The Role of Cut-offs in Integrated Reservoir Studies

Abstract

Abstract There have been many different approaches to quantifying cut-offs, with no single method emerging as the definitive basis for delineating net pay. Yet, each of these approaches yields a different reservoir model, so it is imperative that cut-offs be fit-for-purpose, i.e. they are compatible with the reservoir mechanism and with a systematic methodology for the evaluation of hydrocarbons in place and for the estimation of reserves. These different requirements are accommodated by basing the quantification of cut-offs on reservoir-specific criteria that govern the storage and flow of hydrocarbons. In so doing, particular attention is paid to the relationships between the identification of cut-offs and key elements of the contemporary systemic practice of integrated reservoir studies. The outcome is a structured approach to the use of cut-offs in the estimation of reserves. The principal benefits of a properly conditioned set of petrophysical cut-offs are a more exact characterization of the reservoir with a better synergy between the static and dynamic reservoir models, so that an energy company can more fully realize the asset value. Introduction In a literal sense, cut-offs are simply limiting values. In the context of integrated reservoir studies they become limiting values of formation parameters. Their purpose is to eliminate those rock volumes that do not contribute significantly to the reservoir evaluation product. They have typically been specified in terms of the physical character of a reservoir. If used properly, cut-offs allow the best possible description and characterization of a reservoir as a basis for simulation. Yet, although physical cut-offs have been used for over 50 years, there is still no rationalized procedure for identifying and applying them. The situation is compounded by the diverse approaches to reservoir evaluation that have been taken over that period, so that even the role of cut-offs has been unclear. These matters assume an even greater poignancy in contemporary integrated reservoir studies, which are systemic rather than parallel or sequential in nature, so that all components of the evaluation process are interlinked and therefore the execution of any one of these tasks has ramifications for the others (Fig. 1). The principal use of cut-offs is to delineate net pay, which can be broadly described as the summation of those depth intervals through which hydrocarbons are (economically) producible. In the context of integrated reservoir studies, net pay has an important role to play both directly and through a net-to-gross pay ratio. Net pay demarcates those intervals that are the focus of the reservoir study. It defines an effective flow thickness that is pertinent to the identification of flow units, that identifies target intervals for well completions and stimulation programs, and that is needed to estimate permeability through the analysis of well test data. The net-to-gross pay ratio is input directly to volumetric computations of hydrocarbons in place and thence to "static" estimates of reserves, it is a key indicator of hydrocarbon connectivity, and it contributes to the initializing of a reservoir simulator and thence to "dynamic" estimates of reserves. Unfortunately, there is no universal definition of net pay nor is there general agreement on how it should be delineated. For this reason, net pay has been incorporated within integrated reservoir studies in many different ways that have not always been fit for purpose. In particular, there is no generally accepted method for quantifying net pay cut-offs, without which net pay cannot be delineated. In an attempt to redress some of these shortcomings, this paper is directed at building a systematic foundation for the definition and role of cut-offs in integrated reservoir studies. It tracks the origins of physical cut-offs from both geoscience and engineering perspectives in both Western and Eastern hemispheres. It outlines what they are and why we need them, describes how they should be quantified, and proposes a structured method for incorporating them within integrated reservoir studies for the evaluation of hydrocarbons-in-place and the estimation of reserves. The starting point is some basic terminology.