Water Injection improving deepwater and mature field EOR

Enhanced Oil Recovery (EOR) can increase the volume of recovered oil from a well mainly through the use of two distinct techniques. One is water injection, which is increasingly being used in mature fields and another is CO2 injection which is mostly used in deepwater fields and according to Petrobras will be extensively used at many of the pre-salt fields in Brazil, as these fields usually contain significant amounts of CO2 along with oil and gas. These are secondary production processes. The production process always refers to oil that is recovered naturally from a producing well, that is without the use of pressure or flow stimulants, such as water or CO2.

Water injection simply means drilling injection wells into a reservoir and introducing water into that reservoir to increase oil production. The injected water forces an increase in the depleted pressure, mainly within mature reservoirs, it may also increase the oil´s flow rate by making the oil move within the reservoir. It is considered to be an excellent stimulant for mature fields and any field with low pressure values, including deepwater fields and eventually even pre-salt fields, even though most of these do not show exhibit low pressure values. Injecting water into reservoirs is also known as voidage replacement.

This may increase the O&G recovery factor (percentage of oil usually extracted from a reservoir) by a significant amount. The recovery percentage figure for the average well is something around 30%. Petrobras is rumored to be using this technique, combined with the latest seismic acquisition and interpretation methods, to revamp mature wells in the Middle East, including Saudi Arabia.

What few people know is that this is mainly happening due to recent breakthroughs that occurred during early pre-salt reservoir exploration and research into methods that would guarantee reservoir pressures high enough to force the O&G over 6,000 meters up the pipes to the production FPSOs, with limited or no use of ESPs (Electrical Submersible Pumps).

It is important to note that both the ME fields and the pre-salt are composed of carbonatic rocks.

Using produced water as it decreases the potential formation damage caused by incompatible fluids. The downside is the ever present risk of scaling and/or corrosion in infection flowlines or pipelines. Another factor is the demand to dispose of this water, which is contaminated with various solids, along with hydrocarbons.

Offshore or inshore disposals obviously require costly clean-up processes, much research work is being done aiming at reducing the costs of reprocessing this used injection water in order for it to be deemed fit to re-inject in the well, therefore ending the need for disposal and forming an efficient closed circuit of re-injected water. Salt water from the ocean is also used as it is very convenient offshore. However, filtering, de-oxygenation and biological purification processes are is generally required. It is also necessary to try to use it only at depths that diminish the formation and concentration of algae.

These technologies will increasingly be necessary to enhance productivity in mature and deepwater reservoirs, in heavy oil fields. It may also happen that water injection is needed in some of the one or a few of the individual wells that make up a modern production system (which have various wells linked to one reservoir) and joining at a manifold before being pumped upstream through a single flowline or pipe  to the rigs or FPSOs.

Claudio Paschoa