Questions which need to be addressed when considering the injection of produced water include potential reservoir souring, scaling and clogging of low-quality producing intervals.
A critical concern is the potential souring and scaling of reservoirs and the subsequent reduction in their ability to produce hydrocarbons.
This depends on several factors including:
- the hydrogen sulphide buffering/adsorption potential of the formations
- the presence of organic acids
- the placement of injector wells
- any previous seawater injection history.
However, a tendency to souring can possibly be controlled by introducing nitrate-reducing bacteria to the injection stream. This is currently being investigated on the Norne field operated by Statoil in the Norwegian Sea.
Another concern involves maintaining the injection rate, since produced water contains a significant quantity of fine suspended particles (50-200 milligrams per litre) and oil droplets left over from the separation process.
Both these constituents may plug poor-quality (low-permeable) formations and lead to serious losses in injectivity.
But keeping the injection pressure above the formation fracture pressure will reduce the risk of plugging by inducing artificial fractures.
Thermal effects, the oil-in-water content and the deposition of particles on fracture surfaces can also be detrimental.
In addition, the injection procedure itself could increase undesirable emissions to the air as well as having other negative consequences for topside equipment.
Despite the attention being paid to produced water injection (PWI), it must be appreciated that large volumes of produced water will continue to be discharged to the sea.
This means that the search for suitable purification technologies must also be vigorously pursued.