Page 229 - A Practical Companion to Reservoir Stimulation

PRACTICAL COMPANION TO RESERVOIR STIMULATION

function accurately for nearly 30 years if the electronics are change in density of 1.63 lb/gal). However, a similar change
properly maintained. in density (1.52 lb/gal) from 13.96 lb/gal to 15.48 Ib/gal
Care should be taken to ensure that density measurements corresponds to a change from 15.0 to 23.5 ppga.
are always taken under pressure. Air can become entrained in
a slurry when proppant is added. This air has the effect of P-7.5.2.3: Rate Sensors
increasing the volume of the slurry without a substantial Several styles of rate sensors are used for monitoring high-
increase in mass. Figure P-70 shows that nearly 100 psi of pressure treatments. Some of the common rate sensors in-
pressure is necessary to eliminate most of the effects of air clude pump stroke counters, turbine flowmeters, magnetic
entrainment. flowmeters and venturi flowmeters for gaseous fluids. Pump
A good densitometer will be accurate to within 0.1 ppg stroke counters generally measure the rotational speed of the
over a density range of 8.0 to 25 ppg. This remarkable pump. Each full rotation of the pump moves the pump plunger
accuracy can still lead to errors, especially when using a through one complete cycle. By knowing the volume of each
densitometer to detect proppant totals when treatments go to pump stroke, the number of pump strokes and the pump’s
high proppant concentrations. If the fluid and proppant den- efficiency, a rate can be easily calculated. This type measure-
sities vary by 0.1 ppg and the densitometer has’an accuracy ment is advantageous for high-pressure pumps that are
level of 0.1 ppga, an error of nearly 10% can occur at pumping high volumes of solids, as is the case in pumping
proppant concentrations above 10 ppga because the densito- proppants during a fracturing treatment. The presence of
meter is actually measuring radioactive adsorption, which is solids or varying fluid rheology does not affect this type of
proportional to density. The value of pounds per gallon must measurement. However, if the pumps are not fully primed the
be calculated from the density reading. This calculation at actual rate may be significantly lower than the stroke counters
higher densities is less accurate because a small change in indicate.
absolute density means a larger change in pounds per gallon. Turbine flowmeters are designed primarily for measuring
As an example, the density reading comparing no proppant to clean fluids. As fluid is pumped through the flowmeter hous-
3 ppga sand is 8.43 lb/gal and 10.06 lb/gal, respectively (a ing, turbine blades are forced to rotate. Each of the blades on

Figure P-69-Principle of radioactive densitometers.

P-68