Doe/netl-2012/1540 Mobility And Conformance Control For Carbon Dioxide Enhanced Oil Recovery (Co2-Eor) Via Thickeners, Foams, And Gels - U.s. Department Of Energy Page 204

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five days and gas-lift pressure was maintained on the casing in order to prevent the gel from
forming in the production wellbore
The production of incremental oil and the increase in injection pressures after the treatments
demonstrated that the gels did form in situ. The incremental oil production attributable to these
treatments was 65 BOPD, totaling 25,000 bbl. by November 1985. This project was considered
an economic success in that payout occurred in 1 ½ months, and the ability to effectively block
the entire channel was considered to be a key aspect of the effectiveness of the treatment.
Chevron’s Rangely Weber Sand Unit, Rio Blanco County, Colorado; 1994–1997;
Conformance Control; CO
gel; In situ Gelation of a CC/AP Polymer Solution [Hild and
2
Wackowski, 1998; Friedman et al., 1999; Sydansk and Southwell, 2000].
The Weber Sand Unit (described in the previous section on CO
foams) was tested with foam
2
along with other near-wellbore treatments during the late 1980s. Because the field had matured
and both conformance and mobility control problems were becoming more significant in the
mid-1990s, Chevron decided to employ a technique involving the injection of a very large
volume of a diverting agent that could occupy a substantial portion of the interwell region in the
high permeability, watered out zones. The gel solution was an uncrosslinked polymer solution
that was intended to flow primarily into the high perm zone, gel, and then divert flow into lower
perm zones that (hopefully) did not receive a significant amount of the gel solution.
During CO
flooding, it became apparent that an extensive fracture network existed throughout
2
the field, as evidenced by the identification of over 100 pairs of injector-producer wells that
exhibited early CO
breakthrough. Research conducted at NMIMT [Seright and Liang, 1996]
2
indicated that a Cr (III) acetate polymer gel system requires a permeability of at least several
hundred millidarcies for placement of the matrix. Therefore, this solution was selected because
the high permeability zones and fractures had a greater permeability than the minimum amount
needed for the gel to form, while the zone targeted for post-gelation CO
recovery had a
2
permeability of roughly 10 mD, meaning that the gel solution should not block this oil-rich zone.
Cr (III)-carboxylate/acrylamide polymer gels (CC/AP) are aqueous solutions that form gels when
the polyacrylamide is crosslinked by a Cr (III)-carboxylate-complex, typically in the form of
chromic triacetate. Two major formulations of the gel are used for two distinct applications. If
near-wellbore (<15ft.) total fluid shutoff treatments are desired for unfractured rock, small
volumes (typically < 500 bbl.) of fresh water solutions containing high concentrations (3.5–7.0
wt.%) lower molecular weight (200,000–2,000,000) polymers are used. If fractures or multiple
darcy permeability zones are prevalent, as is the case at Rangely, large volumes (500–40,000
bbl.) of a produced brine solution containing low concentrations (0.3–2.0wt%) of high molecular
weight (>4,000,000) polymer are injected. Details of the chemistry associated with CC/AP gels
are found in Marathon’s review of 12 years experience using this technology in a wide array of
applications, including conformance control, during CO
floods at the Rangely and Wertz fields
2
[Sydansk and Southwell, 2000].
CC/AP can be used in many scenarios, and it was deemed to be appropriate for the Rangely field
because this gel is insensitive to the low pH characteristic of CO
floods if the degree of
2
173

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