Page 225 - Geothermal Energy Systems Exploration, Development, and Utilization

4.7 Verification of Treatment Success 201
with temperature, pressures, and wellbore imaging downhole. This way, the overall
effect of the thermal stimulation is monitored along with observations of local
fracturing leading to productivity improvement.
4.7.2
Evaluation of Chemical Stimulations

As with any stimulation operation, it is important to evaluate the effectiveness of
a matrix stimulation treatment. The effectiveness is gauged by apparent increases
in the PI, without the benefits of posttreatment test. Wellhead pressures and
injection rates are recorded during every matrix stimulation treatment. Ideally,
these variables should be measured at bottomhole. However, the hostile nature of
the stimulation fluids sometimes prevents the use of downhole pressure gauges
and flowmeters. A calculation allows the derivation of bottomhole parameters from
those measured at the surface in terms of progress of the remedial treatment.
Each stage of injection or shut-in during the treatment is considered as a short
individual well test. The transient reservoir pressure response to the injection fluids
is analyzed and interpreted to determine the condition of the wellbore (skin) and
the formation transmissibility (Economides and Nolte, 1989).
The common assumption is that dislodged and migrating fines should be forced
into the formation after an acid stimulation job. However, fine particles may flow a
considerable distance from the wellbore where they can form permanent damage
(Nguyen, 2006). It may become impossible to remove these damages at a later
time. It may be more advisable to backflow the well after the stimulation job and
before routine injection operations. In addition, partial removal of damage with
acid treatment may eventually result in complete damage removal when the treated
well produces back. The high-rate and high-energy backflow from geothermal wells
can blow out damage that was not dissolved by acid. Damage that was softened,
broken up, or detached from downhole tubulars and fracture channels can be
produced back through a large-diameter casing completion. Erosion of production
lines may occur if drill cuttings are produced back during blow down of a well after
stimulation and care must be taken in this regard. A temporary flow line may be
required until solids production has stopped.
The risk seems to be too large to base the job evaluation solely on the increase of
injectivity. It seems to be worthwhile to go to an additional job evaluation method
which can be used if the stimulated well is backflowed after the stimulation job. The
backflowed (i.e., produced) liquids contain the dissolved products of all chemical
reactions taking place during and after the acid job. Using the chemical analyses,
flow rates and produced volumes of the back-produced fluids will allow the operator
to perform some rather accurate and reliable material balances. These material
balances could be used as a valuable aid for the job evaluation. This method has
been used successfully for the evaluation of the acid stimulation of geothermal
production wells.