Gas-cap monitoring in high-flow-rate, low-porosity clastic reservoirs in Colombia gives big economic returns

dCased-hole neutron logging has been in use for many years and is often use d to monitor gas movement in reservoirs behind pipe. This works well in hig h-porosity reservoirs but is rarely successful in low-porosity reservoirs. In Colombia, there are high-flow-rate reservoirs with significant permeabil ity but low porosity [5 to 6 porosity units (p.u.)]. The challenge was to monitor gas-cap expansion, fluid movement owing to re- injection, and voidage. At the same time, the objective was to identify low gas/oil ratio (GOR) intervals behind pipe for possible recompletion. A number of different examples illustrate both the technique and the econom ic benefit of the monitoring program. Many of these wells contributed signi ficant additional oil from bypassed zones that were initially thought to be dry gas. In some wells, the instantaneous incremental production was over 10.000 BOPD from a single recompletion. Gas-cap monitoring in very-low-porosity reservoirs in Colombia proved to be not only feasible, but also extremely profitable by differentiating dry-ga s zones from low-GOR zones.