Sub-horizontal drilling: remedy for underperforming Rotliegend gasfields, L13 block, central offshore Netherlands

Abstract

The first development well in the L13-FE gasfield showed a rapid production decline. Material balance data indicated less than 10% of the expected volumetric reserves to be connected. This poor connectivity is thought to be due to compartmentalization by sealing strike-slip faults, as indicated by faint lineations observed on seismic attribute maps. The presence of only a limited number of scattered, stratigraphically isolated, prolific layers within an overall rather tight and layered reservoir, resulted in a poor overall vertical permeability, which also contributed to the disappointing well performance. The vertical well was subsequently sidetracked sub-horizontally with the aim to connect a larger number of the scattered prolific sands, different fault-compartments and possible open fracture systems. Graded rocksalt drilling mud was used in order to minimize formation impairment. During drilling of the sub-horizontal section numerous problems were encountered due to mechanical failures and the heterogeneous, layered nature of the reservoir. A significant number of prolific, scattered sand layers were encountered and the presence of a small scissor-type fault was recognized from log correlations. Considering that only some 60% of the well could be completed due to mechanical problems, the well is now producing at acceptable rates. The attempt to challenge such a geologically complex, labyrinth-type reservoir by sub-horizontal drilling, has been cost-effective and successful. However, it is recommended that slimhole, sub-horizontal drilling in this type of layered reservoirs should be applied with caution.