Gidley.com - Page Title

The Prefix® Process

Prefix acidizing employs CO2, along with an aromatic solvent, to displace
crude oil from the zone to be acidized. This displacement has two
immediate benefits: first, it prevents contact between spent acid products
and crude oil and avoids the formation of emulsions and sludges that
may result from that contact; secondly, by displacing crude oil from the
producing zone, the path of least resistance for the acid becomes the
producing zone, and the acid becomes concentrated in the region where
it is most effective in increasing oil production. At the same time, the
tendency to stimulate water production preferentially, so prevalent in
conventional acidizing, is often eliminated. The mechanism by which the
latter is accomplished is as follows:

Elimination of preferential water stimulation

In conventional acidizing, illustrated graphically in Figure 1, acid may be
pumped into a zone filled with oil. The relative permeability to acid in a
zone saturated with oil is at a minimum. Hence, the oil zone is resistive to
acid penetration. Acid will follow the path of least resistance. If there is no
physical barrier to prevent it, acid will go under the oil zone and seek out
the water zone, where water saturation and relative permeability to acid
are both high. When this occurs, as it does in many conventional
treatments, acid increases the permeability of the path it travels and
facilitates the movement of water from the water zone to the wellbore. The
result: preferential stimulation of water, and less effective stimulation of
oil. The increase in water cut, which frequently accompanies conventional
acidizing, is a confirmation of this phenomenon.

In the Prefix Process, depicted in Figure 2, CO2 is injected into the
oil-saturated zone ahead of the acid. The CO2, because of its lower
viscosity (less than one-tenth that of acid), fingers through the oil, is
absorbed by the oil, swells the oil, and reduces its viscosity. The oil thus
becomes easier to displace. If sufficient CO2 is used to displace the oil
completely from the near-wellbore region, when acid is introduced it
displaces the low-viscosity CO2 preferentially and remains within the
zone previously occupied by oil. As a consequence, the action of the acid
is on the zone to be treated (the oil producing zone) and not on the water
zone.

If CO2 should stray from the oil zone and into either the water zone or gas
zone during displacement, neither will be stimulated; CO2 is not reactive
with formation materials even though they are readily attacked by
HF-containing acids. The net result of using the Prefix Process is that the
oil producing zone is stimulated more effectively than the water zone.
Hence, water is not preferentially stimulated.

Enhancing productivity improvement

In acidizing oil-bearing formations, conventional acid treatments often
create a zone of productivity impairment that can reduce treatment
effectiveness. As HF-containing acids spend, the materials dissolved
(usually clays and feldspar) are reprecipitated, creating new surface
areas that are coated with whatever they contact in their immediate
environment. In an oil-bearing zone, crude oil is adsorbed on these newly
created surfaces and may form emulsions or sludges. The Prefix
Process, by displacing oil from the acid reaction zone with CO2, prevents
contact between spent acid products and crude oil. By doing so, it
reduces emulsification and sludge formation, provides more effective
damage removal, and yields a treatment markedly better than
conventional acidizing. Examples supporting this point are shown in the
results section of this web site.

Elimination of separation facility upsets

Prefix acidizing almost completely eliminates separation facility upsets
that occur in processing the returns from HF-containing acid treatments.
These so-called spent-acid returns are notorious in upsetting
demulsification operations and in causing incompletely demulsified
crude to be produced from the facility. Conventional acid treatments
frequently upset the separation facility when spent-acid returns are
processed. In some areas, the problem is so serious that operators
collect such returns separately and barge them to an onshore facility to
avoid upsetting the offshore processing facility.

An adequate CO2 formation preconditioner separates spent-acid
precipitates from crude oil and avoids the interaction between them.
Keeping these phases apart prevents the formation of emulsions and
sludges. The majority of the wells treated with an adequate CO2
preconditioner have not required special treatment of spent-acid returns
to avoid separation facility upsets. On most, it has been possible to
process the spent-acid returns along with concurrent production from
other wells without additional chemical treatment or significant
modification of separation facility operation.

Economics of Prefix acidizing

The modest additional cost of the CO2 required to conduct Prefix
acidizing may be offset by savings in reduced afterflush volumes and
pumping costs, reduced additives, and in other modifications that are
enabled by the Prefix Process. Such savings are small, however, in
comparison with the value of accelerated income from better oil
stimulation, reduced water stimulation, and the elimination of extra costs
for spent-acid returns processing.