Abstract

The characterization of Naturally Fractured Reservoirs (NFR) represents additional challenges to that of conventional reservoirs because it is not only required the description of two separate means (matrix and fracture) but also it is necessary to understand their interaction. Among the list of potential parameters that can contribute to understand the complexity of NFR is the fracture intensity. This is defined as the density of fractures per foot of formation. This attribute is a key factor for a quantitative prediction of the porosity and permeability of a NFR. Furthermore, it is directly related to the reservoir productivity and can be used to optimize reservoir management decisions.

In this paper, we present a new methodology to identify fractured intervals in a NFR by combining the answer of a set of conventional electric logs and the information coming from the physical description of the cores of a fractured formation. With this information, a continuous variable fracture intensity track is generated and related with the fracture permeability through two different approaches. In the first approach, the aperture and fracture intensity were used to estimate the fracture permeability. The second approach is indirect. Using a non-parametric regression technique, the measured fracture permeability is modeled from variables such as GR, fracture porosity, Facies, and fracture intensity. In both cases, quantitative models of fracture permeability were obtained under static conditions.

We have successfully applied the proposed technique on a reservoir producing from a Cretaceous Formation in a Colombian Field. The models obtained were satisfactory validated with information coming from core analysis, stratigraphic and structural models, pressure tests, and production data.

Introduction

The fracture intensity is a key factor to predict petrophysical properties in a Naturally Fractured Reservoir. These variables define optimal well productivity and reservoir development strategies.

In this project, by a suitable combination of information such as geologic and engineering concepts, core interpretation, well logs, pressure tests, and production data, we have found a way of modeling fracture permeability from fracturing susceptibility. Our approach was applied to a field producing from a Cretaceous Formation in a Colombian Field.

This paper is divided in three parts. First, a discussion about the core analysis and the petrophysical interpretation is shown. In the second part, the methodology itself is discussed. Finally, we have successfully applied the proposed technique on a reservoir producing from a Cretaceous Formation in a Colombian Field.

Methodology
Core Analysis

Core analyses allowed describing features of fractures such as intensity, dip, length, aperture, mineralization type, and morphology. According to their description, analysis, and environmental interpretation the Formation Facies distribution and their respective Fracture Intensity was determined. Seven hundred and sixty six (766.0) ft of core were analyzed.

Facies Determination

Five Facies were defined. For practical purposes, they were grouped into three Facies, based on the answers obtained from electric logs. Limestones was denominated Facies 1 (considering those of the type grain supported: Grainstone and Packstone and the mud supported: Weakstone and Mudstone). Sandstones was named Facies 2 (including, original Facies 2 of conglomeratic sands and Facies 3 of sandstones) and Clays (represented by Facies 4 of silstones and Facies 5 of mudstones)

This content is only available via PDF.
You can access this article if you purchase or spend a download.