shale gas production
- Only waste produced is a solid salt cake that passes toxicity characteristic leaching procedure (TCLP) testing, suitable for landfill disposal
- Eliminates the need for chemical softening pretreatment
- Recovers greater than 95 percent of the wastewater for reuse
Proprietary crystallization technology benefits:
- Achieves complete desalination of high TDS produced waters
- Provides the lowest operating expense as compared with conventional methods
- Eliminates the need for calcium removal pretreatment, minimizing sludge production
- Produces clean water and stable solids for disposal or reuse
- Simple, robust process with high reliability and availability