Importance of PPG Conformance Control for Mature Oilfields

Most reservoirs from small producers have become mature due to long term water flooding. These reservoirs usually have a high water cut of more than 80% even 90%. Water production becomes a major problem as these oilfields mature. Higher levels of water production result in increased levels of corrosion and scale, increased load on fluid-handling facilities, increased environmental concerns, and eventually well shut-in (with associated workover costs). Consequently, producing zones are often abandoned in an attempt to avoid water contact, even when the intervals still retain large volumes of recoverable hydrocarbons. Controlling water production has been one objective of the oil industry. Reservoir heterogeneity severely affects the flow of gas, oil, and water in the reservoir and thus affects the choice of production strategies, reservoir management, and ultimate oil recovery. Reservoir heterogeneity is the single most important reason for low oil recovery and early excess water production. To maintain reservoir pressure, these reservoirs have usually been developed by water flooding from the early stage of their development. Many of them have been hydraulically-fractured, intentionally or unintentionally, or have channels due to mineral dissolution and production during waterflooding. Reservoirs with induced fractures or high-permeability channels are quite common in the mature oilfields. Gel treatment is one of the most important methods to correct the reservoir heterogeneity.

Using Preformed Gels to Control Conformance Has Become a New Trend for Gel Treatments

Traditionally in-situ gels have been widely used to control conformance. The mixture of polymer and crosslinker called gelant is injected into target formation and react to form gel to fully or partially seal the formation at reservoir temperature. So the gelation occurs in reservoir conditions. However, a newer trend is applying preformed gels for this purpose. Preformed gel is formed at surface facilities before injection, and then gel is injected into reservoirs. So no gelation occurs in reservoirs. Preformed gels have become a newer trend because they can overcome some distinct drawbacks inherent in in-situ gelation system such as lack of gelation time control, uncertainness of gelling due to shear degradation, chromatographic fractionation or change of gelant compositions, and dilution by formation water. Current commercial available preformed gels include preformed bulk gels (Seright 2004), partially preformed gels (Sydansk 2004, 2005), and particle gels which include preformed particle gel – PPG (Coste 2000, Bai 2004), microgels (Chauveteau 2000, 2001, Rousseau 2005, Zaitoun 2007, U.S. patent 6579909) and pH sensitive crosslinked polymer (Al-Anazi 2002, Huh 2005), “colloidal dispersion gels” and swelling micron-sized polymers (Pritchett 2003, Frampton 2004, U.S. patent 6984705). Their major differences are their sizes and swelling times. Published documents indicate that several particle gels were economically applied to reduce water production in mature oilfields. For example, preformed particle gels-PPGs have been applied in about 10,000 wells to reduce fluid channels in waterfloods and polymer floods in China. Recently, Occidental Oil Company (Pyziak 2007) and Kinder-Morgan also used similar product to control CO2 breakthrough for their CO2 flooding areas and promising results have been found. PPG treatment has been widely accepted and is seeing more use by operators due to their unique advantages over traditional in-situ gel, including:

• Particle gels are synthesized prior to formation contact, thus overcoming distinct drawbacks inherent in in-situ gelation systems, such as uncontrolled gelation times and variations in gelation due to shear degradation, and gelant changes from contact with reservoir minerals and fluids.
• Preformed particle gels are strength- and size-controlled, environment-friendly, and they are stable in the presence of almost all reservoirs minerals and formation water salinity.
• Preformed particle gels can preferentially enter into fractures or fracture-feature channels while minimizing gel penetration into low permeable hydrocarbon zones/matrix. Gel particles with the appropriate size and properties should transport through fractures or fracture-feature channels, but they should not penetrate into conventional rock or sand.
• These gels usually have only one component during injection. Thus, it is a simpler process, and does not require many of the injection facilities and instruments that often are needed to dissolve and mix polymer and crosslinker for conventional in-situ gels.
• These gels can be prepared with produced water without influencing gel stability. In contrast, traditional in-situ gels are often very sensitive to salinity, multivalent cations, and H2S in the produced water. This not only can save fresh water but it also can protect our environment.

Further Research is needed for Deploying the Technology

Unless special efforts are made during gel placement (e.g., zone isolation), theoretical studies and field applications demonstrate that gel treatments are most likely to be successful when treating fractures or fracture-like features that cause channeling in reservoirs. Seright et al, (2003) have studied the propagation of preformed bulk gels through open fractures since 1992. They have also performed extensive core flooding experiments and successfully developed a series of theories and methods to characterize the propagation of preformed bulk gels through porous media. However, although the preliminary studies of particle gel propagation through porous media were performed by the product inventors, all core flooding tests used porous media without channels. The smallest gel particles, such as microgels, collodial dispersion gels, and micron-sized swelling polymers, were marketed only for treatment of matrix problems. However, we are interested in whether these particles gels might have applicability to fractures and fracture-like channels. The ultimate purpose of the project is to provide the fundamental information to select gels for mitigating water production and extending field life.Our proposed work features the following innovations.

• The transport of particle gels through fractures and channels will be first tested using screens and core flooding experiments. These experiments will give a design basis for fractured or channeled reservoir gel treatments.
• The analysis of connecting laboratory and field data by modern soft-computing methodologies will result in models to optimize particle treatments in fractured reservoirs.
• Three novel methods will be tested to improve gel treatments, including addition of second crosslinker, inclusion of surfactant, exploration of gravity segregation to aid gel placement.