Reservoir stimulation is a key technology in Enhanced Geothermal Systems (EGS) and
Unconventional Geothermal Resources (UGR) development. Several research projects have
been carried out over the past 30 years at widely different geological conditions and
others are being planned.
Today, chemical and mechanical stimulations are commonly used. Depending on their
chemical reaction behaviour, individual minerals that clog fracture walls can be
dissolved using chemical treatment. The injection of acids is performed with modest
flow rate creating pressure levels below values needed for mechanical stimulation. An
improvement of the well conditions is generally observed, however with largely
varying success rates. The success of permeability improvement by massive hydraulic
injections ("mechanical stimulation") is not easily anticipated. There are two main
mechanisms: shear fracturing (or faulting) and jointing (tensile fracture). Both
methods increase the pore pressure in the rock, however at different levels.
Depending on the stress regime, shear fracturing causes maximum pressures below the
minimum stress component (P ~ smin). Slippage is induced in agreement with the Mohr
(-Coulomb) Criterion on pre-existing mechanical discontinuities. The displacements
generate larger apertures and possibly even new fractures. In contrast, jointing
(tensile fracture) develops perpendicular to the least principal stress (P > smin).
It is mostly applied in sedimentary rocks. Herewith, a single, far extending fracture
can be created.
This presentation aims to give an overview of stimulation techniques used in the
past, classify them in a logical way, precisely define associated mechanisms, and
give a state of comprehension of associated observed phenomenon. A second point
examined in this presentation concerns the results obtained through experiments all
over the world referenced in literature (using IGA and GRC databases, scientific
literature and various reports). As one now knows that the context of each experiment
(history of the site, stress field, temperature…) plays an important role in the
stimulation results, this part of the problem is also integrated in this review in
parallel to the results obtained. The main objective of this study is to examine
conditions in the reservoir under massive flow injections. Thus, chemical stimulation
techniques will be shortly examined, and special attention will be paid to methods
used and results obtained in the petroleum industry.