A Specific Way of Assessing Target Recovery Factor (Chalk Oil Fields)
Authors: Horikx J. , Timmermans P. , Daudin R. , O'Neill N. , Sorriaux F. 
Published: March 2015

An important aspect of oil field management is to compare a field's recovery performance with analogues to assess whether the field is being fully exploited and economic recovery is being maximized. However, this can be difficult when there are many differences in basic parameters that might impact recovery potential. A number of qualitative methods have previously been developed but they tend to be too generic to allow meaningful recovery potential analysis.

The objective of this study has been to develop a tool to assess and understand the recovery performance of offshore Danish fields in relation to world analogues. This can be done meaningfully in a semi-quantitative fashion by limiting the analysis to chalks and by calibrating, where practical, the impact of key parameters with reservoir simulation analysis. The main steps in the study were to establish the principal geological and dynamic drivers determining recovery factors in chalks; build a "calibrated" recovery factor potential model and apply it to worldwide chalk fields, using published data.

The results show that the Danish chalk fields become progressively more complex going from the Tor formations, which have been successfully waterflooded; to the Ekofisk, where there remains technical potential for implementing waterflooding; to the Lower Cretaceous where waterflooding recoveries are theoretically comparable to depletion but due to complexity and expense are unlikely to be attractive. A number of reservoir management activities have been identified to increase recoveries. Overall the Danish field recoveries compare favourably with world analogues. Operators in the Danish sector are likely leaders in exploiting more complex and particularly lower permeability chalk fields.

The Recovery Factor Potential tool developed in this study allows chalk field performance to be compared in a more quantitative manner, making it easier to understand causes of underperformance and to identify reservoir management activities to increase recovery.

Introduction

An important aspect of oil field management is to compare a field's recovery performance with analogues to assess whether the field is being fully exploited and economic recovery is being maximized. However, this can be difficult when there are many differences in basic parameters that might impact recovery potential. To tackle this problem a study has been conducted jointly by Maersk Oil and Foroil aimed at developing correlations that enable comparisons of oil recovery performance of chalk fields around the world.

A number of approaches have been taken in the industry to develop general correlations that can assess a field's recovery against its "complexity index". The complexity index has generally been based on combining a number of key factors (such as structural complexity, reservoir heterogeneity, STOIIP concentration, permeability etc.). Typically each factor has a range of choices (say 1 to 5) of increasing complexity and each factor is given an appropriate weighting. Such tools are useful in giving a qualitative impression of whether the field is being well managed and can highlight the factors that make a field complex so that these complexities can be managed.

This study has focused on chalk reservoirs and as a consequence it has been possible to create a more quantitative model using reservoir modelling and considering chalk-specific factors, such as reservoir compaction, to assess the impact of a number of the important factors. As a consequence the tool has been constituted as a "Recovery Factor Potential" (RFP) tool.

In addition, having established the RFP there has been a focus on investigating the deficit between the actual field projected recovery and its RFP to assess whether any remedial activity can be taken. This has been done for both world chalk analogues and the DUC (Dansk Undergrunds Consortium) Danish fields.