After more than 20 years of exploitation, many of the thick and prolific reservoirs of the Malay basin are depleted. However, field studies indicate that large volumes of hydrocarbons remain located in lower quality but producible layers.
These reserves are the focus of monitoring and recompletion campaigns, to maintain production and extend field life.
Most wells intercept thick accumulations of multi-layered reservoirs, which are produced from dual tubing strings, multiple packers and selectable sliding side doors. In these difficult conditions, special care must be brought to the planning, acquisition and interpretation of monitoring surveys, and to the execution of re-perforation, water shut-off, and other recompletion solutions.
This presentation highlights the practical steps that have led to a successful campaign of identification and production of remaining reserves in the Tinggi field, offshore Peninsular Malaysia.
Through-tubing tools record pulsed neutron Gamma Ray (GR) decay-rate for Sigma log, GR spectroscopy for Carbon-Oxygen log, and oxygen activation for water velocity log (Fig. 1; Ref. 1,2,3,4).
Formation waters are fresh (15,000 ppm NaCl equivalent) and Sigma log does not allow differentiation of formation oil from water, due to their similar capture cross-section: Sigma oil equals 21 capture units (cu) and Sigma water equals 26 cu. However, the Sigma log is well suited for gas evaluation because Sigma gas equals approximately 6 cu.
Instead, Carbon-Oxygen (CO) logging, which is independent of water salinity, is the preferred method of saturation monitoring. The CO log can be recorded in casing - below the tubing shoe - or inside single or dual tubing sections. The CO log is sensitive to the effect of completion items and borehole fluids. The CO response to formation rock and fluids is well characterized in single casing conditions, but logging within single and dual tubing is more complex, because of the extra tool to formation stand-off and the increased effect of borehole fluids and tubing steel.
Cased-hole resistivity can be an alternative to CO logging, but can only be run in single casing. Logging the reservoirs located across the tubing strings would therefore require the dual-tubing to be pulled out. The recently introduced through-tubing version of the tool is suitable for casings up to 7 inch outer diameter, where most completion casing diameters in Tinggi are 9–5/8 inch.
Tinggi field is part of the PM-9 block, located in the southeastern part of the Malay Basin, approximately 280 km offshore east of Kerteh, Malaysia (Fig. 2; Ref. 5,6). The field was first discovered in July 1980 and developed between 1982 and 1984. After production of more than 20 years, the field is at its tail-end of production with more than 97% of its Estimated Ultimate Recovery (EUR) produced. Continuing efforts must be made to ensure production sustenance for such a mature field, and as such, identification of remaining hydrocarbons is vital before any abandonment process is even considered.
The Tinggi structure is a small east-west trending anticline, with early Miocene age sandstone accumulation in a shallow marine environment. The field consists of stacked reservoirs with a large aquifer that provides a strong bottom water drive. The reservoirs are produced under a combination of natural water drive and gas re-injection at the crest of the gas cap, to obtain closure of both the OWC and GOC at the perforations. (Fig. 3)
As part of the Tinggi team's effort to find opportunities for additional production in the field, the openhole logs of all the wells were reviewed to identify previously overlooked zones that might still containing hydrocarbons. However, as the openhole logs were taken more than two decades ago, the team needed to ascertain the current saturation of fluids in the reservoir before any further work could be done. CO logging was chosen as the preferred method of data acquisition for reasons explained in the previous section.