The Lower Sihapas Formation in the M-Field consists of sandstones with subordinate inter-beds of mudstones, shale and coals.

The C sand is a Darcy-permeability blocky sand and part of Lower Sihapas Formation. In the M2-03 well, in 1987, the C sand was perforated at the top of sand to drain oil from the entire sand. After 24 years of production, when the oil rate was 16 BOPD and the water cut was 99%, we ran through-tubing C/O log to look for by-passed oil in any sand. The RST result shows unswept oil in C sand. Other supporting data are also bringing into upswept potential are: Production log data shows small flow contribution from C sand, suggesting small cumulative production; Micro Resistivity Image logs from offset wells suggested the C sand is might be laminated sand that could impacts on low productivity, and also may isolate existing perforation from lower sand body ofthe sand.

The attractive water saturation in the lower sand of 40% from the C/O log might be right.So, we decided to perforate the lower C sand using a through-tubing gun. After perforation, the oil rate spiked at 2000 BOPD and then quickly settled down to 100 BOPD. The cost for the C/O log and perforation was recovered within days. Executing a through-tubing perforation instead of a "stronger" casing gun perforation via a workover saved us more than $250,000.

This data has enabled us to act to slow production decline, save wellwork cost and extend the economic life of platforms. We continue to find that cased-hole logging with Production log and C/O log adds value in mature high water-cut oil wells, by identifying watered out sands and by-passed oil sands.

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