Field Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 503

the test section is followed by a shut-in period at closed valve, resulting in a decrease
of piezometric head in the test section (Cooper et al. 1967, Bredehoeft & Papadopu-
los 1980, Miehe et al. 1993). These tests can only be evaluated if a flow rate is measur-
able in a reasonable time. Drill stem tests are therefore economically applicable only
if T > 10 -7 m2/s (Hekel 1994).
The evaluation of drill stem tests is conducted on the basis of the flow rate measured
during the flow period. Then the shut-in period is evaluated with respect to the equiva-
lent rock mass permeability as with a constant rate test.
Impulse and pulse tests are constant rate injection tests with a short period at open
valve. In an impulse test, the test fluid is injected over a period of a few minutes. In a
pulse test the flow period is attempted to be held as short as possible (Fig. 15.32, upper).
In a constant pressure test the test fluid is injected at constant pressure and the volume
of test fluid V is measured over time (Fig. 15.32, lower).
As with Lugeon tests, constant rate tests and constant pressure tests often are carried
out in stages (Schneider 1981, Schneider 1983, Wittke 1999, Wittke 2000a).
In rock formations with low permeability, a gas such as nitrogen is often used as the
test fluid, since the rock mass permeability is larger for a gas than for a liquid. Pulse
tests and constant pressure tests with gas and brine as test fluids have been successfully
applied in rock salt with equivalent permeability values of some 10 -13 m/s (Stormont
et al. 1991, Wittke 1999, Wittke 2000a). Pulse tests allow rock masses with even lower
equivalent permeability values of < 10 -14 m/s to be investigated (Stormont et al. 1991,
Miehe et al. 1993).
Injection and production tests carried out in low permeability rock formations are fre-
quently evaluated by adjusting the measured pressures or piezometric heads, respec-
tively, and flow rates to characteristic type curves, which are graphical solutions of the
transient flow equation (Stallman 1963, Gringarten et al. 1979, Bourdet et al. 1983,
Horne 1990, Krusemann & De Ridder 1991). Due to the fact that the area surrounding
the borehole does not have the same permeability as the undisturbed rock mass (skin
effects), deviations from ideal flow behavior can be identified in type curves. Skin effects
can arise due to the drilling process itself, such as the formation of new cracks and fis-
sures or the plugging of flow paths by the drilling fluid, as well as due to later activities.
When interpreting production tests the liquid existing in the borehole before the begin-
ning of the test such as the drilling fluid or seepage water also needs to be accounted for
by a so-called “wellbore storage coefficient” (Miehe et al. 1993).
The results of injection tests conducted in low permeability rock formations are also
influenced by effects resulting from the properties of the applied test fluid, the defor-
mations of the borehole wall and the test equipment. These influencing factors need to
be identified and quantified by means of laboratory tests referred to as “system tests”.
If gas is used, the compressibility of the test fluid is the only influencing factor which
needs to be considered. If water and aqueous solutions such as brine are used the defor-
mations of the packers and the injection line also have to be accounted for (Wittke
1999, Wittke 2000a).