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Comprehensive Look at Chemical Flooding Includes Mobility, Surface Gathering

A tour of the pilot and large-scale chemical floods under way in the Daqing field drew the attention of 135 people from 39 organizations and 14 nations at the Applied Technology Workshop (ATW) on Chemical Flooding held in Daqing, China. The keynote address by ATW Cochairperson Wang Demin of Daqing Oilfield Co. described the development of the Daqing field and emphasized enhanced oil recovery (EOR) efforts. There were seven technical sessions. The session was cochaired by Randy Seright of New Mexico Tech.

Objectives of the ATW were:

To review why chemical-flooding projects declined in the mid-1980s.
To review state-of-the-art processes of chemical flooding.
To gain firsthand knowledge of recent chemical-flooding applications in China in recent years.
To identify technical and economic challenges that must be overcome before chemical flooding will become commonly applied.

Session 1 discussed chemical-flooding mechanisms. For polymer flooding, recent research suggests that viscoelasticity may contribute to displacement efficiency. For alkaline-surfactant-polymer (ASP) flooding, optimal salinity and low interfacial tension (IFT) can correlate with soap/surfactant ratio. A gradient of soap/surfactant ratio can assure passage of displacement profile through low IFT. Low tensions may be possible in the lower phase microemulsion region, resulting in a wide low-IFT region. ASP flooding improves recovery by displacement efficiency in high-permeability layers and by sweep efficiency in lower-permeability layers. Enhanced recovery in oil-wet, fractured carbonate-matrix blocks occurs by gravity drainage through alkali and surfactant-altering wettability and moderately lowered IFT. Ultralow IFT is not necessary.

New chemicals were presented in Session 2. Several new polymers were discussed, including hydrophobic associative polymers, a salt-resistant comb-shaped polymer, a cationic polymer with improved high-temperature stability, and swellable microparticles. Improved surfactants for surfactant and ASP flooding also were presented. Surfactant performance can be optimized by proper selection of hydrocarbon-chain length, branched hydrophobe, and surfactant head group (including benzene sulfonates, propoxylated sulfates, betaines, or sulfobetaines). The pros and cons of molecular modeling were discussed when identifying new EOR surfactants.

Chemical-flood simulation was covered in Session 3. Chemical-flooding simulators developed in China include a number of new physical mechanisms for polymer and ASP flooding. Continuous development of the U. of Texas Chemical Compositional Simulator over the last 30 years allowed implementation and laboratory/field validation of a suite of process model options and led to the development of a new chemical simulator, General Purpose Adaptive Simulator, at the U. of Texas at Austin. The Research Inst. of Petroleum E&P described its development of a well-designed geologic modeling technology to meet precise EOR reservoir-description needs. While the need to employ simplified models for quick evaluation of potential chemical-flood projects was widely recognized, a step-by-step simulation approach, as described for the Minas S/P implementation, is a realistic course for a successful field implementation.

Mobility control and profile modification were discussed in Session 4. Foams have potential for mobility control, especially when surfactant injection alternates with gas. Strong foams can exist in two regimes: high quality (dry) and low quality (wet). Research and application of “colloidal dispersion gels” in the Daqing field illustrate a need to resolve the discrepancies between laboratory and field results through better understanding of the target formations. Gel treatments of any kind are not polymer floods. For gel treatments, gel/gelant penetration into oil zones must be minimized, while for polymer floods, polymer penetration into oil zones must be maximized. A clever use of swelling polymers was described. Crosslinked gel particles of submicron size with reversible and stable crosslinks were injected. Subsequently, a rising temperature breaks the reversible crosslink, which triggers expansion (pop) of gel particles and thereby blocks offending pathways in depth. New technology for treating channeling through fractures or fracture-like features was also described. An overall take-home message was that proper selection of candidate wells and treatment method is critical to success of conformance treatments.

Pilot and field test results were described in Session 5. Incremental oil was produced by polymer, alkaline-polymer, surfactant-polymer, and ASP flooding of reservoirs in China, Canada, the U.S., and Indonesia. Polymer-flooding incremental oil recovery ranged from 12 to 16% of original oil in place (OOIP), or 19% of OOIP at high polymer concentration. Daqing produces one-fourth of its oil production by polymer flooding. Alkaline-polymer flooding produced 12% of OOIP, with some wells’ oil cut increasing from 2 to 70% and oil rates increasing from 10 to 60 m3/d. ASP flooding incremental oil recovery ranged from 17 to 33% of OOIP. Six projects in Daqing showed a narrower range from 20 to 26% of OOIP. Polymer, alkaline-polymer, and ASP flooding are economical. Internal rates of return of up to 65% were observed with the cost per barrel of incremental oil as low as U.S. $2.50.

Surface gathering technology for chemical flooding was covered in Session 6. The feasibility of chemical EOR was considered for a high-temperature offshore application. Design parameters were also discussed for polymer/chemical-flooding injection facilities. Compared to Newtonian fluids, viscoelastic polymer fluids exhibited reduced working efficiency in equipment for injecting, producing, and gathering fluids. A nonionic water clarifier and demulsifier was developed that provides less oil carry-over and looser emulsions. Alkali, polymer, and surfactant retained in the produced liquids greatly change the properties of the liquids and increase the difficulty of treatment. For fluids produced from ASP floods, the dehydration and antiscale processes were studied in both the laboratory and the oil field.

Session 7 presented advances in production and injection technology for chemical flooding. A mechanistic study identified the cause of excess wear of sucker rods during polymer flooding at Daqing, and successful solutions were implemented. Two methods were described to increase polymer injectivity during polymer flooding at Daqing, including use of resin-coated sand as a proppant after fracturing and use of surfactant formulations to reduce oil saturation and increase permeability near the wellbore. Facilities were described for processing and reusing water from an alkaline-polymer flood in Canada, including emulsion treating, filtering, softening, chemical blending, and reinjection. To improve polymer-injection profiles in multilayered reservoirs (i.e., Daqing), a ring groove regulator allowed selective degradation of injected polymer in low-permeability zones while causing minimum degradation of polymer injected into high-permeability zones. The stability of gels (for conformance improvement) in the presence of alkaline was examined. A new flow improver reduced the effective viscosity of produced fluids while simultaneously inhibiting paraffin buildup in tubing and allowing crude-oil transport at temperatures below the pour point.

Generous support for the meeting was provided by the Daqing Oilfield Co. Ltd., the Daqing Petroleum Inst., the Heilongjiang Petroleum Soc., and the SPE Daqing Section.