Gas-Cap Monitoring in High-Flow-Rate, Low-Porosity Clastic Reservoirs in Colombia Gives Big Economic Returns

Abstract

Summary Cased-hole neutron logging has been in use for many years and is often used to monitor gas movement in reservoirs behind pipe. This works well in high-porosity reservoirs but is rarely successful in low-porosity reservoirs. In Colombia, there are high-flow-rate reservoirs with significant permeability 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 economic benefit of the monitoring program. Many of these wells contributed significant 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-gas zones from low-GOR zones.