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Abstract

This paper describes novel methods for fracture description and implications by using well logs, core data, well tests and geological interpretation.

The type, amount and orientation of fractures and fracture systems have major impact on field behaviour. We present methods to estimate in situ stress and fractures from logs. Fracture description from cores further enhance the proposed description of the fracture system. The paper also show how well test data give quantitative informations about fracture conductivity initially and during field depletion.

If fracture conductivity exceeds a fracture intensity threshold, separate matrix blocks occur. We identified such systems by a new method which identify individual capillary pressure systems and thereby individual matrix blocks. We also used the method to identify in situ wettability.

The methods presented in this paper we successfully used for some North Sea Chalk fields.

Introduction

The southwestern part of the Norwegian continental shelf, called the Ekofisk Area, contains eleven major chalk fields. The Ekofisk field is the first and main discovery, discovered in 1969 and put on production in 1972.

The productive chalk formations are Hod, Tor and Ekofisk formations of Maatrichtian and Danian age respectively. The formations consist of pelagic and resedimented chalks. Different size of the coccolites, the building block of chalk, give different relationship between porosity and permeability for Tor, Hod and Ekofisk formations.

The traps are salt-induced structures which are medium to highly fractured. This is a result of postdiapiric salt movement, faulting and differential compaction. The fracture system has major influence on the initial state of the field and field production. Wells have penetrated multiple hydrocarbon water contacts.

To enhance field drainage and productivity we need knowledge of the local fault and fracture system. Studies have been undertaken to improve our knowledge. We have studied fracture type, fracture distribution, intensity, fracture and fault orientation, in-situ stress and capillary pressure systems.

FLUID DISTRIBUTION

Gravity separates immisible fluids of different densities. The continuity of the fluids makes individual fluid pressures and pressure differential between the phases. At equilibrium, pressure and capillary force balance. Mathematically, we have:

P = p g h (1)

In this work we used the Brooks and Corey capillary pressure function to describe the capillary force.

Pc = Pe Sw * (2)

P. 371^

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