Net-Gros

Páginas: 9 (2006 palabras) Publicado: 25 de febrero de 2013
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 bedelineated.
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 geosciences and engineering perspectives in both Western and Eastern hemispheres. It outlines what they are and why we need them, describes how theyshould 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.
BASIC TERMINOLOGY
Definitions. Although there is no universal set of definitions of those terms that describe the ability of a rock to store and transmit fluids,there are seven fundamental descriptive terms that are in fairly widespread use. They are grounded in the volumetric analysis of siliciclastics using core and log data, and they may or may not be based on a tie-back to permeability. They all define thickness ratios and they are interrelated. Fig. 2
Gross rock comprises all rocks within the evaluation interval.
Net sand comprises those rocks thatmight have useful reservoir properties. The word sand is a generic oilfield term that historically equates to litho logically clean sedimentary rock. Net sand is usually defined as the summation of those intervals for which the sand content is greater than or equal to a limiting value. This criterion is usually expressed in terms of a shale volume fraction Vsh being less than a limiting value (theshale cut-off). The term shale includes clays and silts (size indicators), clay minerals (compositional indicator, mostly within the clay fraction), and other detritus, usually of a poorly sorted nature. The parameter Vsh is log-derived: it cannot be measured directly in the laboratory.
Net reservoir comprises those net sand intervals that do have useful reservoir properties. This condition isusually expressed in terms of the log-derived fractional porosity (Φ) being greater than or equal to a limiting value (the porosity cut-off). Porosity can be measured down hole and in the laboratory. It is often tied back to core permeability so that the net-reservoir criterion effectively becomes one of any porous and permeable rock that is capable of storing and transmitting hydrocarbons.
Net paycomprises those net reservoir intervals that do contain hydrocarbons. This requirement has been reduced to the log-derived, fractional hydrocarbon saturation Sh being greater than or equal to a limiting value. This condition is tantamount to stating that the water saturation Sw (=1-Sh) is less than a limiting value (the water saturation cut-off). This second option is more commonly used. Theparameter Sw can be measured down hole and also in the laboratory if native state core is available, but reliable core measurements of Sw remain comparatively rare, even though the technology has been around for at least two decades. Where net reservoir is tied back to permeability, net pay describes those net reservoir intervals that contain producible hydrocarbons. In a clear link to reserves, thedefinition of net pay has evolved into those hydrocarbon-bearing reservoir intervals that can be produced economically using a particular recovery method. The term net pay is therefore not merely a descriptor of rock type.
Net-to-gross is a generic term that encompasses three definitions, all derived from the above. Generally, it is the ratio of net thickness to gross thickness. Net-to-gross...
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