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Interim guide to fracture interpretation and flow modelling in fractured reservoirs

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216 pages
Energy research
Coal - hydrocarbons
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ISSN 1018-5593
EUROPEAN COMMISSION
Interim guide to fracture interpretation
and flow modelling in fractured
reservoirs European Commission
Interim guide to fracture interpretation
and flow modelling in fractured
reservoirs
E. S. Aarseth, B. Bourgine, C. Castaing, J. R Chiles, N. R Christensen,
M. Eeles, E. Fillion, A. Genter, R A. Gillespie, E. Håkansson,
K. Zinck Jørgensen, H. F. Lindgaard, L. Madsen, N. E. Odling, C. Olsen,
J. Reffstrup, R. Trice, J. J. Walsh, J. Watterson
JOULE II
Contract No CT93-0334
Research funded in part by the European Commission
in the framework of the JOULE programme
Sub-programme 'Advanced fuel technologies:
security and supply of hydrocarbons'
Directorate-General XII
Science, Research and Development
1997 EUR 17116 EN Published by the
European Commission
Directorate-General XII
Science, Research and Development
B-1049 Brussels
LEGAL NOTICE
Neither the European Commission nor any person acting on
behalf of the Commission is responsible for the use which might be made of the
following information
Cataloguing data can be found at the end of this publication
A great deal of additional information on the European Union is available on the Internet.
It can be accessed through the Europa server (http://europea.eu.int)
Luxembourg: Office for Official Publications of the European Communities, 1997
ISBN 92-827-8812-1
© ECSC-EC-EAEC Brussels · Luxembourg, 1997
Reproduction is authorized, except for commercial purposes, provided the source is acknowledged
Printed in Italy CONTENTS
PREFACE (NH) 1
1 INTRODUCTION (FAG) 3
2 SAMPLING AND ANALYSIS OF FRACTURES 8
2.1 Introduction (FAG)
2.2 Systematics of fractures in outcrop (FAG)
2.3 Sampling of Well Data (EO & FAG) 15
3 GEOMETRICAL FRACTURE MODELLING (BRGM) 21
3.1 Introduction 2
3.2 Step 1. A deterministic and statistical 1-D fracture description of the well 2
3.3 Step. Selection of One (or More) Relevant Templates
3.4 Step 3. Simulation of a 3-D Conceptual,Near-Well Fracture Model 27
3.5 Step 4. Upscaling Of The Fracture Model9
4 FRACTURE SYSTEMS, FLUID FLOW AND EFFECTIVE PERMEABILITY (UiB) 3
4.1 Fracture system connectivity 31
4.2e apertures
4.3 Fracture orientation and permeability anisotropy5
4.4 Semi-quantitative estimates of permeability6
4.5 Discrete fracture flow modelling - the method
4.6 Usinge data in discrete fracture flow models 38
5 RESERVOIR SIMULATIONS ON THE BASIS OF FRACTURE PATTERNS. (GEUS) 4
5.1 Introduction 4
5.2 Determination of permeabilities and matrix block size from a fracture pattern.. 4
5.3 Results of simulation5
6 SUMMARY (UiB & FAG)8
6.1 Overview
6.2 Future Work
References9
GLOSSARYOFTERMS 51
A1 FRACTURE DATASETS5
A1.1 VERTICAL PERSISTENCE AND SCALING OF FRACTURES AT CAPPANAWALLA, THE
BURREN (FAG)
A1.1.1 Introduction2 Stratigraphy
A1.1.3 Mapping Technique 54 Fracture Sets8
A1.1.5 Persistence6 Cliff Section 64
A1.1.7 Joint Architecture6 8 Bedding Thickness and Spacing
A1.1.9 Spatial Distributions9
A1.1.10 Length Populations 71 Vein Thickness Populations7
A1.1.12 Connectivity3 Discussion
A1.2 FAULTS AND JOINTS IN CHALK, DENMARK (GEUS) 81
A1.2.1 Introduction 82 Fracture Data
A1.2.3 Structural Setting ·. 83 4 Concluding Remarks8 A1.3 ANALYSIS OF THE SAUDI ARABIAN FRACTURE DATASET (BRGM) 89
A1.3.1 Introduction 82 The Multiscale Fracture Datasets Of The Western Arabian Sedimentary Cover
Rocks
A1.3.3 Tectonic Interpretation Of The Multiscale Fracture Datasets Related To Red Sea
Rifting 95
A1.3.4 Scaling Analysis Of Fracture Systems 99 5 Discussion 110
A1.3.6 Conclusions2
A1.4 DATASET FROM HORNELEN, WESTERN NORWAY (IBM/UiB) 117
A1.4.1 The Devonian Basin Of Hornelen, Western Norway
A1.4.2 The Data :3 Data Resolution 126
A1.4.4 Data Analysis5 Connectivity9
A1.4.6 Conclusions 13
A1.5 OVERVIEW OF FRACTURE ANALYSIS (FAG) 134
A1.5.1 Introduction
A1.5.2Geological Setting
A1.5.3Sampling
A2 AN ATTEMPT TO SIMULATE FRACTURE NETWORKS FROM BOREHOLE IMAGERY IN A
CLASTIC RESERVOIR (BRGM) 138
A2.1 Introduction
A2.2 Simulation Method For Fracture Networks From 2-D High-Quality Datasets 13
A2.3 Application Of The Simulation Method To A Buried Clastic Reservoir 151
A2.4 Conclusions 157
A3 FLUID FLOW IN FRACTURED ROCKS AT SHALLOW LEVELS IN THE EARTH'S CRUST
AN OVERVIEW (IBM/UiB) 161
A3.1 Introduction
A3.2 Flow In Single Fractures
A3.3 Flow In Fractured Rock Masses And Fracture System Connectivity 16
A3.4 Flow In Fault Zones9
A3.5 Flow In Fractured Rock Masses Away From Fault Zones 172
A3.6 Flow Modelling In Fractured Rocks 178
A3.7 Discussion 18
A4 RESERVOIR SIMULATIONS (GEUS)
A4.1 Simulation Models.....
A4.2 Results Of The Simulations 190
A4.3 Input Parameters For The Reservoir Simulations 193
A5 DERIVATION OF A FRACTURE APERTURE MODEL AND HIERARCHICAL FLOW MODELLING
APPLIED TO THE FRACTURE SYSTEM IN HORNELEN, WESTERN NORWAY (IBM/UiB) 197
A5.1 Resolution And Fracture Length
A5.2 A Fracture Aperture Versus Trace Length Model8
A5.3 Discrete Fracture Flow Modelling
The institutions primarily responsible for each section are indicated by the following abréviations:
FAG Fault Analysis Group
BRGM Bureau de Recherches Géologique et Mineres
UiB Universitet i Bergen
GEUS Geological Survey of Denmark & Greenland (formerlyDGU)
NH Norsk Hydro
BD Enterprise Oil
IV PREFACE
Enterprise Oil Ltd and Norsk Hydro A/S joined this project in 1993 as industrial partners. The
project aims concerned problems which we were facing when producing from high porosity, low
permeability fractured chalk reservoirs and other fractured limestone reservoirs. The aims, goals
and methods put forward in the proposal and later outlined in interim reports, could give very
valuable input to how we build geological models, and how we produce and manage these
complicated reservoirs in which the understanding of 3-D fracture interconnectivity and spatial
distribution is the key to a successful field development.
The dual porosity/permeability system poses two main problems: the siting of production wells
and of injection wells. In this type of reservoir the producers must intersect several of the major
fractures to secure acceptable production rates from the rock volume. Injection wells, if wrongly
placed can result in rapid advance of injected water towards producers, isolating potentially
recoverable hydrocarbons in the fracture-bounded rock matrix. These hydrocarbons are then
almost impossible to recover.
To be able to model/simulate the subsurface fracture systems, there is a need for a better
understanding of the development and spatial distribution of fractures. How these vary with
respect to lithology, bed thickness and tectonic setting (doming, extension or compression) can
only be understood by examination of field analogues. This approach has been used in this
project, where five areas have been studied to try to unravel the significant relationships.
Based on the above considerations, the project has addressed a series of problems and made
good progress in trying to solve them. The results and methods, as presented in this interim guide
are relevant to the industry as they will improve our geological models with respect to how we
model/simulate the spatial distribution of fractures and how we "parameterize" the fracture
properties.
This report outlines a method for modelling fractures in 2-D and 3-D from a 1-D dataset, and how
we can incorporate this in our flow simulation model. This method will assist in planning the
development of such reservoirs and in optimising the reservoir performance during production. 1 INTRODUCTION
This guide is intended to assist the non-specialist reservoir geoscientist or engineer to optimise
the application of limited data to characterisation of a fractured reservoir. A procedure for
simulating fluid flow in fractured reservoirs is outlined in the guide, and flow diagrams are
provided in each chapter in order to clarify the steps which are to be undertaken. A glossary of
terms is included at the end of the guide which defines the terms printed in bold in the main text.
The authors of each chapter are indicated on the contents pages and the guide was collated
and edited by the Liverpool Fault Analysis Group. Technical details of work on which this guide is
based are given in a separate volume of appendices which contain' supplementary information.
Fractured reservoirs are becoming of increased importance as increasing numbers of such
reservoirs are discovered and exploited. The behaviour of the reservoir depends on many factors
including the relative permeabilities of the fractures and of the matrix. Typically, the effects on
the production history of fractures intersected by the production well are instantaneous, but
this reserve of hydrocarbon is quickly depleted. At this point the hydrocarbon may be drawn
from the matrix at flow rates limited by the matrix permeability. The matrix hydrocarbon may
be voluminous and therefore represent the bulk of the storage capacity of the reservoir. In
order to produce a fractured reservoir efficiently it is necessary carefully to control the
production rates and so encourage the transfer of hydrocarbons from the rock matrix to the
fracture system and on into the production well. Therefore, accurate reservoir simulations are
required. A review of the literature on fluid flow in fractured rocks is given in Appendix A3.
Fractured reservoirs represent a major challenge to reservoir geologists. Firstly, the fracturing
is frequently complex and difficult to predict and may occur on a variety of scales. Secondly,
once fractures are defined, they are usually too numerous to be incorporated individually into a
reservoir simulation, and so indirect means have to be devised for bringing this geological
information into the simulation.
The general method which we are suggesting is outlined in the flow diagram (Fig. 1.1) and in the
flow diagrams given at the beginning of each chapter which indicate the break-down into sub-
tasks. Conventional reservoir simulators cannot model large numbers of fractures explicitly
and so the discrete fracture flow model is used to provide values for the permeabilities of
the matrix plus fractures for input to the reservoir simulator. Well fracture data and knowledge
of fractures in outcrop are combined in order to produce a fracture model. This fracture model
is then used as input to an upscaling flow simulation in which the fractures are explicitly defined.
This discrete fracture flow model is used to produce upscaled permeability values. The results
given are for single phase flow only and their application to multiphase flow is uncertain.
The fractures which are dealt with here are opening-mode fractures, such as joints and veins:
faults and other shear fractures are not considered. Well fracture data are the most
important direct source of information about reservoir fractures (Chapter 2). However well data
are not sufficient to define the 3-D fracture geometry and so cannot be used alone to predict
features important for flow, such as the connectivity and the size of unfractured blocks. It is
proposed, therefore, that characteristic end-member fracture geometries defined from
outcrops should be used as an additional source of information. Thus, well data can be used to
choose between various fracture templates, which then provide the required information
about the sizes, spatial distributions and connectivities of the fractures. Because of the
importance of outcrop data, large fracture databases have been collected and are presented in
Appendix 1. It has been found that individual fractures tend either to be confined to individual
strata, in which case the spacing between thes is a simple function of the thickness of
the layer or, alternatively, that in the absence of layering they are vertically unconstrained. In
this second case the fractures typically occur on a wide range of scales and have fractal
properties. This scaling behaviour causes problems in creating fracture models and flow
simulations because the fractures are not characterised by a single scale.
As every fracture in a reservoir cannot be known explicitly, it is necessary to create a
geometricale model (Chapter 3 & Appendix A2). This model is created by combining well
data with fracture templates, and the model is conditioned to the wells so that the model
fractures and the known fractures coincide. The resulting fracture model is then used as input
to the discrete fracture flow model, which simulates flow in the modelled fractures FLOW MODELLING IN FRACTURED RESERVOIRS
Outcrop Well Fracture
Templates Data
(1-D) (2) (2-D) (2)
Fracture Modelling (3)
Discrete Fracture Flow
Row Modelling (4) Templates
t
Permeabilities
J
Fteservoir Simulation (5)
( Calibration J
Good
Well Tests
Comparison?
T
End
Figure 1.1 Organogram showing overview of procedure for flow modelling in
fractured reservoirs. Numbers refer to chapter numbers of this volume.

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