Using 4000 ft Long Induced Fractures to Water Flood the Dan Field
Authors: Larsen Frank P. , Cowie D.R. , Ovens Jonathan V. 
Published: December 2007

Dan, a tight chalk field, was brought on production in 1972. The initial field development consisted of primary depletion, first undertaken through deviated, hydraulic fracture stimulated wells and later by employing horizontal wells with multiple hydraulic fractures.

A full field water flood is presently being implemented. To increase injectivity water is being injected above the fracture propagation pressure into fractures which have grown up to 4000 ft in length. To determine the viability of this process, a number of trials have been performed prior to initiating full field implementation.

Water is being injected in conventional deviated wells, with a single fracture, and horizontal injectors with multiple injection fractures. Management of the water flood is conducted by using the selective completions that are installed in horizontal producers and injectors. The completions allow isolation of individual fractures and control of the injection rate into individual fractures by means of downhole chokes. This paper discusses the water flood concept, the initial water injection trials undertaken, and the status of the full field water flood implementation.

Introduction

The Dan Field, located in the Danish part of the Central Graben in the North Sea (see Fig. 1), was discovered in 1971 and was brought on production in 1972 as the first oil field in the entire North Sea.

Initial development consisted of primary depletion through conventional deviated wells. During depletion, the dominant drive mechanisms were the expansion of the gas cap and solution gas drive. The aquifer underlying the field played a minor role due to the low permeability and relatively limited energy when compared to the gas cap.

The production wells were initially acid fracced. To increase the fractures' effective radius, production wells are now mainly provided with sand propped fractures.

By 1987, a total of 42 conventional production wells had been drilled in the Dan field. To enhance the primary recovery, in-fill drilling of horizontal production wells was initiated. To achieve a significant productivity improvement over sand propped fractured conventional wells, the horizontal wells were stimulated with multiple sand propped fractures. The higher productivity of horizontal wells also allowed the development of flank areas, which had previously been considered noncommercial due to limited pay thickness.

In the Dan field, horizontal wells with a reservoir section in excess of 15000 ft have been drilled (North Sea Record). A typical horizontal production well is provided with some 15 sand propped fractures, see Fig. 2. To allow control of the stimulation and the production from each fracture, cemented liners and selective completions are installed, see Fig. 3. Each zone is perforated, stimulated and isolated in a single trip. Access to each zone is provided by a sliding sleeve, operated by coiled tubing. All producers are provided with Side Pocket Mandrels for gas lift valve installation.

The primary development was completed by the end of 1994 when 22 conventional wells and 34 horizontal production wells were on production. The number of conventional producers had been reduced from 42 as a result of horizontal sidetracks and conversions for water injection trials. Following encouraging pilot results, a field wide water flood is being implemented incorporating propagating injection fractures of up to 4000 ft in length.

This paper presents the water flood concept and the water injection trials undertaken prior to initiating implementation of the full field water flood. This is followed by sections covering the status of the full field flood and the reservoir management operations used to control the impact of communication through induced fractures between injectors and producers. P. 583