Many reservoirs in the Gulf of Guinea are sequences of permeable sands interbedded with variable proportions of silt and clay. The sands are typically permeable, but the silt and clay are not, determining a high degree of vertical heterogeneity. Some of these reservoirs are also highly depleted, with thin oil columns delimited by encroaching gas caps and aquifers.
Horizontal wells are a proven way to produce mature reservoirs with high permeability contrast. However, the higher construction costs and technical complexity demand the utmost care in their placement. To this end, pilot holes are drilled in order to exactly identify the target zones. The task to precisely identify the sweet spots where to place the horizontal wellbore is sometimes challenging.
For start, thin-beds reservoirs are traditionally difficult to evaluate quantitatively. This is especially true in depleted reservoirs, where hydrocarbons identification is complicated by their reduced volumes and corresponding diminished effect on typical logs. Fresh connate water increases the uncertainty of traditional resistivity-based saturation methods. The highly variable permeability complicates the task of acquiring and interpreting formation pressure. Lastly, operational reasons may prevent the use of wire-line-conveyed logging devices and associated methods.
This paper presents a case where multiple formation evaluation challenges were addressed in real time and quasi-real time by the integrated interpretation of advanced logging-while-drilling (LWD) data.