While waterfloods to improve oil production were recorded in the early 1900s, it wasn’t until the early 1960s that researchers began studying adding chemicals to injection water to improve sweep efficiency. Initially the idea was to increase the viscosity of the water to give a more favorable mobility ratio. But it was also realized that blocking or plugging materials could divert water flow from high permeability flow channels into lower permeability rock.

Bulk Gels are designed for high permeability thiefs or fractured channels.

By the late 1960s, water-soluble polymers began to be used to control water mobility. Although polymers are mainly intended to help maintain a more even fluid front, they also affect the rock. Like water, polymer solutions tend to move through the most permeable flow paths if the rock is heterogeneous. Unlike water, polymer adsorbs to the rock and has a permanent effect on the flow of subsequent water through the rock. This residual resistance factor (RRF) is beneficial and adequate to control mild rock heterogeneity.

In cases where heterogeneity is strong, a stronger process is needed. Higher concentrations of polymer can help, but when heterogeneity is great, the polymer eventually breaks through to producing wells, causing problems in separating produced fluids. It is also typically not economical to inject large volumes of chemical.

CAT-AN® Process

In the early 1970s, layering of cationic and anionic polymer was developed to increase the residual resistance effect. This process, termed the CAT-AN® process, begins with the injection of cationic polymer solution followed by anionic polymer. The result is a thicker layer of adsorbed polymer in the most permeable rock and a higher RRF. Because chemical concentrations are low enough to be economical and the process reaches deeply into the formation resulting in residual resistance factors high enough to strongly impact flow, CAT-AN® is still in use today.

In the mid 1970s, Phillips Petroleum developed a new process of layering aluminum citrate gels for in-depth reservoir heterogeneity correction. The process involved injecting a large volume of anionic polymer followed with a small volume of concentrated aluminum citrate, then resuming anionic polymer injection. This “layering” resulted in higher resistance to flow than the CAT-AN® process alone due to a greater thickness of the adsorbed polymer inside pore throats. In addition the aluminum citrate injection could be repeated as many times as necessary, tailoring the resistance necessary to achieve maximum sweep efficiency.

Colloidal Dispersion Gels

Colloidal dispersion gel (CDG) technology evolved from layered aluminum citrate gel technology in the mid-1980s when it was discovered that small amounts of aluminum citrate mixed directly with low concentrations of polymer resulted in solutions that had a much higher resistance to flow than uncrosslinked polymer.

The CDG process has several advantages over layered aluminum citrate gels:

• The process provides a higher RRF.
  
  
• It is simpler to inject and control than layered gels.
  
  
• The process is more economical because less aluminum is needed.
  
  
• Laboratory design of optimum gel strength is much simpler.
  

Oil producing reservoirs contemplated for secondary recovery must be studied prior to waterflooding to characterize the rock properties and determine how efficiently the reservoir will flood. Many exhibit a non-uniform permeability contrast that results in rapid water breakthrough at the offset producing well(s), with resulting inefficient oil recovery. If a reservoir shows a Dykstra-Parsons factor greater than 0.55 and has a fresh water (< 20,000 ppm TDS) injection supply source, then a long-term injection side application of colloidal dispersion polymer gel should be considered early in the life of the waterflood to improve flood efficiency.

TIORCO's UNI PERM® Colloidal Dispersion Gel (CDG) is a specialized polyacrylamide gel that forms deep in the reservoir within the most permeable flow paths. These gels are formed from low concentration of polymer and are capable of entering matrix rock and flowing in-depth, while being adsorbed onto the rock surfaces. Thus, high permeability flow paths are physically altered to reduce permeability. This makes the reservoir more uniform to the drive fluid resulting in more of the low permeability oil bearing rock being contacted.

Case history data indicates one may expect recovery improvements up to 10% of original-oil-in-place, with less water injected over a shorter flood life.

Bulk Gel Use

The 1960s also saw the development of crosslinked bulk gel systems designed for near wellbore problems due to bottom water encroachment in producers and channeling of water out-of-zone in injectors. As with polymer processes, bulk gel technology has improved greatly over the past 35 years.

Bulk gels are a rigid gel system designed for high permeability thiefs or fractured channels and are used with both injection and production wells. The robust chemistry of the gels allows them to be used in most waters and at relatively high temperatures (<120 C). They are also resistant to H2S and CO2.

While water is still quite effective in homogeneous reservoirs with light oils, most reservoirs exhibit some degree of heterogeneity and a polymer or gel system will increase oil recovery. What fluid is most effective is dictated by rock and oil properties. A fractured high permeability matrix may require a bulk gel process, whereas a low permeability rock with viscous oils may need a low concentration polymer. Fortunately, a process is available for most reservoir conditions