ESG: Faster, larger, better?

Roughly six years ago, electrical technology moved into the well stimulation market to drive hydraulic fracturing pumps. To date, there are two primary electric fracturing approaches. Reflecting the upstream equipment, the first approach incorporates 600 VAC motors to drive its frac fleet. The second approach uses larger, single or dual shaft AC induction motors to drive one or two pumps simultaneously, but at a higher voltage. Both systems drive conventional frac pumps rated from 2,500HP to 3,000HP.

To reduce carbon footprint, OPEX and maximize HP density, frac pump designs are growing in size and HP capacity as a result of Environmental Social and Government (ESG) emphasis, increasing politicization of climate change, the push for E-frac has increased. In order to reduce the fleet size of frac spreads, most, if not all frac pump manufacturers are now developing, testing, or marketing pumps ranging from 4,000HP to 7,000HP. In contrast to diesel prime mover options, a direct-coupled electric prime mover rated for 5,000HP can drive a 5,000HP hydraulic pump, Fig. 1. Electrical losses in power supply to the motor do not require an increase in motor rating. Conversely to a conventional spread of 16 to 24 pump trailers @ 2,000 brake HP, an electrically-powered spread can yield a theoretical range of equivalent brake HP with roughly half of the number of pump trailers with significantly less maintenance cost, fuel cost, and environmental impact. Can it be more convincing that the conventional radiator + engine +transmission will be phased out?

Fig. 1. A 5,000-hp prime mover can drive a 5,000-hp hydraulic pump without the parasitic losses of a diesel prime mover.

The electric frac design has achieved standard acceptance industry wide. Development of new designs should incorporate the following key considerations to size and select the optimal components to drive the new high HP pumps:

Weather parameters. Given the range of elevations, climate conditions, and environmental differences in various continental shale plays, frac motors must meet requirements to operate -45^oC to +45^oC and weather-related ingress protection (min IP23W with filters) to mitigate potential contamination. The chosen unit should not de-rate to less than the power generation or pumping equipment during operation under extreme conditions.

Consider the available power sources when determining ideal frac motor voltage. Today’s electric frac spreads are powered by high-capacity natural gas turbines that generate between 2.6 kVAC and 14.4 kVAC. Other applications, such as drilling, production (ESP), gas compression, and more, are considering turbine generation to achieve ESG-mandated emission goals. These power sources generate medium voltages (MV) that provide greater power density, lower amperage, and easier distribution, using lighter cables and fewer electrical connections.

After voltage is determined, sizing a motor correctly for well stimulation must include extra HP and torque to overcome pump mechanical inefficiencies to match nameplated true hydraulic HP downhole. Using a Variable Frequency Drive (VFD) control, it’s simple enough to match a motor to a pump speed torque profile. The gearbox on the pump should allow the motor to run at a high speed range; this reduces the torque load and the corresponding size of the motor.  The maximum operating motor speed must stay below the “limiting speed” of the antifriction bearings and lubrication used. For example, the large bearings in a motor rated over 2,000HP should not exceed ~2,100rpm with high speed motor grease. The highest speed capability can be achieved with sleeve bearings. However, there can be problems associated with slow speeds (as when pressurizing in fracking operations).  Also sleeve bearings operate in a very narrow temperature range; they must be protected from ambient conditions and motor self-heating. Sleeve bearings in frac applications would add a necessary level of cost and complexity if high speed motors are mandatory.

In conjunction with the motor chosen, the design must incorporate a system of VFD, auxiliary support, operating controls, and physical footprint of the complete unit. Although parasitic losses are minimized in the motor to pump connection, parasitic power costs must be accounted for in the total power budget. Some variation of power source capacity, distribution techniques and number of assets per job may be afforded by the common industry practice to operate a frac spread within a comfortable range below maximum capacity to manage asset deployment and maintenance costs. As with any electrical integration, identify trade-off issues as early as possible to avoid costly remedies to irreversible design choices.