CBIL logs: vital for evaluating disappointing well and reservoir performance, K15-FG field, central offshore Netherlands

Abstract

The first two development wells of the K15-FG Rotliegend gas reservoir showed a large variation in productivity and sharply declining flow rates. Material balance data indicated only 30% of the field’s expected Gas Initially In Place (GIIP) to be connected to the wells. This poor performance reflects stratigraphie and structural reservoir compartmentalization. A Circumferential Borehole Imaging Log (CBIL) was run in the second development well. The CBIL enabled identification of small-scale reservoir heterogeneity and thin, otherwise undetected highly productive aeolian layers. The log analysis also revealed the presence of a conjugate set of shear fractures, confirmed by cores. Furthermore, small-scale, sub-seismic resolution reverse faults were detected intersecting the well path, as well as anomalous structural dips around the well, indicating fault drag. The presence of near wellbore reverse faults and sub-vertical shear fractures with cataclastic fill, restricting horizontal inflow, explains the disappointing production rates of the second well. Moreover, continuous borehole break-outs were observed from the log, indicating present-day stress anisotropy. The CBIL data in combination with lineations observed on seismic-attribute maps contribute to a concept of dextral wrenching across the field, with related reverse faulting and strike-slip faults or shear zones. Such a configuration of partly and completely sealing faults is a major cause of the poor reservoir connectivity as confirmed by pressure behaviour. The CBIL is an effective tool for evaluating both depositional and structural reservoir heterogeneity. The resulting interpretations created scope for optimization of recovery by resolving the causes of anomalous well behaviour and field architecture. Pressure analysis of a recently drilled third development well has once more confirmed the presence and sealing character of strike-slip faults within the field.