Changing one part of produced-water management achieves better results.
Produced-water management is a growing concern in the Permian Basin. As infrastructure grows, the cost of managing produced water is declining. One operator is taking a unique approach to increasing efficiency and reducing the cost of managing produced water.
The operator is in West Texas, focused on the Delaware Basin, developing the Bone Spring and Wolfcamp reservoirs. They also developed several midstream assets in this basin.
It is these midstream assets where the operator has been looking to optimize their infrastructure to improve efficiency while reducing cost. The goal was to evaluate the produced-water gathering system and look for ways to improve efficiency or reduce costs.
In 2017, the operator contracted with Hydrozonix to complete the evaluation of their produced water-gathering systems. Consistent with the operator’s strategy of continuous improvement, the team identified a potential area where both treatment efficiency could be improved while reducing costs. The operator had been employing a chlorine-dioxide strategy to provide bacteria, iron and sulfide control. Let’s briefly discuss why these prevention steps are important.
IRON AND SULFIDE CONTROL
Oxidizers like chlorine dioxide and ozone convert ferrous iron to ferric iron. This oxidation step changes soluble iron to an insoluble form, which is easier to separate from the water by filtration, settling, coagulation or flocculation. It is important to remove iron because it can form scale, act as a coagulant and cause formation damage. Iron-oxide particles can also cause wear of surface equipment and pipe. If reuse of the produced water is considered an option, then iron should be removed because iron can interfere with friction reducers and scale inhibitors.
Hydrogen sulfide is typically generated from sulfate-reducing bacteria. So, sulfide control can be preventing bacterial growth or oxidation of existing sulfides. Hydrogen sulfide is a deadly gas, which is why so much attention is spent eliminating and controlling it. Not all sulfides are hydrogen sulfide. There is also iron sulfide. Iron sulfide can be present in the form of scale, usually requiring the presence of hydrogen sulfide and iron.
Bacteria is responsible for microbiologically induced corrosion (MIC), hydrogen-sulfide formation, and plugging and fouling surface equipment and pipe, as well as downhole pipe. Bacteria is present in sessile and planktonic forms. Sessile bacteria are colonies that form on surfaces because of bacterial adhesion. Biofilms are an example of this. Planktonic bacteria are free floating in the water that is released from sessile locations due to their natural growth cycles or because of some disruption to the bacterial colony. Bacteria growth occurs in distinct phases, starting with the lag phase, then log or exponential, stationary and finally, the death phase.
During the lag phase, bacteria is just arriving and settling in, while getting used to its environment. There is little to no growth during this phase. The presence of growth means it has entered the log or exponential phase. The more suitable the environment, the quicker you go from lag to the log or exponential stage.
The log or exponential phase is where bacteria counts double over a unit of time. This unit is dependent on conditions and bacteria type, but can be as little as 20 minutes to as long as 20 hours. The goal here is to create an environment where the lag phase never ends or is very long by providing a harsh environment for bacterial growth. Oxidizers create this harsh environment while killing planktonic bacteria and oxidizing some food sources.
One thing to consider is when we test water for bacteria, we are only measuring planktonic bacteria. The assumption is if I see continuous low levels of planktonic bacteria, I should not have growing colonies of sessile bacteria. Although this may not be a perfectly accurate assumption, it sure beats climbing into a tank, and scraping the sidewalls and bottom looking for sessile bacteria colonies.
STANDARD GATHERING SYSTEM
Now, let’s get back to business. In your typical produced-water gathering system, the water goes from the wellhead to tank batteries, followed by a secondary oil/water separation step and then into a saltwater disposal (SWD) injection well. The primary oil/water separation is at the wellhead and then again at the tank batteries. Gun barrel tanks are typically used in an additional oil/water separation step before injection. When water flow slows down and becomes more static, the environment for bacteria growth is improved. That makes tank batteries and gun barrel tanks your biggest concerns for bacteria growth and where you should focus your bacteria testing and implement your bacterial-control strategy.
Tank storage and design is an important consideration for bacteria control. A gun barrel system requires a specific retention time to achieve good oil/water separation, but long residence time means a more static water environment. That tends to support bacterial growth. Another concern is tank short-circuiting. If the outlet of one tank is directly below the inlet, water can pass straight through the tanks letting some water to sit in the tank or tank system for much longer than expected or designed. This will lead to an ineffective oil/water separation system and a static water volume that can grow bacteria regardless of how effective your bacterial-control program is. Tank systems must be designed to prevent short-circuiting the tanks and pipework dead legs. Dead legs are abandoned or infrequently used lines, which if not properly isolated, become breeding grounds for bacteria. They can also cause water hammers.
CONCENTRATING ON GUN BARRELS
After evaluating the gathering system, the gun barrels became the focus. This was also the focus of their existing chlorine-dioxide program for bacteria, iron and sulfide control. Chlorine dioxide was introduced just prior to the gun barrel oil/water separators at a continuous dose rate. Produced water quality tends to fluctuate. To optimize an oxidation system, you would want your dose rate to change with your water quality. However, the operator wanted cost control. A continuous dose rate allowed for a controlled cost, but inconsistent water quality. A ceiling was set at $0.10/bbl.
The operator had a perfectly functioning program that provided the bacteria, iron and sulfide control they needed. But their goal was continuous improvement by evaluating all their practices and looking for more efficiency or opportunities for reducing cost by continually enhancing fundamentals. The operator’s technical team developed a scope of work for their West Texas water-gathering system serving the Scott Field area in Ward and Reeves counties. Hydrozonix was selected and proposed a fully automated ozone-treatment system. A comparison follows:
|Oxidant||$/BBL||Automation||Remote 2-hr Monitoring||Met Bacteria, Iron & Sulfide Goals|
The fully automated ozone system came to be known as HYDRO3CIDE. It was built to the operator’s requirements and incorporated with their existing automation and control. The automated oxidation system included weekly monitoring and water-quality testing as part of an ongoing partnership. The ozone system consists of an air compressor, which takes surrounding air and sends it to an oxygen concentrator. The concentrator separates the oxygen from the air by pressure swing adsorption. The oxygen stream then passes into an ozone generator where ozone gas is sent via venturi eductor into a slipstream from the gathering system.
The system is sustainable requiring only electricity and air to make the ozone used for oxidation. No chemical totes that can spill and need to be replaced. No concerns of degradation of chemicals in the harsh West Texas heat. No chemical storage. A simple low-cost alternative that comes with full automation, remote control and monitoring. Data is logged and stored for custom reports and trend analysis of water-quality parameters over time. All this valauble additional data at a lower cost.
The technical team required a 60-day review. During this period, the system had to have a greater than 90-percent uptime goal while maintaining bacteria, iron and sulfide control. At the completion of a successful review, the operator purchased the first HYDRO3CIDE system. The operator purchased a second HYDRO3CIDE, which was installed in the fourth quarter of 2017. A third system is in the planning stages for this summer.
Authored by Shale Play Water Management Staff