Total Rock Characterisation Of North Sea Sandstones With Particular Reference To Interstitial Clays
Authors: Todd Adrian C. , Tweedie John , English Brian 
Published: December 2007

Abstract
The need for a more detailed understanding of the nature and behaviour of hydrocarbon bearing rock material is becoming increasingly important with regard to the design of development plans for primary recovery, waterflooding and enhanced recovery systems.
The significance of the microscopic properties of sandstone rocks, for example composition and pore size distribution, is discussed with reference to those bulk properties which are normally measured.
A total core characterisation procedure is outlined and the analytical techniques involved are briefly discussed. The philosophy of the total characterisation approach is used in an examination of a series of sandstone cores from a North Sea well. Particular attention is given to the role clays are likely to play during a water flood, and the analysis is directed towards this application. Kaolinite, the predominant clay mineral component, is shown to contribute significantly to rock microporosity. Permeability contrasts between cores analysed can be explained by reference to the structure of kaolinite crystal aggregates and their location within pores and pore throats.

1. INTRODUCTION
Oil and gas development programmes require substantial technological and financial resources, and must proceed despite significant geological, technological and economic uncertainty. Furthermore, when the discovery is offshore, opportunities to make major changes in development plans become strictly limited, once production platforms have been installed. Data gathering and interpretation playa vital role in minimising risk at all stages.
The habitat of hydrocarbons is complex and knowledge of reservoir geology is generally incomplete, with the result that. the most significant source of uncertainty is typically in the reservoir description itself. This reservoir description forms the basis for planning, including: platform location and design, location and completion of drainage and injection wells, mathematical models of the reservoir, enhanced oil recovery programmes, pressure depletion policy and reservoir unitisation.
This paper describes a procedure for rock characterisation, designed to optimise data collection from core material for the purpose of improving the quality of reservoir rock description. The method is applied to the specific problems of waterflooding, and in this regard the role of interstitial clays is assessed.
The work has been conducted as part of a continuing programme of research into the problems of well and reservoir damage which result from drilling, completion and injection practises.

2. CHARACTERISATION TOTAL OF RESERVOIR ROCK SYSTEMS
The value of extensive coring during the development of an onshore field has recently been emphasised by Shirer et al.(1) The data presented included standard core analysis plus special compressibility and capillary pressure studies. The greater the knowledge of the core material itself, the better will be the understanding of the reservoir and the more accurate will be the prediction of reservoir performance.
Reservoir rock systems can be characterised by the consideration of three distinct attributes: microscopic, bulk and behavioural properties. In Table I, under these three headings, are listed examples of properties which relate to reservoir systems.