Caltex Pacific Indonesia integrated carbon-oxygen (C/O) logging into a total reservoir management program for the Bekasap field waterflood in central Sumatra.

Extending the life of a mature field is accomplished by maximizing production, increasing recovery and reducing costs. The costs of gathering the information necessary to efficiently implement improved recovery programs can be significant. If these investments are not made, however, up-front savings can result in added costs and poorer performance later.
Caltex Pacific Indonesia (CPI) made a significant investment in a comprehensive program of through-casing C/O logging to accurately characterize the remaining oil in a multiple-zone waterflood in Indonesia. Halliburton Energy Services' new through-tubing version of its through-casing C/O logging tool is making it possible for CPI to capitalize on this investment by monitoring the flood's performance at lower costs.
Bekasap field
In the Pertamina/CPI production-sharing contract area about 6 miles (10 km) west of the Duri field, Bekasap began producing in 1965 (Figure 1). About 42% of its 1.19 billion bbl of original oil in place has been produced. A peripheral waterflood was implemented in 1997 to increase recovery and extend field life. According to CPI, cumulative production through 2000 was about 481 million bbl of oil. Average injection of 250,000 b/d of water from 16 injectors was established into 10 distinct reservoir intervals. Field production is about 27,500 b/d of oil, 2,400 b/d of condensate and 150,000 b/d of water from 75 producing wells.
The geology of the field includes multiple reservoirs within three main formations: Menggala, Bangko and Bekasap. All are estuarine to shallow marine sandstones with a range of reservoir quality dependent on shaliness (Tran, 2000).
Recognizing the number of intervals involved, the age of the wells and the likelihood of oil being bypassed due to behind-pipe communication or geological barriers, CPI initiated a total field reservoir management project in 1999 to optimize the waterflood operation and maximize recovery. This project included a program of C/O logging to identify zones with oil saturations where additional recovery was likely, and also to monitor the progress of the waterflood.
C/O logging effective through casing
In many of the low-salinity, medium- to high-porosity Indonesian oil reservoirs, C/O logging has provided an effective method of through-casing reservoir surveillance (Tran, 2000). In the Bekasap field, CPI selected several well candidates across the field for initial C/O runs to determine those portions of the field with oil saturations that could potentially benefit from the waterflood. The resulting saturations were mapped and compared to saturation maps based on reservoir simulation, with good agreement. This characterization provided a starting point for the integrated, log-based field reservoir management.
The ongoing C/O logging program, as well as continued simulation, have been the basis for identifying well workovers, infill drilling locations and injection well workovers (recompletions and perforation squeezes). C/O logging undertaken in about 44 wells from 1999 through 2001 resulted in an increase in oil production of more than 5,000 b/d and a reduction in water production of more than 100,000 b/d, said Thanh B. Tran, special studies engineer for Bekasap AMT with CPI.
For example, a high watercut in Bekasap No. 7 was diagnosed as a reservoir coning problem, and the completion zones were isolated. The subsequent C/O log (Figure 2) revealed a relatively significant amount of bypassed oil remaining in the Bekasap 2,050-ft (625-m) B-1 and B-2 sands at 2,030 ft to 2,220 ft (619 m to 677 m). The well was retested and flowed at 1,485 b/d of oil and 395 b/d of water. A new completion design incorporated a reverse, coning-downhole design to allow the well to be maintained on production.
Through-tubing tool introduced
The success of the C/O program prompted CPI and Halliburton to try to determine ways to broaden its application. One way was by introducing Halliburton's RMT Elite Reservoir Monitor Tool (RMT-E). The RMT-E is a slimhole pulsed neutron logging system for obtaining through-tubing C/O measurements with two to three times higher measurement resolution than other systems. The tool has been used in six Bekasap wells since 2001, with good results.
Skip Reed, global nuclear product champion with Halliburton, said "Using the RMT-E avoids the cost of killing the well, pulling the tubing and rerunning the tubing. Another benefit is that existing perforations don't need to be squeezed off prior to running the RMT-E as with previous C/O logging tools." State-of-the art interpretation techniques are able to factor out the borehole fluids using fluid holdups calculated from the borehole C/O or by running production logs in combination with the RMT-E. This reduces well downtime by about 10 days and can amount to thousands of dollars in savings. In addition, the risk of damaging the formation from kill fluids and cement is decreased.
The RMT-E has a maximum 21/8-in. OD, permitting it to be run in tubing with a 23/8-in. ID. Its maximum logging speed is 5 ft/min in C/O mode (30 ft/min in Sigma mode), a speed two to five times faster than competing systems.
"The tool relies on high-density bismuth germanium oxide (BGO) detectors that have a much higher average count rate than the traditional sodium iodide detectors," Reed said. "This gives the BGO detectors a more prominent peak structure in both inelastic and capture energy spectrums. In addition, a specialized tool housing material reduces the magnetic field at the surface of the detector housing, allowing the detector crystal diameter to be maximized. This combination of design factors permits the tool to achieve resolutions previously available only with larger-diameter C/O systems."
The RMT-E tool is able to evaluate formations with porosities as low as 8% with a continuous log, while comparable tools are limited by a dynamic range of 18% to 20% porosity.
Reservoir monitoring
CPI has an active reservoir surveillance program using a combination of C/O and cased-hole formation resistivity to monitor waterflood performance as well as select horizontal locations. Said Tran, "We use C/O logging, specifically the Pulsed Spectral Gamma Tool and the RMT-E, as one of several tools for reservoir management and reservoir surveillance in the Bekasap field. Information from C/O logs helps us identify bypassed reserves resulting from water coning and optimize our workover program to recover those reserves."
CPI also use the results of the C/O logging program to optimize waterflood operations and injection well locations based on a better understanding of where unswept oil reserves remain. Tran added, "CPI has also relied on the results of the logging program to optimize development drilling. We have drilled nine vertical development wells and 14 of a total of 40 to 50 horizontal wells so far."
The success of C/O reservoir monitoring at Bekasap appears to support the notion that improvements in the capabilities of through-tubing logging tools are making it easier for companies to justify investing in the data collection necessary to extend the lives of mature fields.
Reference
Thanh Binh Tran, T.B., et al., "Critical Wellbore Considerations for Successful Carbon-Oxygen Log Applications: Benefits of a Teamwork Approach," SPE 64405, 2000.

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