Modeling of a Naturally Fractured Carbonate Reservoir Based on Petrophysical Rock Types

The Permian Zechstein (Ca2) carbonates form important gas reservoirs in Northern Germany. We present a case study from a naturally-fractured, partly dolomitized sour gas field in the Lower Saxony Basin. In order to prepare an accelerated field development plan, identify suitable infill well locations and improve production forecasting, detailed sedimentological core descriptions (incl. fracture analysis), petrophysical log interpretations (fracture plausibility calculations) as well as test and pressure data were integrated into a fit-for-purpose static reservoir model. A rock-type based reservoir modeling approach was chosen. Emphasis was put on defining petrophysical rock types that combine flow properties of fractures and matrix, providing a single permeability description for all grid cells.

Based on material balance analysis, the initial gas in place of the field was found to be significantly higher than assumed so far due to previously underrated, but extensive porous/permeable dolomite bodies mainly present in the western part of the field (Type-III NFR). In contrast, the eastern part of the field is almost entirely composed of tight, but naturally fractured limestones (Type-II NFR).

Outcomes of the study may help to better understand reservoir quality heterogeneity and production performance of the field as basis for targeted development drilling.