Page 208 - Geothermal Energy Systems Exploration, Development, and Utilization

184 4 Enhancing Geothermal Reservoirs
minimum horizontal including the thermoelastic stress, a fracture can propagate
from the wellbore.
Various numerical models have been developed to explain and predict thermally
induced fracturing in sedimentary rocks. A numerical model to calculate the
change in horizontal stress resulting from a change in temperature across a region
of elliptical cross section and finite thickness has been developed by Perkins and
Gonzales (1985) simulating the thermoelastic processes associated to the injection
of cold fluids during waterflooding. Conditions, under which secondary fractures
perpendicular to the primary main fracture may open, are also discussed. In fact,
these considerations explain, how a line crack, representing a two wing vertical
hydraulic fracture might gradually evolve into a fracture network of elliptical
geometry, confocal with the line crack, by injection of cold fluid. Gadde and
Sharma (2001) further refine the Perkins and Gonzales model also including
fracture growth in injection wells due to particle plugging, thermal, and pore
pressure effects.

4.5.3
Chemical Stimulation

Matrix acidizing treatments are designed to remove or bypass solid flow obstruc-
tions (damage material) from the wellbore or from the reservoir in the vicinity of the
wellbore by injecting fluids of low pH into the wells. There are two main stimulation
techniques: matrix stimulation and fracturing. Matrix stimulation is accomplished,
in sandstones, by injecting a fluid (e.g., acid or solvent) to dissolve and/or disperse
materials that impair well production. In carbonate formations, the goal of matrix
stimulation is to create new, unimpaired flow channels from the formation to the
wellbore. Matrix stimulation, typically called matrix acidizing when the stimulation
fluid is an acid, generally is used to treat only the near-wellbore region. In a matrix
acidizing treatment, the acid used is injected at a pressure low enough to prevent
formation fracturing (Economides and Nolte, 1989). Very often carbonates show
low matrix permeability and just creating wormholes in the near-wellbore area may
not be sufficient to produce the reservoir economically. Fracture acidizing is the
technique that is being used to achieve the task of providing a conductive path
deeper into the formation (Burgos et al., 2005).
Matrix acidizing is one of the oldest well-stimulation techniques used to remove
damage near the wellbore. It was initially applied in carbonate reservoirs and
over the years it has been extended to more complex mineralogies. So far, matrix
acidizing is often considered by many people as risky to undertake, primarily due
to the heterogeneous nature of formation minerals and an appreciable degree of
unpredictability of their response to acid formulations. However, it is a relatively
simple stimulation technique that has became one of the most cost-effective method
to improve significantly the well productivity and/or injectivity.
In sandstone formations, matrix acidizing may enhance significantly the well
performance by removing the near-wellbore damage, primarily associated with
plugging of pores by siliceous particles as the consequence of drilling, completion,