Summary
This paper covers the successful pilot field application of polymer gels for
reservoir conformance improvement in the ongoing CO2 injection
project at Bati Raman heavy-oil field in southeastern Turkey. Bati Raman is a
naturally fractured carbonate reservoir in which the heterogeneities and the
unfavorable mobility ratios between CO2 and the heavy oil cause
inefficient sweep of the reservoir. These conditions prompted the pilot
application of a conformance-improvement fracture-plugging (flowing) gel system
in three wells in July 2002. Based on injection tests performed in the field,
approximate treatment volumes were estimated to be on the order of 10,000 bbl
for each well. Volumes actually pumped ranged from approximately 6,500 to
11,000 bbl. All three of the wells showed a gradual increase in injection
pressure during treatment, indicating a decrease in injectivity index as
treatment progressed. During one treatment, an offset producer experienced
changes in fluid level consistent with rapid pressure transmission via the
connecting fracture early in the treatment, with later loss of such
communication. This behavior provides direct evidence of fracture plugging
during treatment (Lane 2002).
A mechanistic semianalytical model based on previously published laboratory
work (Lane and Seright 2000) obtained a good match with the field data. The
rate/pressure data were fed into the model, and effective fracture widths were
backcalculated. Comparisons of results with the Formation MicroImager (FMI) log
findings are explained. Gel-monitor well responses were scaled based on field
data using a Fetkovich type decline-curve analysis. These studies enabled the
incorporation of the effect of reservoir heterogeneities on the gel propagation
radius so that future gel-treatment design parameters could be optimized.
Pre and post-treatment CO2 injection pressures and the rates are
as shown in Table 1. Sweep efficiency was increased as defined by produced
oil/injected gas ratio. The 1-year average post-gel oil rate from 19 offset
producers is 720 STB/D, as compared with a pre-gel oil rate of 645 STB/D. The
rate of increase from the treatments is thus 75 B/D, or 12%, which indicates a
payout time of 12 months. Keeping this enlightened approach and seizing on the
key concepts, four more CO2 injector wells were treated in 2004 to
follow up on the encouraging results.
Introduction
Bati Raman field is the largest oil field in Turkey, having an estimated
1.85 billion bbl of heavy-oil reserves. It is located in southeast Turkey (Fig.
1) and contains low-pressure, low-gravity (10 to 13°API) oil at an average
depth of 4,300 ft. Its reservoir rock, the Garzan formation, is heterogeneous,
fractured, vugular limestone. Average matrix porosity is 18%, with mainly vugs
and fissures, and secondary porosity is 1 to 2%. The typical matrix
permeability by core analysis is 10 to 100 md; however, well tests show
effective permeabilities in the range of 200 to 500 md, confirming the
contribution of secondary porosity. The field was first placed on production in
1961 and had produced 1.5% of its reserves by 1986, when Turkish Petroleum
Corp. (TPAO) began immiscible CO2 injection. Through 2003, 5% of the
reserves had been produced, which is still an unexpectedly low value.
Production rate has declined drastically since 2000. TPAO is seeking the most
applicable methods to impede or reverse the decline. Polymer gel treatments
were an obvious enhanced-oil-recovery (EOR) method to increase CO2
sweep efficiency.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
9 January 2004
- Meeting paper published:
17 April 2004
- Revised manuscript received:
2 November 2006
- Manuscript approved:
2 November 2006
- Version of record:
20 April 2007
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