Page 230 - A Practical Companion to Reservoir Stimulation

PRACTICAL CONSIDERATIONS FOR FRACTURE TREATMENT DESIGN

the rotor creates a small pulse as it passes beneath a magnetic by the changing viscosities or proppant addition. The electro-
pickup. Each revolution of the rotor generates a distinct magnetic flowmeter operates on Faraday’s law of electro-
pattern of pulses equal to the number of turbine blades. The magnetic induction. When a conductor is moved across a
rotational speed of the rotor defines the flow rate, while each magnetic field, an electromagnetic force is induced in the
pulse defines a volume of liquid passing through the meter. conductor. The electromagnetic force is orthogonal to both
Most turbine meters are calibrated with fresh water. As the the direction of movement and to the magnetic field itself.
viscosity of the measured fluid increases, the rotor speed The electromagnetic flowmeter uses a pair of coils mounted
decreases proportionally. Therefore, the turbine flowmeter on the outside of a nonmagnetic pipe. An electrical current
should be calibrated with the actual fluid to be measured to flows through the coils to produce the magnetic field. The
ensure accuracy. These meters are useful for measuring liq- electromagnetic force is created when fluid (a conductive
uid additive rates but have limited value when used in proppant material) passes through the coils. The biggest limitation of
slurries. Fracturing fluid viscosities often vary throughout a the electromagnetic flowmeter is that it cannot be used with
treatment, especially when proppant is introduced. The oil-base fluids.
proppant also tends to erode the turbine blades, further; reduc- To accurately measure nitrogen or carbon dioxide rates, a
ing the accuracy of the flow measurement. venturi flowmeter should be used. The velocity of the gas
Magnetic flowmeters are becoming more popular for through the venturi is determined by measuring the pressure
measuring treating fluid rates. These flowmeters measure the drop across a reduced area (Fig. P-71). The line pressure and
rate of fluid flow in a line, similarly to a turbine flowmeter, temperature must also be measured to compensate for any

I I

1 I

0 50 100 150 200 250 300
Pressure (psi)

Figure P-70-Effect of entrained air on slurry density.

P- 69