An Effective Approach for Automatic Rock-Typing and Calibration Using Single-Well Models
Authors: Sharma Sachin Kumar , Carrillat Alexis Vincent , Friedel Torsten 
Published: May 2016

Rock-typing is an essential ingredient for any static or dynamic modeling; it is the link between depo- or lithofacies and their pore geometries and the fluid flow properties. The rock types determine porosity-permeability transforms and initial water saturation in static models and govern properties that directly impact fluid flow behavior (such as relative permeabilities or rock compressibilities). With this in mind, they are guiding elements for most decisions on well and infill-well placement today, at least wherever reservoir models are used.

As for many engineering applications in oil and gas E&P, a key issue is the nonuniqueness of the rock-typing methodology, including the definition of each rock type. This is particularly the case in carbonate reservoirs, which are heavily impacted by diagenetic processes. Despite use of advanced technologies such as nuclear magnetic resonance (NMR) logging on the pore scale, in carbonate reservoirs it is practically impossible to derive permeabilities that are valid for reservoir simulation directly from core analyses or to derive rock types directly from electrofacies characterization.

A methodology has been developed that effectively addresses both aspects. It conventionally integrates special core analysis (SCAL), well log interpretation, and iterative binning based on flow zone indicators (FZIs). Porosity-permeability transforms and an initial water saturation model are built for the FZI bins and tested using a single-well, full physics model (SWM) that has been created using a commercial reservoir simulator to match the initial production rates. Based on feedback from the SWM, FZI bins are fine-tuned and the SWM is set to give a fieldwide optimized solution. The loop, including the SWM, is fully automated and uses a gradient method for fieldwide iterative optimization of the permeability transformation.

The method provides an integrated way of rock-typing for giant carbonate fields. The SWM produces a robust rock-type model with an optimal number of rock types, validated porosity-permeability transforms, and an initialized water saturation model at low numerical cost.

Introduction

Simulating the dynamic behavior of a reservoir requires accurate permeability, initial water saturation, and relative permeability functions. These parameters can be best modeled using rock-typing, which is an effective tool for permeability and saturation modeling in carbonates. When properly classified, a given rock type is imprinted by a unique porosity-permeability relationship, capillary pressure profile, and a set of relative permeability curves (Gunter et al. 1997; Amaefule et al. 1993). Rock-typing thus provides the architecture and means of data population (porosity conditioning, permeability, and saturation), integration and field heterogeneity mapping in static geomodeling, which are key inputs to the dynamic simulation.

In that respect, the objective of rock-typing is to integrate all relevant data, from SCAL to well logs, into a coherent and meaningful reservoir description and relevant porosity-permeability transform, initial water saturation function, and relative permeability curves for the field. These rock types will serve as a deterministic basis for property propagation in static modeling, reservoir history matching, infill-well planning, and reservoir management optimization.