The Application of Cutoffs in Integrated Reservoir Studies
- Paul F. Worthington (Gaffney, Cline & Associates)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- December 2008
- Document Type
- Journal Paper
- 968 - 975
- 2008. Society of Petroleum Engineers
- 7.5.3 Professional Registration/Cetification, 5.5.3 Scaling Methods, 5.5 Reservoir Simulation, 5.6.1 Open hole/cased hole log analysis, 2.4.3 Sand/Solids Control, 4.3.4 Scale, 5.5.2 Core Analysis, 5.6.3 Deterministic Methods, 1.2.3 Rock properties, 1.6.9 Coring, Fishing, 5.4.1 Waterflooding, 5.1 Reservoir Characterisation, 4.1.2 Separation and Treating, 5.2 Reservoir Fluid Dynamics, 5.5.8 History Matching, 5.1.5 Geologic Modeling, 4.1.5 Processing Equipment, 5.2.1 Phase Behavior and PVT Measurements, 1.8 Formation Damage
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Methodologies have been developed for applying net reservoir and thence net pay cutoffs in cases of primary and waterflood depletion. The cutoffs are dynamically-conditioned to be reservoir-specific (i.e., they are tied back to a reference permeability parameter in a way that is driven by the reservoir data themselves). They also honor scale, where feasible, and are conformable with any pertinent rock-typing. For primary depletion, the Leverett equivalent circular pore diameter has been used to distinguish between reservoir and nonreservoir rock. This composite parameter can incorporate mobility for multiphase work. It is linked to a discrete core porosity under simulated reservoir conditions, so the adopted net reservoir cutoff is expressed as a limiting porosity. For waterflood depletion, an extrapolated endpoint relative permeability to oil has proved effective for the same purpose. Here, the zero endpoint relative permeability is linked to a conventional "absolute" core permeability corrected to reservoir conditions. Porosity is tied back to this limiting absolute permeability so that the net reservoir cutoff is again expressed in terms of porosity. Consequently, in both cases, the discrete porosity cutoffs have a dynamic significance. Other cutoff parameters, such as shale volume fraction and water saturation, can be tied back to these. A proposed workflow for applying dynamically-conditioned cutoffs allows data character to drive their use in integrated reservoir studies. The workflow constitutes a basis for greater technical consistency in the estimation of ultimate hydrocarbon recovery. However, the treatment is not exclusive, and other approaches to the determination of net pay will emerge as more reservoirs are considered.
The rationale for using net sand, net reservoir, and net pay cutoffs in integrated reservoir studies was outlined in an earlier paper, to which the reader is referred for background and an informative bibliography (Worthington and Cosentino 2005). However, that same paper also revealed a variety of interpretations of the concepts of net sand, net reservoir, and net pay, with at least five different classification schemes appearing in the technical literature. Those disparities manifest themselves in the associated cutoffs, a situation that is clearly unsatisfactory given that the identification of cutoffs is an important step towards the estimation of effective hydrocarbon volumes. The situation is not improved by reverting to the longstanding industry default (net-reservoir) cutoffs of 0.1 md for gas reservoirs and 1.0 md for oil reservoirs [e.g., Phillips and Liwanag (2006)]. Even if those historic cutoffs were universally viable, there has been a plethora of interpretations as to what they actually mean (e.g., as regards the nature of the permeating fluid(s), correction for any gas-slippage effects, and conversion to reservoir-stress conditions). Putting these matters together, there is an evident need for technically-sound systemic methods for quantifying and applying cutoffs in integrated reservoir studies. Moreover, the cutoffs should be dynamically conditioned, so that they reflect flow as well as storage criteria, and be fit for purpose, in that they are based on reservoir-specific data rather than being imported quantities.
This paper describes a practical approach to the application of dynamically-conditioned, fit-for-purpose cutoffs in integrated reservoir studies that are directed at the estimation of ultimate hydrocarbon recovery. The treatment is set within the context of two reservoir-depletion mechanisms, primary depletion and waterflood depletion. The aim has been to develop a methodology for quantifying and implementing cutoffs for each of these situations in a way that honors scale of measurement, where feasible, and conforms to rock typing criteria. The proposed methods have been adopted because they have already proved beneficial in field studies. However, they are not exclusive and there may be different reservoir situations where alternative approaches are more appropriate. That outcome will be determined by the reservoir data themselves, an appropriate prognosis because most formation-evaluation procedures are empirical.
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