Saltwater is a byproduct of oil and gas production. Producers dispose of this byproduct by injecting it back into porous rock formations deep in the Earth. This not only protects groundwater and surface water from contamination but can aid in the artificial lift and production of oil and gas. For saltwater injection, high-pressure pumps are used. This article discusses how to properly apply alternating-current (AC) variable frequency drives (VFDs) to optimize this process and save energy when using multistage centrifugal pumps.
Saltwater disposal typically requires either positive displacement (PD) plunger pumps or multistage centrifugal pumps to achieve the high injection pressures. PD pumps are constant torque and require heavy-duty AC drive ratings. Multistage centrifugal pumps are variable torque and can use normal-duty AC drive ratings, which are more cost effective. The multistage centrifugal pump (H-pump, HPS) has become a popular choice because of its reliability, reduced maintenance and efficiency. When coupled with VFDs, it provides significant energy savings when the process allows for reduced flow rates. Other benefits include:
- Process automation
- Reduced starting current
- Increased production
Minimize Consumed Energy
The main reasons AC drives are used in saltwater injection applications is for process control and energy savings. Saltwater is collected and delivered to injection well sites. Once delivered, it is stored in pits or tanks until it is injected into the well. AC drives can match the injection and collection rates by varying the pump speed, which also results in reduced energy consumption. Anytime the injection flow rates can be reduced, energy consumption is also reduced because of the affinity laws.
For example, an injection well with a 500 horsepower (HP) centrifugal pump (which uses approximately 370 kilowatts) can handle the maximum permitted injection rate for the well. However, what if an end user only needs 80 percent capacity? By operating the pump at full speed 80 percent of the time, the system will use 80 percent of the energy and produce 80 percent of the maximum flow (see Equation 1). However, by operating the pump at 80 percent speed 100 percent of the time, the user will still get 80 percent flow but it will only consume 51.2 percent of the energy (see Equation 2). At 8 cents/kWh, the energy savings equal about $74,677.
Reduce Starting Current
Often, disposal wells are in remote areas with limited power grid capability, making full-voltage startup impossible. Additionally, power companies frequently place restrictions on maximum starting current. VFDs are often the only means of operation available. AC drives can limit motor starting current to less than 120 percent of the motor rating. This can reduce system demand charges from the utility, further reducing operational costs.
Automate & Maximize Production
Regulatory agencies may limit the amount of fluid that can be disposed in each well. By adjusting the speed, an AC drive can optimize the amount of fluid injected while staying within the permitted parameters. Normally, fluid viscosity and well head pressure change the fluid flow rates. By using an AC drive, the speed can compensate for these changes and automate the process . By using the drive’s built in setpoint, proportional-integral-derivative (PID) control, the process can be automated to tank or pit level, permitted flow or injection pressure. Minimum and maximum speeds can be entered to operate the pump within the required guidelines and restrictions.
AC drives require a clean, dry environment for optimum reliability. The drive must be placed in a suitable environment or installed in a protective enclosure. The most common enclosure used in saltwater disposal applications is National Electrical Manufacturers Association (NEMA) 3R. Additionally, an end user may consider adding the following devices to the panel:
- Input circuit breaker for short circuit protection
- AC line or direct current bus reactor to filter the input power
- Surge suppression to protect the drive from line-side power disturbances
- 50 C ambient rating for hot climates
- Space heaters for cold climates
- Filtering or multi-pulse rectification for harmonic attenuation
AC drives offer significant energy savings and process control advantages when used in saltwater disposal applications. In addition, drives can operate in remote areas with inadequate power to support full-voltage startup by eliminating inrush current.