by C. Jim 'Jamie' Stewart IV

In a fracturing operation, the blender's job is to mix sand, water and additives precisely, then feed the mixture to the fracturing pumps at pressures high enough to keep cavitation from occurring in the fluid ends. Blender underperformance can lead to multiple costly failures and poor fracture quality. The blender's importance is often overlooked in favor of the pumps because a site usually has many pumps but only one or two blenders. Most modern blenders are fully automated, highly complex pieces of machinery and can be operated manually at the blender or treating van if necessary. The blender is a key piece of equipment to the success of any fracturing job.

Risks of Underperforming Blenders

One type of blender failure is called a screen out. This occurs when the blend of sand, water and additives forms a plug or screen downhole where the pressure required to continue pumping exceeds the safe limitations of the wellbore or wellhead equipment. This can happen for several reasons, but almost all are related to improper operation or underperformance of the blender itself. A screen out usually requires the operator to stop pumping and the wellbore to be cleaned out before pumping can begin again. A screen out can be very costly in both dollars and downtime. The second and most common type of blender failure occurs when the blender cannot provide slurry at a high enough pressure at the fluid end of the fracturing pump, which causes cavitation that leads to fluid end damage. The most common cause of cavitation is running the tub or mixing chamber dry, lack of horsepower (HP) or a worn out booster pump. Extreme cavitation can lead to simultaneous failure of some or all of the fracturing pump fluid ends. This type of failure can cost the service provider hundreds of thousands of dollars, depending on how many pumps are working at the time. Evidence of a cavitation event may not materialize until hours or even days later, making it harder to pinpoint the timing of the occurrence. Skilled operators and good blender designs can help avoid these costly failures.

Blender Design

A properly designed blender has adequate HP to provide rate and pressure at a desired density. The denser the fluid, the more HP is required to move a given volume at the desired pressure.

Figure 1. A properly designed blender has adequate HP to provide rate and pressure at a desired density. (Courtesy of Surefire Industries)Figure 1. A properly designed blender has adequate HP to provide rate and pressure at a desired density. (Courtesy of Surefire Industries)

Blenders are typically rated for a maximum flow rate, but major differences in performance can be uncovered when flow, pressure and density are examined together. Ask the manufacturer for a performance curve showing performance at typical operating parameters and find out how much HP is installed. Make sure that the system has adequate HP plus some in reserve to account for wear. Many modern blenders in the 120-barrel-per-minute and above class should have 1,300-1,400 HP to handle the rate and density required for some of today\'92s typical shale jobs. A properly designed blender has a mixing tub/pump designed to mix widely varying sand concentrations at different flow rates without sanding off. The two main types of mixers are the tub type, which is typically characterized by an open top, and the mix pump design, which combines sand and water at the vane of the pump. Tub type blender designs should also have a level sensor that can read accurately in the presence of foam. The advantage of the tub type is that it can generally handle larger volumes of sand and water. However, the tub can be overfilled if the tub level system is not properly calibrated. The main advantage of the mix pump is that it has no fluid level system, and it also acts as the discharge pump. But this design usually cannot handle the higher rates and sand concentrations commonly seen in slickwater fracture treatments very well. Before making a selection, users should understand what the typical job for the blender will be. Will they be pumping high-rate slickwater jobs, or is this blender going to be used for more low rate, low pressure, linear gel fluids? Favor tubs in cases where flow rates and sand concentrations are higher. Lastly, a properly designed blender has a modern control system that can be controlled locally, remotely and in an automated fashion from a predetermined job schedule coming from the data van or remote site. Today's sophisticated electronics have intelligence built into the local controller for critical safety concerns but maintain robust communication systems that can quickly manage the operation of the unit. The hardware and networking infrastructure of older technology systems cannot handle the demands of the more complex controls architecture and the data-intensive communication requirements. Modern blenders can handle more than 250 different data points from the blender. Many blenders have Ethernet communication rather than serial, Modbus, CAN Bus or similar communication architecture. This equipment should also have modularity and scalability built in to the control system to grow with the blender and the application. Traditional programmable logic controller (PLC) technology often falls some of the more feature-rich embedded PC-based automation systems. Demand for enhanced automation, tons of data and remote access to that information in real time can quickly exceed the limits of older legacy systems.