Reservoir Rock Typing from Crest to Flank is There a Link?

Abstract

Abstract Minimizing uncertainty associated with predicted log water saturation can be best achieved through the integration of log analysis (formation evaluation) and capillarity (core measured high pressure mercury and oil/water drainage capillary pressure data). Further reduction of uncertainty in predicted formation water saturation requires; first the most probable log porosity solution interpretation; second the most probable log porosity/permeability relationship (match core measured values); and third a quantitative capillary pressure model that represents measured high pressure mercury and oil/water drainage capillary pressure data. This paper is a continuation of previous work done using the classical Leverett-J method (Leverett 1941) in integrating capillary pressure data with routine core analysis and petrophysical rock types to generate a robust model for predicting formation water saturation profiles. Two datasets were used in the application of this method in two phases. Phase 1 was accomplished by using the first dataset, which is for a crestal well, in characterizing reservoir rock types by the assignment of capillary pressure curves to their corresponding rock types. This process was accomplished by grouping rock types according to discrete ranges of the interval speed which is defined as the square root of core measured permeability divided by porosity (k/ϕ) data. Each range of the interval speed will have a corresponding Leverett J function from which water saturations will be interpolated according to its respective height above the Free Water Level (FWL) (using the log predicted permeability-porosity ratios). In phase 2, the second dataset, which is from a flank well, was used to quality check the rock typing interpretation and to aerially map the change in reservoir rock types when going from crest to flank. One of the main findings from this work is the importance of quantifying the cementation process that took place in rocks' post-deposition. In the absence of clear fluid contact, a trial and error method was followed to adjust the FWL until a good agreement between the log predicted water saturation and the predicted water saturation from the capillary pressure correlation was reached. The error in averaging was minimized by increasing the number of rock types (minimizing the ranges of interval speed) using a large number of measured high pressure mercury and oil-water drainage capillary pressure data. Furthermore, this paper presents a comprehensive comparison of the log derived and model predicted water saturations.