Prior to performing a fracture stimulation, a series of tests should be conducted to
optimise the hydraulic fracture treatment design. Real time data recording and
analysis should bed used. Where possible, water inflatable packers should be used
when testing in open hole (less compressible than nitrogen). If possible down-hole
pressure and temperature transducers should be included for more accurate
measurement.
17.1 In Situ State of Stress Tests.
Used to determine which formations can contain a hydraulic fracture height growth.
Isolate the selected zone in open hole and inject small volume of fluid (10 gallons to
2 barrels freshwater) at low rate (2 GPM to 0.5 BPM) until breakdown of formation
occurs or a stabilised injection pressure is established. Record the bottom hole
pressure during the test. Repeat the test several times to overcome near well-bore
effects or until repeatable results are obtained. Carry out tests on each horizon in
the open hole section to determine barriers to fracture height propagation.
17.2 Step Rate Tests.
Used to determine breakdown and fracture extension pressures. Fracture extension
pressure is the stress that must be exerted on the rock to open a fracture and cause
it to grow. Pressure during the test is measured and plotted against time. Analysis of
the plot should show an inflection point in the rate of change of pressure which
indicates the pressure at which fracture extension took place. Where two inflection
points are seen at different pressures this indicates that the fracture has grown out
of zone. These should be consistent in magnitude with minimum in situ stresses.
Step rate tests can also be used to determine the magnitude of fracture tortuosity
effects in the near well-bore, by differentiating between measured friction pressures
and calculated perforation pressure drop, where no abrupt changes in net pressure
and closure pressure can take place.
17.3 Pump In/Flow Back Tests.
Used to measure fracture closure pressure. Fracture closure pressure is the
minimum horizontal stress in the rock less the fluid pressure in the fracture, and is
one of the determinants of fracture conductivity. Fluid is pumped into the well at
sufficient rate to cause fracture extension. The pumps are then shut down and the
well allowed to flow at a constant rate, with the pressure being plotted as a function
of time. An inflection point on this plot from concave upward to concave downward is
interpreted as the fracture closure pressure. This test should be repeated several
times to verify the closure pressure. Where two inflection points (closures) are seen
this can indicate the presence of natural fractures or a horizontal component ("T"
shape
17.4 Mini-Frac Treatments.
Used with actual fracturing fluid to measure fluid leak-off, fluid efficiency and gross
vertical fracture height. The gross vertical fracture height determines the treatment
size that must be pumped to achieve a given length and conductivity. Fracture fluid
efficiency is the volume of the created fracture at the termination of pumping divided
by the total volume of fluid pumped, and is a measure of fluid leak-off across the
fracture face. Since fracture volume cannot be physically measured, fracture fluid
efficiency and leak-off are derived from post fracturing pressure decay analysis.
Tests are usually conducted with using a volume ranging from 10% to 20% of the
planned fluid volume without proppant at the planned injection rate. Radio active
materials can be used to facilitate logging of the vertical fracture height.
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