The membrane treatment techniques use pressure-driven or electrical power. This flow diagram illustrates the basic process. (Image courtesy of MI-SWACO)

Combinations of chemical treatments, filtration, reverse osmosis (RO), and thermal, each playing a role, will become more standard operating practice in water treatment of oilfield brines. Dilution also will be used (if it is cost-effective and socially acceptable) to obtain the required water properties for reuse in frac fluid fabrication or environmentally acceptable discharge. All of these approaches will be applied in the effort to ensure that the water is reused to the maximum extent, reducing costs defined as financial, environmental, and social. In the process, these techniques will reduce the footprint on the locations where they are used and limit impact on the local communities.

Water treatment processes

Desalination refers to the process of removing excess salts from salt water (i.e., brackish, seawater, brine, produced water) to produce potable water. Originally, desalination technology was applied to produce drinking water, but for the oil and gas industry, the technology has as its objective the treatment of water used in drilling and production operations.

A desalination process separates salt water into two streams:

• Product freshwater (nearly salt-free water with a minimal concentration of dissolved solids); and

• Brine concentrate (high concentration of dissolved solids).

Desalination in the upstream oil and gas industry has come to prominence during the last two to five years in great part due to the increased use of water in the completion and stimulation of the unconventional shale gas plays across North America. The three driving objectives for treating, recycling, and reusing water obtained from fracture flowback are economical, environmental, and social.

Three categories of water purification technologies are used to desalinate the returned water: chemical treatment, membrane technologies, and distillation processes.

A closer look

Chemical treatment, as the name implies, makes use of chemicals that remove by precipitation ions that cause hardness. Ion exchange can also be considered a chemical process, but its use has been more limited to fluids with low levels of total dissolved solids (TDS).

The membrane treatment techniques use pressure-driven or electrical power. Pressure-driven membrane technologies include microfiltration, ultrafiltration, nanofiltration, and hyperfiltration, also known as RO. Both RO and nanofiltration can remove salts. The electrical-driven techniques are electrodialysis (ED) and electrodialysis reversal (EDR).

ED makes use of an electromotive force applied to electrodes adjacent to both sides of a membrane to separate dissolved salts in water. EDR is a very similar process except that the cation and anion reverse to routinely alternate current flow. Both ED and EDR remove or reduce contaminants from feed water, and the process is not sensitive to pH.

The most common types of membranes used for RO are polymeric, but over the last few years the use of ceramic membranes has increased greatly. Combined with the use of cross-flow filtration, this has increased the life of the membrane and in turn reduced operating costs. Membrane treatments have become more robust and more affordable in a range of applications. Ceramic materials also are being applied to the suite of filters in the pretreatment package, again allowing for improvements in nonproductive time and cleanup of the process.

The pressure required for the RO is directly related to the TDS concentration of the feed water. As an example, for brackish water, the pump pressure required is 140 to 400 psi. For seawater, this needs to increase to 1,200 psi. For oilfield brines, the maximum concentration of TDS for treatment with RO is approximately 35,000 ppm.

Thermal technologies, as the name implies, involve heating saline water and collecting the condensed vapor (distillate) to produce pure water. All thermal processes produce a high-purity distillate product with a salinity of less than 10 ppm. The capacity of the thermal desalination processes varies over a wide range from 3,145 b/d (500 cu m/d) to 345,939 b/d (55,000 cu m/day). Thermal technologies have rarely been used for brackish water desalination because of the high costs involved. For TDS concentration greater than 35,000 ppm, thermal techniques are more economical and practical than membrane-based technologies.

Thermal techniques can be sub-divided into three groups:

• Multistage Flash Distillation (MSF);

• Multi-Effect Distillation (MED); and

• Vapor Compression Distillation (VCD).

Thermal technologies also have been improving; the use of improved design in heat exchangers has greatly increased the effective operating efficiencies and related operating costs.

Desalination in action

After fracturing a well, approximately 25% of the injected fracing fluid returns to the surface where it is collected in holding tanks. The impurities include organic materials (bacteria present in the rock formation and fracturing chemicals), polymers (the friction reducers and cross-linked gels), residual hydrocarbons (trace oil and volatile organic compounds such as benzene and toluene), contaminants present in the rock formation (barium, calcium bicarbonate, iron, magnesium sulfate, sodium chloride, and strontium) that the fracturing fluid picks up in the formation, and suspended solids (clay, iron oxides, and silica).

The pretreatment of the return frac fluids is very important to the efficiency of all of the desalination techniques. The total suspended solids must be removed as well as residual traces of hydrocarbons. The organic material in terms of polymers for friction reduction and viscosity control also can be very detrimental to the removal of salts in the desalination processes.

Because of the homogeneity of the feedstocks used in the oil field, the pretreatment section of the total desalination process is critically important. Many failures in operation have been, on root cause analysis, attributed to the underperformance or misapplication of the pretreatment procedures. To investigate this process and the technology available for filtration and the range of material and techniques used, M-I SWACO has formed a research agreement with Texas A&M University under the auspices of the petroleum engineering department.

In general, membrane pretreatment requires all or some of the following treatment steps:

• Clarification;

• Hardness reduction;

• Iron removal;

• Polymer removal;

• Reduction of alkalinity by pH adjustment;

• Addition of scale inhibitor; and

• Filtration.

Some of the pretreatment steps can be removed in the distillation techniques.

What is required in the upstream oil and gas business in terms of the desalination equipment? In the environment where it is being widely used, the packages need to be mobile, flexible, robust (especially for transportation to remote locations), and reliable.

In desalination processes globally, cost is a major factor, and it is usually site- specific. The normal factors affecting desalination cost are the quality of the feedstock. The higher the TDS, the more energy is required for treatment. Pretreatment costs to remove or reduce the levels of contaminants also can be significant. This is particularly true for the processing of oilfield brines as the homogeneity of the feedstock is so variable.

Plant capacity is very important because it affects the size of the treatment units, pumping, water storage, and related equipment. Regulatory requirement costs associated with meeting local, state, and federal permits also can be significant.

The cost per unit volume treated is directly related to the concentration of TDS, energy costs, capital, and other related costs. In oilfield applications, the cost drivers are as stated for any desalination process. With the requirement for mobility, the capacity per treatment unit is reduced to allow for transportation. Mobilization and demobilization also are major costs in the oilfield operation.

A normal oilfield location is such that the desalination process can be working on flowback from a single well, multiple wells, or pads with multiple wells per pad. There has been a move in many locations to have central processing stations where water is either pumped directly or trucked. The fluid is stored in pits or suitable tanks prior to treatment. This allows continuous operation on a blended feedstock that can allow optimization of the operating parameters to ensure maximum efficiency, minimum downtime, and more cost-effective solutions.