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Mike Spearing, Tim Allen and Gavin McAulay (AEA Technology)

NET PAY - WHAT IS IT AND WHY IS IT IMPORTANT?

Net pay is the thickness of the porous, permeable interval containing commercial hydrocarbon saturations. The thickness of net pay compared to the total pay zone thickness is known asthe “net-to-gross” ratio. This is an important factor in hydrocarbon volumetric calculations of reserves, and the design of facilities. Net pay definition is particularly difficult in low permeability gas reservoirs, since sands which are typically excluded as being “tight” may, in fact, contribute significantly to gas movement.

• With no net pay cut-off, gas reserves may be over estimated. Thismight mean

daily contract quantities are not met, with the consequent implication of financial penalties.

• With a net pay cut-off that is too high, gas reserves would be under estimated.

This could lead to the production facilities being designed for too short a lifetime, with possible consequent loss in gas reserves.

Given the importance of net pay definition, it is stillsurprisingly arbitrary being normally defined by a single porosity or permeability value. Rarely is information found in the literature about the basis of net pay cut-off. Dake (1994) made an apt statement on the subject: “Application of petrophysical cut-offs is a somewhat arbitrary exercise and is highly subjective in that the practitioner in his office makes decisions about fluid movement in reservoirswithout the slightest resort to any quantitative technique.” Laurie Dake, in The Practice of Reservoir Engineering, 1994.

THE WINLAND R35 METHOD

Dale Winland of Amoco developed the following empirical equation: log R35 = 0.732 + 0.588 log Kair – 0.864 log ϕ core [1]

where R35 is the pore aperture radius corresponding to the 35th percentile of mercury saturation in a mercury porosimetry test,Kair is the uncorrected air permeability (in mD), and ϕ is porosity (in %). The equation was originally defined from mercury porosimetry measurements on some 300 samples from the Spindle Field in Colorado. Winland correlated porosity and permeability to pore throat radii corresponding to different mercury saturations

and found that the 35th percentile gave the best correlation.. The 35thpercentile was taken to approximate the modal class of pore throat size where the pore network becomes interconnected forming a continuous fluid path through the sample. More accurately, the above is only true at the pore throat size corresponding to the point of inflexion of the pore throat size versus mercury saturation plot (Katz, 1986). The R35 method can used as a tool to assign flow units, andalso as a net pay cut-off to exclude very low porosity-permeability, using a slightly more scientific approach than simply selecting a certain porosity, permeability and/or gamma ray/Vshale cut-off. Winland used an R35 value of 0.5µm as the definition of net pay for the Spindle Field due to evidence he had seen of dry wells having an R35 of 0.5µm. The value of 0.5µm has since been used in otherreservoirs to define pay.

OBJECTIVES OF THE STUDY

This method of net-pay definition is not widely used and the UK Department of Trade and Industry (DTI) were interested in assessing its value for wider use in defining net pay for low permeability gas sands. The study set out to answer two key issues: 1. Pittman (1992) suggested there may not be a modal pore throat size in low permeability samplesand hence no point of inflexion on the semilog plot illustrated in Figure 1. Therefore the R35 method may be inappropriate for low permeability samples. 2. R35 values are calculated from a correlation rather than measured on the particular rock type in question. We consider the correlation could change markedly for different rock types, therefore giving different cut-off points.

EXPERIMENTAL...