Gas Cap Monitoring in high flow rate, low porosity clastic reservoirs in Colombia gives big economic returns

Abstract

Cased hole neutron logging has been around for a long time and is often used to monitor gas movement in reservoirs behind pipe. This often 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, that have low porosity (5–6 PU). The challenge was to monitor gas cap expansion, fluid movement due to re-injection, and voidage. At the same time the objective was to identify low GOR intervals behind pipe for possible recompletion. A number of different examples are shown to 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 ovr 10,000 BOPD from a single recompletion. Gas cap monitoring in very low porosity reservoirs in Colombia proved to be not only feasible, but extremely profitable by differentiating dry gas zones from low GOR zones. Introduction-Cusiana Field, Colombia Cusiana is a field in Colombia (Figure 1) that produces high rates of oil and gas from sandstone formations in the Andes foothills. Tectonic evolution of the area dates back to an early Paleozoic rifting period forming grabens where a thicksequenceofsedimentswasdeposited. The Llanos (which is the current plains area east of the Andes in Colombia) was an extensional subsidence basin in the Triassic and early Cretaceous, becoming a passive subsidence basin in the late Cretaceous (Figure 2). Tertiary, Cretaceous and Paleozoic silico-clastic sedimentary sequences overlie the pre-Cambric basement consisting of crystalline and metamorphic rocks. The maximum sedimentary thickness reaches 12,000 m.(40,000 feet).