Upper Zone flow control
valve, diverting treatment
toward failed frac pack
assembly
Upper Zone,
prone to sand
production
Lower Zone,
free from sand
Lower Zone flow control
valve, isolating sand-free
zone
WEST AFRICA
Chemical remedial sand control using
the subsea flowline succeeds
First-ever project done from an FPSO offshore Nigeria
Michel Piemontese
Marco Colombo
Giuseppe Ripa
Dario Pellicano
ENI E&P
Pratyush Singh
John Murphy
Weatherford
Sand production is a major concern to the industry and is more critical today, especially in offshore environments where tolerance to sand production is low. Sand infux into the wellbore may
lead to problems such as valve and pipeline erosion, sand deposits in separators, and economic
loss from production limitations. Mechanical
sand control methods are considered reliable
and there are options available, from simple
to sophisticated, depending on the situation.
When sand control is predicted to be a life of well issue, operators
often install sand control from the outset.
So it was with ENI E&P in an offshore feld offshore northwest-
ern Nigeria which was frst produced in 2003 and subsequently up-
graded in 2009. Production is from subsea wells at a water depth
of about 2,000 ft (610 m), where the temperature is as low as 5°C
After reviewing available data, it was determined that to
minimize the risk on the subsea asset and to increase the
maximum sand free rate (MSFR), some form of sand con-
trol remediation was required. The obvious approach was to
work the well over, but this would involve costs and delays
in addressing the problem. So, several chemical sand-control
solutions were considered. The
use of chemicals such as resins
has had some success but when
treating wells with resin, once the
treatment has cured over the sub-
strate, it cements the sand grains
together and reduces formation
permeability and consequently loss of production. However,
another alternative was the zeta potential altering system
(ZPAS). This has a limited risk of
reservoir permeability impair-
ment and can be applied by mix-
ing the chemical with seawater
and simply bullheading it into the
formation through the fowline.
Zeta potential altering chemistry
Zeta potential modifcation is not new and has been
used in the oil feld for years in such applications as cor-
rosion and scale inhibition, as well as in drilling
fuids to improve solids dispersion. The active
ingredient of the ZPAS is an inner salt of a very
low molecular-weight polymer, that when add-
ed to the base fuid disperses and rapidly coats
any metal oxide substrate such as sandstone or
any other anionic downhole formation. It also
contains a component that can disrupt the wa-
ter layer that coats solid surfaces in the forma-
tion. It is not an absolute sand-control solution,
such as an expandable screen, gravel pack, or
a frac pack, but it creates an ionic attraction between the sand and
fnes grains, and modifes the relative permeability of the formation.
The main features of the system are that it enables an increase in
MSFR and eliminates or signifcantly reduces sand fnes migration.
ZPAS is a matrix treatment that modifes the zeta potential of the
anionic formation particles to within a range of + 3 to -5mV, where
the sand and fnes particles become attracted and agglomerate.
ZPAS works on the anionic substrate. It modifes the zeta potential
of most fnes and reduces or stops fnes migration, thus preventing
or signifcantly reducing the near wellbore damage they cause. This
process presents a low risk to the reservoir, and can be pumped us-
ing rate diversion into long producing intervals.
Treatment design
The well in question was a dual-zone intelligent completion
and its selectivity feature enabled the operator to identify the
fact that the upper zone was producing sand while the lower
one was sand free.
The well had been a high producer since February 2009
and had a gross fow rate of approximately 1,450 b/d, with 50%
BS&W and a GOR of 750 cf/bbl. Both zones were completed
with a frac pack for sand control, but the upper one failed and
needed remediation to allow continued production. By manipulating the zonal fow control valves, it was possible to direct
fow via the fowline to the damaged upper zone, facilitating
the pumping of treatment from
the FPSO, using in place pumping equipment, while isolating the
sand-free lower zone.
To assess the suitability of
ZPAS, chemical lab tests were performed with formation cores and sand fnes to verify that the
treatment squeezed into the formation would not cause any
formation damage. Based on these tests, the operator
decided to proceed with a ZPAS treatment, and detailed
plans for the performance of it
were drawn in cooperation with
the service company. The base
Completion schematic. (Image courtesy Weatherford)
