The Role of Cut-offs in Integrated Reservoir Studies

Summary There have been many different approaches to quantifying cutoffs, 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 cutoffs 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 the estimation of ultimate hydrocarbon recovery).These different requirements are accommodated by basing the quantification of cutoffs 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 cutoffs and key elements of the contemporary systemic practice of integrated reservoir studies. The outcome is a structured approach to the use of cutoffs in the estimation of ultimate hydrocarbon recovery. The principal benefits of a properly conditioned set of petrophysical cutoffs 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, cutoffs 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. Typically, they have been specified in terms of the physical character of a reservoir. If used properly, cutoffs allow the best possible description and characterization of a reservoir as a basis for simulation. Yet, although physical cutoffs have been used for more than 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 cutoffs 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 anyone of these tasks has ramifications for the others (Fig. 1). A particular aspect of the systemic approach is the provision for iteration as the reservoir knowledge-base advances. For example, simulation may be used in development studies to identify the most appropriate reservoir-depletion mechanism, but, once the development plan has been formulated, the dynamic model is retuned and progressively updated as development gets under way. The principal use of cutoffs is to delineate net pay, which can be described broadly 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 around a well that are the focus of the reservoir study. It defines an effective thickness that is pertinent to the identification of hydrocarbon 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 ultimate hydrocarbon recovery; it is a key indicator of hydrocarbon connectivity, and it contributes to the initializing of a reservoir simulator and thence to "dynamic" estimates of ultimate hydrocarbon recovery.