Page 305 - Hydrocarbon Exploration and Production Second Edition

292 Facilities

generally need to be treated before it can be injected into a reservoir, whether it is
‘cleaned’ seawater or produced water. Once treated it is injected into the reservoir,
often at high pressures. Therefore, to design a PFS for water injection one needs
specifications of the source water and injected water.
Possible water sources for injection are seawater, fresh surface water, produced
water or aquifer water (not from the producing reservoir). Once it has been
established that there is enough water to meet demand (not an issue in the case of
seawater), it is important to determine what type of treatment is required to make
the water suitable for injection. This is investigated by performing laboratory tests
on representative water samples.
The principle parameters studied in an analysis are

(1) Dissolved solids to determine whether precipitates (such as calcium carbonate)
will form under injection conditions or due to mixing with formation
waters.
(2) Suspended solids (such as clays or living organisms) which may reduce injection
potential or reservoir permeability.
(3) Suspended oil content where produced water is considered for re-injection. Oil
particles can behave like suspended solids.
(4) Bacteria which may contribute to formation impairment or lead to reservoir
souring (generation of H 2 S).
(5) Dissolved gases which may encourage corrosion and lead to reservoir impairment
by corrosion products.

The likely impact of each of these parameters on injection rates or formation
damage can be simulated in the laboratory, tested in a pilot scheme or predicted by
analogy with similar field conditions (Table 11.2).
Once injection water treatment requirements have been established, process
equipment must be sized to deal with the anticipated throughput. In a situation
where water injection is the primary source of reservoir energy it is common to
apply a voidage replacement policy, that is produced volumes are replaced by injected
volumes. An allowance above this capacity would be specified to cover equipment
downtime (Figure 11.24).

Table 11.2 Water treatment considerations
Problems Possible E¡ect Solution

Suspended solids Formation plugging Filtration
Suspended oil Formation plugging Flotation or filtration
Dissolved precipitates Scaling and plugging Scale inhibitors
Bacteria Loss of injectivity Biocides and selection of
(corrosion products) and sour service materials
reservoir souring
Dissolved gas Facilities corrosion and Degasification
loss of injectivity