In hydraulic fracturing, the current practice of pumping proppant-laden fluids through high-pressure reciprocating pumps exposes the fluid end components of those pumps to severe erosion damage. This can be a significant contributor to operational downtime for that equipment and added costs.
A new technology for hydraulic fracturing can significantly reduce or even eliminate erosion damage caused by proppants to fluid ends and related components. This technology employs isobaric pressure exchangers incorporated into a manifold trailer, also known as a missile, which allows the proppant-filled fluid to bypass the high-pressure pumps and instead travel through the pressure exchangers. The pressure exchangers send the fluid downhole at the required pressures and flow rates without requiring the high-pressure pumps to touch the sand that can cause abrasion and erosion.
Pressure Exchanger Technology
The principle of transferring energy from one medium to another is used in many devices. For example, in gas and hydraulic turbochargers, an exhaust or waste medium (hot exhaust gases from an internal combustion process) at high pressure transfers its energy via a shaft to a second medium, such as air. These devices command efficiencies in the range of 60 to 80 percent. An isobaric pressure exchanger, on the other hand, operates with efficiencies in the 93 to 97 percent range.
These devices consist of four main components: two end covers, a free-spinning rotor driven by fluid flow and a sleeve encompassing the rotor. This device has four ports, allowing for fluid inlet and discharge at high and low pressures. End covers are designed to perform multiple functions. The fluid flow path through the end covers determines the torque imparted on the free-spinning rotor. The end covers also provide axial bearing support for the rotor, as well as sealing between rotor ducts and end cover inlets and discharge. The fluid and clearance between the rotor and sleeve provide radial bearing support for the rotor.
An isobaric pressure exchanger employs a positive displacement mechanism that incorporates two separate fluid flows. Designed without a piston, the device uses equal fluid flow rates of low pressure and high pressure. Controlling flows ensures the transfer of pressure from high-pressure fluid to low-pressure fluid is efficient (see Figure 1).
As the fluid passes into the multiple ducts of the rotor—the pressure exchanger's only moving part—energy from a high-pressure fluid is transferred to a second low-pressure fluid. The fluid flows and fluid travel distance within each rotor duct control the speed of the rotor and determine the level of mixing between the two fluids. In the current design, mixing levels can be as low as 3 percent. Some of the other design parameters that affect the performance of the pressure exchanger include rotor duct length, open and closed location of rotor ducts relative to end covers, and the angles at which fluids enter and leave the end covers. This iteration of pressure exchanger, designed as a hydraulic pump and integrated into a missile, performs at up to 95 percent efficiency.
Mechanical & Structural Design
The pressure exchanger hydraulic pump cartridge components are manufactured from a proprietary grade of tungsten carbide-binder composite, also known as cermet. Primary reasons for using tungsten carbide include performance in abrasive environments, structural integrity at extreme pressures and high stiffness to maintain tight clearances.
Considerable effort is made in designing the pressure exchanger's primary and secondary flow path. Primary or bulk flow can determine the performance of the pressure exchanger. Further, secondary fluid flow determines rotor bearing performance and load support, allowing the device to operate at required loads. Computational fluid dynamics (CFD) and finite element analysis (FEA) are employed to achieve these design considerations. Analysis-based designs are validated by extensive testing in a fully simulative in-house test loop that replicates service pressures and flow conditions.
Cartridge Housing Design
Cartridge housing is an integral part of the pressure exchanger hydraulic pump (see Figure 2). It is highly engineered to contain the extreme pressures used in hydraulic fracturing. This component links the pressure exchanger cartridge to the missile. Housing design considerations include:
- ability to withstand pressures of 15,000 psi
- internal flow path
- high- and low-pressure connections to pressure exchanger hydraulic pump
- valves and pipe connections to missile
- weight and cost
The traditional layout of a hydraulic fracturing operation involves equipment and a process flow as shown in Figure 3 (see page 8). Figure 4 shows the equipment and flow path that allow operators to bypass pumps using the new missile with pressure exchangers (see page 8).
The traditional layout exposes the pumps directly to proppant-laden fluid coming from the blender. Life of pump components is primarily a function of proppant concentration, fluid type and treating pressure. Pump component failures are initially concentrated around valves and valve seats. The presence of proppants, high closing forces between valve and seat and high velocity of the proppant-laden fluid entering as soon as the valve opens combine to create an environment where valves and seats become severely damaged very quickly. As a result, pumpers have rigorous schedules to replace valves and seats to avoid the migration of damage to pump fluid ends. This puts a burden on productivity and operating costs during treatment.
The new missile design and layout eliminate the issues with worn pump valves and seats through bypassing the proppant-laden frac fluid from the frac pumps. The new design requires two streams of fluids flowing into the missile:
- high-pressure, clean fluid from the frac pumps into the missile and into each pressure exchanger through a high-pressure common manifold
- low-pressure frac fluid from the blender into the missile and into each pressure exchanger device through a low pressure common manifold
The clean fluid stream at high pressure provides the energy that is transferred to the low-pressure frac fluid stream within each isobaric pressure exchanger on the missile.
High-pressure frac fluid leaves the missile downhole through a high-pressure manifold, and clean fluid leaves through a low-pressure manifold back to the water tank.
Until modifications are incorporated into existing equipment on location, the use of the new missile will require additional skid-mounted components as part of the rig. The first of the skids incorporates a diesel engine and boost pump to provide fluid to the high-pressure pumps through a low-pressure manifold.
In the traditional fracture treatment configuration, the blender provides the boost to the frac pumps. The second skid provides additional boosts from the blender to the low-pressure portion of the missile manifold to the pressure exchangers. The additional boost of about 50 to 100 psi is required to maintain a positive pressure on the discharge of the low-pressure port of the pressure exchanger.
The pressure exchanger is extremely efficient, but it is not perfect. The pressure exchanger performs at up to 95 percent efficiency, but this means roughly 5 percent is lost in the energy transfer process. So the flow rate on the clean side of the system has to be approximately 5 percent higher than the flow rate on the slurry side of the system. For example, if the desired downhole rate for the dirty fluid was 100 barrels per minute (bbl/min), a clean fluid rate of 105 bbl/min at the same pressure would be required. Because the clean flow rate is higher than the amount of water required at the blender, a portion of the water must be diverted. Two options are being tested for managing the extra fluid.
The first option uses a proportioning valve on the skid between the missile and the blender. The controllable valve enables some portion of the clean fluid stream to be diverted back to the clean water tanks, with the remainder going to the hydration unit or blender. The disadvantage is the potential need to manage any small amounts of sand that end up in the clean fluid stream, possibly requiring a small centrifuge between the proportioning valve and the water tanks.
The second option uses an additional high-pressure pump tied into the second low-pressure manifold. Rather than direct all of the clean fluid from the pressure exchanger back to the hydration unit or blender, some of it is used to feed a high-pressure pump. The high-pressure clean fluid from that pump would then be mixed with the high-pressure slurry stream from the pressure exchangers. In this scenario, it may be necessary to correct for the dilution of the slurry stream when designing the chemical and proppant addition schedules at the blender, depending on the relative rates, similar to the idea of split-stream fracturing.
Matthew Mandarich and Greg Neal1 reported results in a 2011 publication from a split-fluid fracturing process where some of the pumps handled proppant-free fluid while the rest pumped double the concentration of proppant for 54 days. The pumps with proppant-free fluid had about 96 percent fewer valve replacements and 100 percent fewer seat replacements.
The same publication also proposed that the number of valves and seat replacements did not change significantly for the pumps with double the proppant concentration as opposed to those that pumped normal proppant concentration. Therefore, the new missile design would provide a similar benefit to pumps in split-fluid fracturing. Additional benefits would include reductions in the crew for regular maintenance, the cost of consumables and downtime, and the addition of equipment redundancy.
Mandarich, Matthew and Neal, Greg. "Split-Fluid Method Improves Fracturing Operational Efficiency." Paper 141523 presented at SPE Production and Operations Symposium, Oklahoma City, OK, 27 – 29 March 2011.