Abstract

Modelling of naturally fractured reservoirs is the first step to develop best scenarios for hydraulic fracture treatment, design an optimum production method and evaluate reservoir potential. This paper reviews the state of the art in current methods and presents an integrated modelling methodology utilizing object-based modelling, stochastic simulation and global optimization. Firstly, as an object-based model, each fracture was presented and treated as a discrete object. A stochastic simulation was carried out to generate an initial fracture network. An objective function was then formulated as the difference in statistics between the initial network and the target. Semi-variogram and other spatial statistical properties (cross variogram, multi-histogram mean and variogram distance) of fracture parameters were included, so that the objective function was able to statistically describe representative field data. Subsequently, a global optimization (simulated annealing) algorithm was used to optimize the objective function. An application study of a 2D fracture network showed that the modelling methodology have mapped discrete fracture network very closely to the observed physical distribution of fractures and their properties.

Introduction
Background

Due to geological reasons, many of the naturally fractured reservoirs possess very low permeability, which is inadequate for economic production. Therefore, some permeability enhancement techniques are essential for these reservoirs. However, several authors(1)(2) have identified that underlying principles of such techniques, such as hydraulic fracture stimulation, are complex and progress is hindered due to lack of appropriate geo statistical fracture description models. Thus, there are three main reasons for a detailed fracture distribution:

  • To site best locations for production wells

  • To study the response of natural fractures under stimulation pressure, hence, to develop a best scenario for hydraulic fracture treatment.

  • To design an optimum production method and evaluate reservoir potential

In order to achieve the prescribed objectives, naturally fractured reservoirs need to be characterized and modeled, which include description of reservoir boundaries, rock heterogeneity, major faults and medium to small-scale fracture networks (3).

Fracture Modelling Techniques: State of the Art

Mathematical and Geo-Mechanical Models

Several techniques to simulate naturally fractured reservoirs have been documented in literature. Firstly, there were mathematical and geo-mechanical models. Earlier mathematical methods to simulate natural fracture networks and fluid flow through them included equivalent continuum (4), discrete network (5)(6) and hybrid models (7)(8). The usual approach relied on simplistic geometrical description of fracture systems (e.g. homogeneous reservoirs, parallel plate fractures, etc.), with primary efforts spent on flow behaviors(9). Many authors have used different geo-mechanical approaches for modelling fractured reservoirs, such as simple curvature analysis, field stress and fracture growth mechanism, and numerical analysis by solving systems of non-linear continuum mechanics equations using finite element methods (10) (11).

However, due to the complex nature of fracture systems, most studies so far have succeeded only in modelling of unrealistic homogeneous reservoirs. Moreover, most of these techniques used only a limited source of data, such as seismic or well logs. Hence, in modelling naturally fracture reservoirs, it is necessary

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