The exploitation of an oilfield essentially occurs in three stages. In the first, the oil is drained naturally into the wells under the effect of pressure gradient existing between the bottom of the wells within the oilfield.

When pressure medium is inadequate or when being produced significant amounts of other fluids (e.g. water), then starts the second phase, which consists of injecting into the oilfield a less expensive than raw fluid to maintain a pressure gradient.

In these first two stages is able to recover an approximate average of 25% to 30% of original oil in place, with the remaining trapped in the pores of the structure of the reservoir due to viscous and capillary forces in addition to the presence of regions natural fractures or causing the injected water to flow through lower resistance potential channels and leaving substantial amounts of oil trapped in the high permeability formation.

After the primary and secondary recoveries, the oilfield still contains an estimated 60-80% of the original oil in place. Numerous methods have been studied for the recovery, at least partially, of these large amounts of oil remaining in the wells.

Among them are methods involving injection of fluids miscible with oil and gas under high pressure, either separately or in combination, all as part of the third phase raw recovery. Also under optimum conditions, which surfactant solution can contain co-surfactants, electrolytes, polymers, among others injected into the reservoir has the potential to solubilize the oil by dispersing it effectively in the form of an emulsion.

Other methods belonging to the third stage of recovery known as enhanced recovery methods with chemical additives, which have been extensively studied for representing a successful, despite that have been discarded in times when oil prices are low, where the main argument above is the low profitability of the process, mainly due to the costs of chemical additives.

ENHANCED RECOVERY METHODS WITH CHEMICAL ADDITIVES

The methods of enhanced recovery by chemical methods include:

l       Injection of polymers and polymer micellar solutions.

l       Surfactant injection processes.

l       Injection alkaline solutions or alkaline additives combined with mixtures of alkali-surfactant or alkali-surfactant-polymer.

Because each oilfield is unique in relation to the properties of the raw and the porous medium, chemical characteristic must be designed for each application systems. The chemical reagents used, their concentrations in injection processes and the size thereof, will depend on fluid properties and the porous medium of the formation and, to the economic considerations.

Injection of polymers

The basic principle of this method is below the water can become more viscous after the addition of a water soluble polymer, which leads to improvement in the mobility ratio water / oil and thus can improve the efficiency of sweeping and therefore a higher percentage of recovery.

The following figure shows schematically the operation of this method of enhanced recovery:

Figure: The polymer injection process.

Source: PDVSA-CIED, 1998.

Among the polymers used for this method are polysaccharides (or biopolymers) and polyacrylamide (PAM) and its derivatives.

At low salinities the PAM exhibit greater mobility ratio by increasing water viscosity and decreasing the water permeability of the formation. The biopolymers are less sensitive to the effects of salinity, but they are more expensive by virtue of requiring pretreatment processes.

Ultimately, select polymers at low concentrations and oilfield conditions maintain a high viscosity, are not susceptible to degradation and that are thermally stable. It should be noted that the mobility decreases with increasing salinity, due to the high concentration of divalent ions such as Ca+2 and Mg+2. With respect to the degradation, the main effect is a reduction in viscosity which directly results in a change in mobility and thus the oilfield sweep efficiency (PDVSA-CRED, 1998).