Abstract
Methodologies have been developed for applying net reservoir and thence net pay cut-offs in cases of primary and waterflood depletion. The cut-offs 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 non-reservoir 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 cut-off is expressed as a limiting porosity. For waterflood depletion, an extrapolated end-point relative permeability 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 cut-off is again expressed in terms of porosity. Consequently, in both cases, the discrete porosity cut-offs have a dynamic significance. Other cut-off parameters, such as shale volume fraction and water saturation, can be tied back to these. A proposed workflow for applying dynamically-conditioned cut-offs 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. The treatment is not exclusive, and other approaches to the determination of net pay will emerge as more reservoirs are considered.
