High Permeability Layers in Carbonates: Innovative 3D modeling Approach in Lower Cretaceous reservoirs, offshore Abu Dhabi.
Authors: Ruelland Philippe Jacques , Bu-Hindi Hani 
Published: May 2016

High Permeability layers are a common feature of most carbonate series, and contribute to heterogeneous sweeps and early breakthroughs when injecting gas or water in an oil reservoir.

The innovative workflow proposed to model the high permeability layers, on the basis of strong constraints from geological data and concepts, enables to be more predictive on the dynamic behaviour of the reservoir. It also enables a better selection of the most adapted recovery mechanism and a better strategy for optimizing the positioning of future perforations.

Increasing efforts are made to track and represent high permeability layers, also named streaks, in 3D dynamic models. Well data (core description, porosity-permeability plug measurements and well tests) allow the recognition of such heterogeneity at the well. Attributing the high permeability layers to fractures or to matrix heterogeneity can be achieved by cross-checking the well data with production tests and sometimes with 3D seismic. The 3D representation of these layers, however, does not necessarily incorporate all the geological understanding that has been gathered.

In the case of the studied field, offshore Abu Dhabi, the heterogeneity that is related to high permeability layers is linked to dissolution diagenesis. Dissolution diagenesis occurs preferentially just below the sequence boundaries in carbonate series. Although there are uncertainties on the dissolution trends, a stochastic modeling approach based on geological observations and concepts can help capture the effects of diagenesis on the rock properties. Multiple realizations present various equi-probable images of the field heterogeneity and will guide the field development strategy.

In the first stage of the workflow, depositional environments and lithofacies have been modeled in 3D. They are constrained by the well data and the geological concept related to slope profile and to paleobathymetric control over depositional environments. Diagenesis overprints the rock texture by cementing or dissolving. Vugs are typically the product of dissolution diagenesis. No abundant dolomitization is observed in the studied formation. Dissolution diagenesis is one of the major causes of permeability enhancements in carbonates. Dissolution diagenesis preferentially occurs in the shallowest depositional environments, just below sequence boundaries when relative sea level is lowest. It is related to the infiltration of meteoric water in the carbonate series. There are three possible controls over the pathway which meteoric water can follow: the slope, the "weakest" lithofacies and early fissures or fractures in the rocks.

The approach proposed to represent the effects of diagenesis is explicit. It is not currently driven by any fracture orientation. Pathways where alteration occurs are stochastically modeled on the basis of well observations and hypotheses on their orientation. Inside these modeled pathways, specific permeability ranges are entered to represent the rock properties. An interesting aspect of using an explicit approach is the quest for predictiveness of the reservoir behaviour. Most approaches related to high permeability streaks modeling today are implicit: high permeabilities observed at the wells are extrapolated away from the well by kriging or stochastic simulation methods without much geological constraints. In some cases a very fine vertical layering is chosen in order to capture even the slightest permeability variations vertically along the well.