Oil recovery: squeezing the last drop
In tertiary recovery, enhanced oil recovery (EOR) techniques are employed to alter the capillary forces, viscosity, interfacial tension and wettability of the reservoir rock matrix, to drive the trapped oil out of the rock pores. There are a number of ways to aid the recovery of this residual oil. Among chemical methods, application of polymers and surfactants either as individual treatments or in combination is particularly useful.
For more than 30 years the oil and gas industry has been examining polymer applications and their economic effectiveness in obtaining more hydrocarbon from oil-bearing rocks and reservoirs. Polymer injection can be very useful in enhancing the effectiveness of waterflooding in EOR. However, the use of polymers can face technical challenges, such as the development of gels that can easily flow for several hundred feet into a reservoir rock, then harden to block water flow.
Polymers can offer biodegradable options for these challenges, but often biopolymers and natural alternatives do not have the required strength and resistance to other forms of degradation. Indeed, having polymer stability for several months may render the polymer extremely stable to any biodegradation processes.
In order to impart the required properties and stability to polymers, and to form appropriate gels, many are cross-linked in situ by the application of appropriate cross-linkers. A good example of the problems with gel forming cross-linkers is illustrated by the strengthening of guar gum used extensively in hydraulic fracturing. The most common cross-linkers are borate-based systems and can be formed by boric acid, borax, an alkaline earth metal borate or an alkali metal alkaline earth metal borate. It is essential that the borate source has around 30% boric acid. The boric acid forms a complex with the hydroxyl units of the guar gum polysaccharide, cross-linking the polymer units.
A number of other systems – mainly exotic organo-metallic- based systems such as titanium – have also been found to be useful cross-linkers.
A major polymer group that is commonly used in EOR applications is polyacrylamides (PAM), where high viscosity aqueous solutions can be injected to improve the economics of conventional water flooding. PAM-injected solution assists in sweeping (or pushing) oil locked in a reservoir towards the producing well(s). The result is improved volumetric sweep efficiency – more oil is produced for a given volume of water injected into the well.
However, PAM polymers are susceptible to chemical, thermal and mechanical degradation. Chemical degradation occurs when the labile amine moiety hydrolyses at elevated temperature or pH, resulting in the evolution of ammonia and a remaining carboxyl group. Thermal degradation of the vinyl backbone can occur through several radical mechanisms, including the auto-oxidation of small amounts of iron and reactions between oxygen and residual impurities from polymerisation at elevated temperature. Mechanical degradation can also be an issue at the high shear rates experienced in the near-wellbore region. However, cross-linked variants of PAM have shown greater resistance to all of these methods of degradation and have proved to be much more stable.
It is important to recognise the need for specialist design and application in this area, the effect of polymer and associated ingredients on the properties of formulated products, their environmental fate and their impacts on environmental acceptability.