Page 228 - A Practical Companion to Reservoir Stimulation

PRACTICAL CONSIDERATIONS FOR FRACTURE TREATMENT DESIGN

Several individual pump rates can be added together to
show the total pump rate.
Time scales can be easily changed so that data are
plotted against a log of time or square root of time rather
than linear time.
Surface pressures, fluid frictions and fluid densities are
combined to calculate bottomhole pressures.
The real-time treatment of data has proved to be invaluable
when monitoring complex treatments.
In some cases these same computers not only monitor but
also control additive rates based on a predetermined sched-
ule. Process control of blending and pumping operations is
proving to be a major advancement in treatment execution.
Proppant schedules are now ramped rather than added in Figure P-68-Computer system with multiple processors
capable of simultaneous monitoring, control and simulation.
stages. The field computer uses input from density sensors to
control the amount of proppant added at the blender. Very
small but precise changes can be made in the proppant measures pressures over an entire pressure span. The resolu-
addition, providing the accuracy needed for the always in- tion indicates the size of pressure increment needed to affect
creasing ramped schedule. Viscosities of fracturing fluids the measurement.
may also be changed by process control. Polymer loadings Earlier pressure transducers used a hydraulic separator
are tailored throughout the job to deliver the desired fluid hose between the high-pressure connection and the sensor
properties. The computer-controlled execution helps elimi- electronics. The pressure actually moved a pressure indicator
nate human-induced errors that can be detrimental to the in a sight glass, and electronics measured the movement of
treatment (Fig. P-68).
the pressure indicator. This type of sensor was very reliable
P-7.5.2: Sensors but did not provide the accuracy or resolution required for
Sensors acquire input data so the numerous operations taking detailed evaluation.
place on location can be tracked and accounted for. Most any P-7.5.2.2: Density Sensors
parameter needed for evaluating a fracturing treatment can be
followed with sensors. Pressure, rate, density, temperature, For years radioactive densitometers have been used to mea-
pH and viscosity are some of the most common parameters sure density. This technique provides a nonintrusive, con-
requiring sensor output. tinuous density measurement for any fluid flowing in a pipe.
Two basic types of signals are sent by most sensors to the The technique is based on the absorption of gamma rays by
data-acquisition device. Frequency sensors deliver a pulsed the measured fluid.
signal. This type of sensor is often used in rate measurements The densitometer consists of
where the frequency of these pulses is directly proportional to A radioactive source on one side of a pipe.
the rotational speed of a pump. Analog sensors deliver a A gamma ray detector on the other side of the pipe.
current signal with a defined range. Common signal ranges An electronic panel to provide a signal reading.
are 0-20 milliamp or 4-20 milliamp. The sensor is then
calibrated to give minimum and maximum values based on As fluid passes through the pipe, gamma rays emitted by the
the signal range. source are attenuated in proportion to the fluid density. The
detector senses the gamma rays transmitted through the fluid
P-7.5.2.1: Pressure Sensors and converts this signal into an electrical signal (Fig. P-69).
Pressure transducers measure the deformation of a sensing The electronic panel processes the electric signal into a
material to provide a pressure reading. Specially designed density indication. A denser material absorbs more radiation,
strain gauges are bonded to precision-machined metals that resulting in the detection of fewer gamma rays. Thus, the
subtly deform as they are exposed to pressure. The quality of signal output of the detector varies inversely with respect to
pressure transducers varies both in accuracy and resolution. density.
The accuracy of a gauge is determined by how closely it Most densitometers use a radioactive isotope with an
extended half-life. A densitometer using cesium 137 can

P-67