Resources exploitation of gas, oil and heat from deep reservoirs needs sometimes the
development of the permeability around the production wells to ensure an efficient
production. Different techniques can be used to enhance the reservoir productivity
but the main ones are the hydraulic fracturation and the chemical treatment. This
presentation will be focused on the methods of chemical stimulation.
This technology, developed for more than one century by the oil industry for the
stimulation of oil and gas wells, is also used in geothermal wells. The aim of this
technology is to enhance the well productivity and to reduce the skin factor by
removing near-wellbore damage and by dissolving scales in natural fractures. It
consists to pump into the reservoir reactants such as strong acids (hydrochloric
acid, HCl-HF mixture), organic acids (acetic acid, chloroacetic acid, formic acid,
sulfamic acid) or chelatants (EDTA). These reactants can be pumped into the reservoir
according to two procedures: below the fracturing flow rate and pressure of the
reservoir (matrix acidizing) or above the fracturing flow rate and pressure
(fracture acidizing). The matrix acidizing allows the reaction of acids with minerals
present in existing pores and natural fractures.
Normally used for the removal of skin damage associated with work-over, well killing
or fluids injection in damaged wells, matrix acidizing is also used to increase the
formation permeability in undamaged wells. It is performed in three steps: firstly,
an adequate preflush with HCl to dissolve associated carbonates, secondly, a
mainflush with a correct HCl-HF mixture formulation depending on the rock composition
and at last, an overflush with weak HCl or freshwater.
This technique is one of the mostly used for oil and gas wells but also for
geothermal reservoirs. It was performed with success for damage removing from
geothermal wells in Philippines, where in some cases the boreholes productivity
increased by a factor 3 to 10 before and after treatment. The fracture acidizing
method, also called acid fraccing, consists to inject first a viscous fluid at a rate
higher than the reservoir matrix can accept, opening new fractures and then, to
inject HCl acid reacting all along the created fractures.
This technology provides well stimulation and not just damage removal. In numerous
cases, acid treatments have shown their high efficiency for near-wellbore damage
removing.
Nevertheless, it should be noted that, most often, the high reactivity of chemical
agents prevents a deep penetration into the formation, creating wormholes which
increase the porosity around the well but not necessary its productivity. Even if
this drawback can be limited by retardants, many studies demonstrate that the
influence radius of acid treatments in a homogeneous system does not exceed some
metres around the injection well (maximum 2 to 5 metres).
The major disadvantage of acid treatments is linked to the corrosion risks of the
casing in particular with strong acids. Nevertheless, this risk can be reduced by
using less-corrosive agents as chelatants or by addition of corrosion inhibitors, but
their use increases the treatment cost. In the field of EGS, few chemical treatments
have been applied to stimulate reservoirs. In November 1976, at Los Alamos Scientific
Laboratory (USA), 190,000l of 1 N carbonate sodium base solution was used to dissolve
quartz from the formation and to reduce the impedance of the existing system. About
1,000kg of silica were dissolved and removed from the reservoir but without impedance
reduction. In 1988, a matrix acidizing was performed on the Fjällbacka reservoir
(Sweden): major and minor fractures of the granitic reservoir were filled with
calcite, chlorite and clay minerals. About 2,000l of HCl-HF acid were
injected in Fjb3 to leach fracture filling. Returning rock particles showed some
efficiency of this acid injection. Recently acid treatments on EGS were performed at
Soultz-sous-Forêts (France). This deep granitic reservoir contains fractures
partially filled with secondary carbonates (calcite and dolomite). In order to
dissolve these carbonates and to enhance the productivity around the wells, each of
the three boreholes (GPK2, 3 and 4) were treated with different amounts of
hydrochloric acid. If GPK2 and GPK3 have shown weak variations of
their productivity, GPK4 presented a real increase of its productivity after
treatment: the wellhead pressure was reduced by about 40% due to the acidification
treatment and a decrease of the reservoir impedance of a factor 2 (0.2 to 0.4l/s/bar)
has been estimated.

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