Abstract
The wells of this study have produced from two stacked Carbonate reservoirs which are characterized by variation in interparticle porosity along with a variable secondary porosity system of dissolution from vugs and fractures. The objective was to characterize these heterogeneous reservoirs in the absence of core data and to define consistent petrophysical models including porosity, permeability, rock quality indicators and saturation height models that considers the complexity of the reservoirs to be simulated.
Since no core data is available, the NMR T2 distribution and derived permeability (scaled to mobility from well tests) were correlated with borehole image features in key well to define the rock typing scheme. The saturation height functions were developed directly from the SW from resistivity logs by transforming and adjusting NMR T2 distribution to saturation height. In wells with only conventional logs, the SHF was used to back calculate permeability within the transition zone. In horizontal wells, a layering framework was built using electrical image logs for modeling high conductive features (vugs and fractures).
In the key well, the NMR T2 distribution reveals important variations in pore size distribution that enabled to establish a robust rock typing index based on Lucia Rock Fabric. The permeability estimated from NMR log was adjusted to match mobility from MDT. By transforming the pore size distribution from NMR to saturation height, a close correlation with the SW from resistivity logs is observed. The saturation height function allows within the transition zone to back calculate permeability from log derived SW in wells where NMR log is not available thus capturing heterogeneities in all wells.
For building a 3D property model which can include the layers contributing to dual porosity and permeability, a layer based model was defined in horizontal wells using borehole image dips and textural characteristics, petrophysical interpretation and production data as inputs. The correlation between wells indicate that the shallow most layer shows highest secondary porosity and conductive fractures. Despite the absence of core analysis, limited special logs and a mixture of vertical and horizontal wells with only conventional logs, the petrophysical properties of carbonate reservoirs with complex depositional and diagenetic features can still be established by associating the existing information into a coherent petrophysical model.
The novelty of this study is the construction of consistent permeability and saturation height model for a complex pore network carbonates formation using NMR logs in a key well that honors the log derived saturation profile and allow to back calculate permeability in wells with absence of core and limited log dataset. The construction of detailed layering along horizontal wells based on electrical borehole images is highly beneficial for developing the reservoirs models.
