Late last year, operators of a water injection well in Kansas started having problems. They noticed a reduction in injectivity, and the well was failing. They’d been operating this well for a while, and the machinery was in good shape. Their personnel on site were experienced. So what was the problem? Water quality.
Companies in the U.S. started injecting industrial waste into deep wells during the 1950s. Injection was the safest and least expensive choice for disposing of industrial byproducts. There are now about 180,000 of these wells in the U.S. They’re mostly in Texas, Oklahoma, California, and Kansas—and operators inject 2 billion gallons of brine every day.
With so many saltwater disposal wells in operation, what’s the No. 1 factor in their failure? Water quality. Poor water quality causes a laundry list of problems. Bad water leads to mechanically induced harm, damage from water/rock interactions, permeability issues, biological impairments, and interactions between fluids.
Mechanically induced harm
Because of poor water quality, the operators of the water injection well in Kansas experienced damage that was mechanically induced. Injecting bad brine downhole meant solids were getting into the formation. These solids caused plugging in the injection face area. They also experienced velocity-induced harm—aka fines migration.
Fines migration in water injection well
Usually smaller than 40 microns, these loose particles in sandstone’s pore spaces cause big problems in salt water disposal wells.
Though they’re present in every hydrocarbon formation, fines migrate when water quality is poor. According to research, fines migration happens when low salinity or high pH fluids are injected into the formation. When you’ve got a fines migration problem, it reduces the permeability of your formation by up to 22 percent.
Solids and fines migration in the Kansas well reduced productivity.
Damage from water/rock interactions
Injecting water with poor quality led to more problems for the Kansas salt water disposal well operators. Because their water wasn’t extensively treated, they ended up with interactions between the water and rocks in the formation. The low-quality fluid led to changes in wettability and chemical absorption of the formation. It also caused swelling, deflocculation, and ultimately the formation started to break up. Cost savings from low-quality treatments ended up leading to expensive and well-killing damage.
In addition to the permeability problems caused by fines migration, the Kansas water injection well experienced serious relative permeability effects. Skim oil and free gas entrainment drastically reduced injectivity. Research shows that when operators inject water with just 10 to 15 percent entrained skim oil or free gas, it reduces permeability by up to 95 percent in the affected area. If the operators continue to inject the bad water, the damage zone just gets bigger. Injecting the cleanest water possible keeps salt water disposal wells healthier.
No matter how clean your injection water is, it’s going to contain bacteria. But poor-quality, bacteria-rich water—like operators were injecting into the Kansas well—does serious damage. And under the right conditions, the bacteria can create lethal H2S gas. Bacterial growth on metal equipment leads to pitting in machinery and pipes. Also, when bacteria-rich water is injected, over time it will plug the pore throat, leading to reduced injectivity. Ensuring your water is high quality before injecting it will keep your workers safe and prolong the life of your equipment and well.
Interactions between fluids
Besides all the other negative interactions it causes, poor water quality leads to damaging interactions between injected and in situ waters. The two fluids in the Kansas well interacted and led to insoluble scales and problems with emulsification. Operators also ended up with iron precipitates that plugged the pore system. They were lucky, however, because the interaction didn’t end up creating sulfate and carbonate precipitates as well, or wax and asphaltene deposition problems. Low-quality injected fluid leads to damaging fluid interactions.
Effectively treating fluid before injecting it into your water injection well may not seem cost-effective. There’s always something that sounds more important. But poor water quality is the No. 1 cause of saltwater disposal well failure. If you want to avoid mechanically induced harm, damage from water/rock interactions, permeability issues, biological impairments, and interactions between fluids, partner with a reputable company for effective pre-injection fluid treatment.