Summary
The first large-scale colloidal dispersion gel (CDG) pilot test was
conducted in the largest oil field in China, Daqing oil field. The project was
initiated in May 1999, and injection of chemical slugs was completed in May
2003.
This paper provides detailed descriptions of the gel-system
characterization, chemical-slug optimization, project execution, performance
analysis, injection facility design, and economics. The improvements of
permeability variation and sweep efficiency were demonstrated by lower water
cut, higher oil rate, improved injection profiles, and the increase of the
total dissolved solids (TDS) in production wells.
The ultimate incremental oil recovery (defined as the amount of oil
recovered above the projected waterflood recovery at 98% water cut) in the
pilot area would be approximately 15% of the original oil in place (OOIP). The
economic analysis showed that the chemical costs were approximately U.S. $2.72
per barrel of incremental oil recovered. Results are presented in 15 tables and
8 figures.
Introduction
Achieving mobility control by increasing the injection fluid viscosity and
achieving profile modification by adjusting the permeability variation in depth
are two main methods of improving the sweep efficiency in highly heterogeneous
and moderate viscous-oil reservoirs. In recent years (Wang et al. 1995, 2000,
2002; Guo et al. 2000), the addition of high-molecular-weight (MW)
water-soluble polymers to injection water to increase viscosity has been
applied successfully in the field on commercial scales. Weak gels, such as
CDGs, formed with low-concentration polymers and small amounts of crosslinkers
such as the trivalent cations aluminum (Al3+) and chromium
(Cr3+) also have been applied successfully for in-depth profile
modification (Fielding et al. 1994; Smith 1995; Smith and Mack 1997). Typical
behaviors of CDGs and testing methods are given in the literature (Smith 1989;
Ranganathan et al. 1997; Rocha et al. 1989; Seright 1994).
The giant Daqing oil field is located in the far northeast part of China.
The majority of the reservoir belongs to a lacustrine sedimentary deposit with
multiple intervals. The combination of heterogeneous sand layers
[Dykstra-Parsons (1950) heterogeneity indices above 0.5], medium oil
viscosities (9 to 11 cp), mild reservoir temperatures (~45°C), and low-salinity
reservoir brines [5,000 to 7,000 parts per million (ppm)] makes it a good
candidate for chemical enhanced-oil-recovery processes.
Daqing has successfully implemented commercial-scale polymer flooding (PF)
since the early 1990s (Chang et al. 2006). Because the PF process is designed
primarily to improve the mobility ratio (Chang 1978), additional oil may be
recovered by using weak gels to further improve the vertical sweep. Along with
the successes of PF in the Daqing oil field, two undesirable results were also
observed: (1) high concentrations of polymer produced in production wells owing
to the injection of large amounts of polymer (~1000 ppm and 50% pore volume)
and (2) the fast decline in oil rates and increase in water cuts after polymer
injection was terminated.
In 1997, a joint laboratory study between the Daqing oil field and Tiorco
Inc. was conducted to investigate the potential of using the CDG process, or
the CDG process with PF, to further improve the recovery efficiency, lower the
polymer production in producing wells, and prolong the flood life.
The joint laboratory study was completed in 1998 with encouraging results
(Smith et al. 2000). Additional laboratory studies to further characterize the
CDG gellation process, optimize the formulation, and investigate the
degradation mechanisms were conducted in the Daqing field laboratories before
the pilot test. A simplistic model was used to optimize the slug designs and
predict incremental oil recovery. Initial designs called for a 25% pore volume
(Vp ) CDG slug with 700 ppm polymer and the
polymer-to-crosslinker ratio (P/X) of 20 in a single inverted five-spot
patten. Predicted incremental recovery was approximately 9% of OOIP.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
12 January 2004
- Revised manuscript received:
19 July 2006
- Manuscript approved:
31 July 2006
- Version of record:
20 December 2006
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