Oil and Gas Recovery Behaviour in the UKCS Basins

Abstract

Abstract Company management, industry analysts and governments like to have measures of portfolio or national resource performance. Recovery factors are one such means of assessing relative performance. The UK DTI's large database has been employed to assess the distribution of recovery factors with basin parameters. Assessed over time, ultimate recovery factors indicate how expectations have changed for the UK North Sea fields and how new technology and cost reduction have broadly influenced exploitation. Recovery factors also seem to relate to geological facies. Forward prediction of overall growth in recovery factors is of concern for gauging the security of local and world supply. Of course there are caveats or limitations and these are discussed. Introduction "What's the recovery factor" used to be the regular challenge to junior company staff when they presented a new field for development to senior management. In those days this simple volume measure was widely used to rank the technical status of the project before computing permitted the modern variegated display of more sophisticated measures. It is fully appreciated that every hydrocarbon accumulation is unique (1) requiring its own digital models of some degree of sophistication. Nevertheless for those concerned to control a portfolio of fields or husband a nation's resources, the recovery factor remains an important yardstick. A field with a low recovery factor attracts regulatory attention and enquiry regarding further extraction - if not by the incumbent operator then perhaps by a new interested party with fresh ideas. Applicable average recovery factors are also needed to gauge likely future ‘reserves’ in yet to find hydrocarbon resources as part of national planning, and to justify exploration budgets. In this paper we review recovery factors for offshore oil and gas fields on the UK Continental Shelf (UKCS). The UKCS has extremely varied geology and consequently a very wide range of hydrocarbon traps. Strangely this is then simplified by a very dominant source rock for oil (Jurassic Kimmeridge shale) so that the common approach of relating recovery factor to oil specific gravity gives no useful correlation. We look therefore to relationships with reservoir type, geological horizon and, perhaps the less sophisticated parameters, field size and development history. For those seeking correlations it is fortunate that the UK has not imposed a deliberate depletion policy but has instead allowed price and demand to control output(2). We can therefore check for the influence of historic technology changes on recovery factor over some 35 and 25 years of respective gas and oil production. Recovery Factor as a Concept Recovery factor is a term properly applied to a single hydrocarbon reservoir only. A few significant UKCS fields have more than one effective reservoir and even quite different hydrocarbons types in each. The North Alwyn Field (3) is the best example with a conventional Brent sand oil reservoir overlying a Statfjord sand and deeper still a Triassic reservoir both holding gas condensate. Ascribing a single recovery factor to such a field is obviously incorrect and this error was avoided. The majority of fields have one or one dominant reservoir with one hydrocarbon type, and thus the field recovery factor is an appropriate measure of performance. Gas reservoirs usually have much higher recovery factors than oil reservoirs. This is especially true in the absence of aquifer drive. Much of the gas is produced by depressurisation and this process results in thorough production as every part of the producing rock formation will equalise pressure in due course. Whereas oil recovery factors rarely exceed 75%, it is quite common for gas fields to exceed 85%. This difference makes review of recovery from a basin in oil equivalent terms a non-starter. Recovery Factors - the UK DTI Approach Every year the DTI reviews with operating companies the reserves status for each UK field as at 31st December of the year preceding and this paper is distilled from the database. Fields that straddle the median lines between the UK and Norway or the Netherlands have been excluded from this paper together with on-land fields.