Advanced Vertical Interference Test Modeling by Considering Petrophysical and Image Based Rock Typing for a Better Kv Estimation in a Heterogeneous Reservoir-A New Workflow

Abstract

Vertical Interference Tests (VIT) using wireline formation testers are industry standard tests to estimate the vertical permeability of reservoir pay zones. In general, the test interval is considered homogeneous for the interpretation, leading to an inaccurate estimation of vertical permeability (Kv) in complex geological systems like thin laminated beds, channel sands, etc. This paper presents a novel approach of accounting for this heterogeneity through use of petrophysical and borehole image-based rock-typing methods, thus leading to a more realistic characterization of vertical permeability. Advanced petrophysical logs and images are used to generate rock types through Artificial Neural Network (ANN) and Stratigraphic Modified Lorentz Plot (SMLP) techniques. These rock types are then used as an input into vertical interference test interpretation model, thereby factoring in the reservoir heterogeneity for deriving the vertical permeability. This is followed by a sensitivity analysis to examine the impact of the permeability results in multiple geological systems like channel sands, thin bed lamination, near to fault, pinch outs etc. Vertical permeability (Kv) is a major input in majority of the advanced reservoir engineering calculations and has a significant impact on the field development plan and IOR/EOR techniques. This unique approach of accounting for rock types in a VIT interpretation model leads to a relatively good estimation of vertical permeability. The rock typing techniques used here, allow the user to define the number of layers and minimum interval thickness, which is extremely useful in highly laminated reservoirs. The sensitivity analysis plays a key role in understanding the utility limitations of both conventional and new approach in complex geological systems. In case of thick homogeneous reservoir, sand units, the conventional approach could be used with fairly accurate results. However, in cases of thin sand-shale units with low net-to-gross ratio, this approach gives a good estimation of layer-wise permeability distribution. This paper presents a unique blend of petrophysical and dynamic workflows into a novel workflow. The results from the sensitivity study, discussed in the paper, can be used as standard criteria in determining the best suitable technique for interpretation of a vertical interference test. This unique approach allows the user to optimize on the interpretation time and to simultaneously ensure the accuracy of results.