Page 222 - Geothermal Energy Systems Exploration, Development, and Utilization

198 4 Enhancing Geothermal Reservoirs
Pressure build-up surveys measure the bottomhole pressure response during the
shut-in period which follows a pressure drawdown. This is useful for measuring
reservoir properties and near well effects such as skin. In this test, the flow rate is
known (zero).
Interference tests between two wells are used to estimate the transmissibility
(k h) of the formation in the interval between the wells. A pressure change is
created at the active well by shutting in or opening up the well, and a pressure
gauge in the closed-in observation well awaits a pressure response, the arrival time
of which can be used to estimate transmissibility.
A pulse test is a version of the interference test, but attempts to provide enough
information to allow the interpreter to eliminate the effects of noise and gauge drift
in pressures (to which the interference test is prone) as measured at the observation
well.
A Stepped flow and stimulation test essentially is a small hydraulic stimulation,
aimed at reducing near-wellbore inlet and outlet hydraulic impedances. It consists
of injecting into a well at a constant flow rate, and recording the pressure rise
within time.
In a multi-rate-pre-fracturing-hydraulic-test, the injection flow rate is increased
in steps. In each step the flow is continued at a constant rate until the injection
pressure attains an asymptotic value. The test delivers valuable information on
transmissibility, the significance of turbulence, and details of fracture dilation
(Murphy et al., 1999).
A multi-rate-post-fracture-performance-test is performed to determine the hy-
draulic properties of the stimulated fracture system. It is performed like the
corresponding prefracture tests except that the flow rates are usually much higher,
and the test duration is longer.
Long-term injection and production tests in a single well are especially useful
for determining the outer hydraulic boundary conditions of the stimulate fracture
system. They can help to predict the long-term fluid loss from the reservoir during
operation.
Generally, pressure differences during the test phase should be kept sufficiently
small, in order to remain in the hydraulic mode instead of frac mode.
All of the aforementioned test procedures were developed in order to test wells
which produce a single well fluid. In the more general case of multiphase flow,
multiphase effects have to be taken into account in test interpretation. After Horne
(1995), it is almost always better to design a well test ahead of time, to avoid
multiphase conditions during the test.

4.7.1.3 Tracer Testing
Tracer testing is a technique which is frequently applied in geothermal development
projects in order to detect and characterize hydraulic connections between deep
geothermal wells, to understand the migration of injected and natural fluids, and
to estimate their proportions in discharged fluids, their velocities, flow rates,and
residence times. The method also enables to quantify fluid-rock contact surfaces,
swept volumes, circulation paths, and reservoir volumes. It can provide very useful