When it is time to design a fracture treatment, one of the biggest concerns is where to get enough water. Logistically, desert areas pose the largest problem, but the shortage of water for fracturing is almost universal. Offshore, stimulation companies have figured out how to use seawater to supply their needs, but this brings up another problem – disposal.

Once a well is treated, it is allowed to flow back and clean up, but even though seawater was used in the treatment, the effluent from the flowback simply cannot be dumped overboard. It may contain oil or other undesirable elements from the well that must be removed until the liquid is of sufficient purity to be disposed at sea.

Within seconds of treatment, coagulants begin to separate. Heavier ones sink to the bottom, where they are removed, and lighter ones float to the surface, where they are skimmed off, leaving clean, pure water suitable for reuse in stimulation. (Images courtesy Halliburton)

A new treatment has been developed that promises to solve both problems – that of supplying suitable volumes of water for fracing and cleaning up flowback fluids so they can be disposed.

What is the incentive?
Two factors make water treatment attractive. On land, a single treatment can require 1,000 truck trips hauling water from a licensed source to a temporary impoundment at the well site or hauling flowback water away to a treatment facility or disposal site. Transportation costs alone can exceed US $250,000 for a single well. In areas where there are few natural water resources, such as the desert, produced water from existing wells, or even brine from specially drilled wells that tap saltwater aquifers, can be treated and used for fracing. Essentially, reusing produced water or flowback water from previous treatments kills two birds with one stone. It gets rid of a disposal problem while supplying a much-needed resource for stimulation treatment.

The main impediments to onsite water treatment have been cost and volume. One process produces drinkable water at a reasonable cost, but only at about 100 b/d. Another mimics the hydrological cycle to create distilled water that is pure enough, but at a high cost.

Modular treatment units with high throughput offer flexibility and cost reduction by providing recycled treatment water.

Another issue has been mobility. Frac treatments are conducted all over the world, often at very remote sites. If water treatment is required, whether for use in fracing or for disposal of effluent, treatment equipment must be portable. Currently, several modular units are available or under evaluation that fit on a standard 53-ft (16-m) long trailer. For offshore use, the modules could be packaged in a standard sea container or integrated into a well-treatment vessel’s equipment.

Treatment approaches vary. Some use a distillation process, while others use an electrostatic separation process. Both techniques are capable of producing water that is acceptably pure for reuse in subsequent fracing jobs or disposal. Some units have the capability to “polish” processed water to remove traces of oil-in-water below the mandate-required 29 ppm.

Most units accumulate a concentrated “sludge” similar in appearance to mud cake that must be disposed in an approved landfill. But the volume of solid waste removed during the treatment process is small compared to the volume of clean, treated water that can be produced. A few techniques have the ability to remove specific minerals from treated water, allowing benign minerals to pass through. This means that the customer can specify the degree to which the flowback water or produced brine must be treated by removing only those elements that would impair the subsequent treatment design.

A new system makes its debut
Halliburton’s Cleanwave Water Treatment Service is designed to address all water needs – treatment for reuse and/or disposal in an automatic process. It removes suspended solids, oil, other insoluble organics, and bacteria from the water.

The unit is modular and transportable, either on land or on sea. A single unit is capable of outputting 20 bbl/min (28,800 b/d) of treated water. Scalability is achieved by using multiple modules side-by-side. The unit uses a combined electrocoagulation and electroflotation technique and can handle effluents with 100 mg/l total dissolved solids (TDS) up to 300,000 mg/l TDS. This amounts to 99% TDS, and the system can coagulate particles less than one micron in size. Turbidity of the treated water is less than 10 nephalometric turbidity units.

In principle, electrocoagulation destabilizes and coagulates suspended colloidal matter in water. At the same time, gas

Modular treatment units with high throughput offer flexibility and cost reduction by providing recycled treatment water.

bubbles produced by the process attach themselves to the coagulated matter, causing it to float to the surface, where it is skimmed off continuously. Heavier coagulants sink to the bottom, where they can be removed, leaving a clear water product. This product is routed to the electrostatic polisher, where remaining oil is stripped out, making the product suitable for disposal at sea.

A tough test is passed
Recently, an operator planned a well stimulation in a remote area of Utah. Performed in winter, the operation took place atop a mesa that was accessible only by a single-lane road 40 miles (64 km) long. Much of the time, water trucks were required to be pulled to the well site behind snow plows. The operator required a reliable source of treatment water. A Cleanwave treatment pad was established nearby so the operator could deliver produced or flowback water for treatment and haul clean water back for subsequent use in the completion procedure. Using the water treatment service, the operator was able to achieve reduction in transportation costs while ensuring an ample water supplyto support drilling and completion activities. More than 1,000 truck trips were eliminated for a net savings of $250,000. In addition to economic benefits, the operator noted:
• An extremely small footprint – less than 2,500 sq ft (232 sq m);
• Elimination of bacterial growth;
• Reduction of H2S to nondetectable levels;
• Overall reduction in volatile organic carbon emissions;
• A system flexible to fluctuation in inlet volume and TDS concentration;
• A system capable of handling both high concentrations of TDS and hydrocarbon contamination; and
• Reduction in risk of road accidents and environmental damage.

Every barrel of oil produced in the world is accompanied by 3 bbl of water, and 10% to 40% of treatment water flows back from each fracing operation contaminated with formation fines and minerals along with polymers from the frac treatment. Obviously, there exists a clear mandate for an effective and economical treatment technique. Recycling treatment and produced water is a solution that could find favor with the industry as well as environmentalists.

Treatment water chemistry and bacteria content must be controlled. Even if an abundant source of natural water is readily available, it might not be optimally compatible with the treatment formula or the formation being treated. Logs and core analysis are used to develop a clear foreknowledge of downhole conditions before a treatment is designed. Frac fluid can be formulated for maximum compatibility as it interacts with formation minerals and conditions.

The ability to remove undesirable elements and compounds from flowback or produced water from adjacent wells gives operators the opportunity to optimize each treatment for the well it is designed for, whether in the desert or the deep blue sea.