Reservoir compartmentalization: an introduction

Abstract

Reservoir Compartmentalization – the segregation of a petroleum accumulation into a number of individual fluid/pressure compartments – occurs when flow is prevented across ‘sealed’ boundaries in the reservoir. These boundaries are caused by a variety of geological and fluid dynamic factors, but there are two basic types: ‘static seals’ that are completely sealed and capable of withholding (trapping) petroleum columns over geological time; and ‘dynamic seals’ that are low to very low permeability flow baffles that reduce petroleum crossflow to infinitesimally slow rates. The latter allow fluids and pressures to equilibrate across a boundary over geological time-scales, but act as seals over production time-scales, because they prevent crossflow at normal production rates – such that fluid contacts, saturations and pressures progressively segregate into ‘dynamic’ compartments. Thus, reservoir compartmentalization impacts the volume of moveable (produceable) oil or gas that might be connected to any given well drilled in a field, which restricts the volume of reserves that can be ‘booked’ for that field. Booking of reserves is tightly regulated by government authorities because it is a key measure used by stock analysts and investors to value an oil company. This places reservoir compartmentalization studies, and the predictive science and technology applied to them, at the heart of company valuation. Unexpected compartmentalization can also seriously impact the profitability of a field: with more data acquisition, more study, more wells, more time being required to produce less oil and gas than was originally anticipated. In extreme cases, this might even lead to early field abandonment. However, unexpected or misunderstood reservoir compartmentalization has been an industrywide experience for over 30 years, and it is clear that there is great value in learning from past, often expensive mistakes (e.g. Smith 2008). This learning process has often driven developments in geoscience, engineering and related technology – enabling operating companies to identify and predict ‘new’ untapped volumes in old fields, make general improvements to field management (e.g. Gainski et al.; Wonham et al.), and apply this knowledge to other similar, but lessmature fields in their portfolio. Reservoir compartmentalization is therefore a major uncertainty that should be accurately assessed during the appraisal of petroleum reservoirs, in order to avoid unexpected compartmentalization at the production stage. In the past few decades, there have been major advances in data, detection and surveillance methods, and our geoscience and reservoir engineering approaches. However, reservoir compartmentalization can still be underestimated during field appraisal, and can still give surprises that force a re-think of the field development and production plan (e.g. Smith 2008). This is not necessarily a defect in the science or technology – but, rather, it may also be a result of ineffectual data appraisal or discipline integration within subsurface workflows. Thus, it is not unusual for subsurface teams to place too much emphasis on one aspect of the evidence, or to make early assumptions that bias data acquisition, analyses and interpretations later on. For example, Fisher & Jolley (2007) point out that non-existent faults or improbable fault seal capacities might be invoked to explain variable petroleum contacts and dynamic fluid/pressure behaviour – when an alternative or combination of factors, or other unrelated explanations might be more appropriate for the data (e.g. hydrodynamic tilting of petroleum contacts, Tozer & Borthwick). Research initiatives and collaborations between oil companies, service groups and academic institutions vary considerably in scope and content, and there is often a creative tension between the competitive advantages, motivations and knowledge gaps perceived by the sponsoring companies and their research partners. In addition to this, the scientific insights that can be gained from the