Upstream process: Well Stimulation

Fracturing truck with pump unit

After a perforating gun has punctured holes in the formation, it is time to crack the shale rock and create the fissures that will allow oil and natural gas to flow into the well. Fracturing equipment – mounted on a truck, trailer, or skid for easy mobility – include high-pressure pumps that send the liquid from the fluid end down the shaft and into the perforations. A high-horsepower diesel engine with cooling system and heavy-duty transmission provide power for the triplex or quintuplex pump, whose pressure also prevents the fracturing fluid from shooting back up the well.

Specialized pressure transmitters containing rugged measuring elements are designed to measure fracturing fluids that are corrosive, abrasive, and under very high pressures. Also, these instruments must withstand difficult working conditions, which include extreme weather, high vibration, and severe pulsation from the pump.

Fracturing blending unit

The fluid used for hydraulic fracturing is custom-mixed onsite for the specific conditions of each well and the type of fracturing desired. Fracturing blending units are the control centers for receiving the right quantities of hydrated fluid from the hydration unit, proppant (usually sand, treated sand, or ceramic material) to keep the cracked fissures propped open, and chemical additives to reduce corrosion, inhibit bacterial growth, maintain viscosity, and so on. This tailor-blended fluid is then transferred to the fracturing truck with pump units for injecting down the wellbore.

This blending system comprises a sand hopper and dry-chem and liquid-chem feeders. Specialized pressure transmitters help control and monitor the suction and discharge pumps, while pressure gauges on the control panel allows technicians to easily oversee operations. Temperature transmitters and compact resistance thermometers monitor the condition of the fracturing blending unit’s hydraulics, engines, and cooling systems.

Hydration unit

Water comprises about 90% of fracturing fluid. But before it reaches the fracturing blending unit, it is first mixed with certain polymers:

• Friction reducers so the fluid can be injected down a well at a higher velocity
• Viscosifiers like guar gum, as more viscous fluids can carry more proppant

In a mobile hydration unit, the polymer – in gel or powder form – is combined with water to produce a compound that is higher performance than ungelled fluid. This process uses load cells to accurately measure the weight of guar gum or another polymer in a hopper before it is hydrated. Pressure transmitters help control the suction and discharge pressures, while pressure gauges allow operators to monitor the hydraulically driven pumps.

Frac tank

Hydraulic fracturing calls for circulating an enormous amount liquids – mostly water but also chemicals and drilling mud – in and out of wells. Frac tanks are capacious yet mobile vessels designed to store vast quantities of fluids. A variety of level sensors can be used to ensure that a frac tank remains full enough for efficient operations, but not so full that it overflows.

There are many considerations when choosing level sensors for frac tanks, including:

• The fluid’s turbulence, viscosity, and compatibility with the sensor’s wetted parts.
• The desired accuracy and resolution.
• The mounting location and placement of the sensor.

Float sensors are commonly used in frac tanks due to the instruments’ high accuracy, ease of installation, and low maintenance. Usually mounted at the top of a tank, float sensors have a guide tube that extends all the way to the bottom. Around the tube is a hollow float that moves vertically along the shaft with the liquid level. Float sensors come in two types:

• Float switches incorporate one or several floats to detect a fluid interface, or are used simply to alert operators when minimum and maximum levels are reached.
• Float transmitters continuously monitor liquid levels, determining the fluid volume of tanks in real time, and are ideal for remote monitoring.

If top mounting is an issue, use a submersible pressure sensor for hydrostatic level measurement. This specialized type of level sensor, placed at the bottom of a tank, measures the pressure of a liquid column (e.g., inches of water), which is then converted into the liquid’s volume. When turbulence or sludge is an issue, an attachment like the LevelGuard^TM provides extra durability and stability.

If the fluid is too turbulent or viscous for a submersible pressure sensor, or if the fluid is corrosive or otherwise incompatible with the sensor, a foolproof solution is to move level measurement from inside the tank to outside of it. In this type of measurement, load sensors – like load pins, bending beams, or shear beams – are mounted to the tank’s support structure for the static and dynamic weighing of liquids; the measurements are then converted into a volume measurement.