Approaches for Estimating Carbonate Fractured Reservoir Property Cutoffs

Kolganov, V. (JSC Giprovostokneft) | Kovaleva, Galina Anatolievna (JSC Giprovostokneft)

OnePetro 

In Russia, since 1972, core correlation relationship between connate water saturation awc and porosity awc=f(m) is used for porosity cutoff estimation based on methods which have been generated to find property cutoffs:

1)     method of effective porosity me=m(1-awc)

2)     method of dynamic porosity md=m(1-awc-aor), where awc and aor-connate water and residual oil saturation

Estimation method of property cutoffs is as follows: plot petrophysical correlation relationship me=f(m) and md=f(m), which are usual as line functions with high correlation coefficient. Find graphically or analytically cross points of porosity axis and functions me=f(m) and md =f(m). Sections on this axis intercepted with points me=0 and md=0 are accepted numerically as porosity cutoffs.

Judging by foreign publications, correlation relationships me=f(m) and md=f(m) are used only in domestic practice of estimating hydrocarbon reserves and thus this method can be considered as Russian know-how.

Oil reserves estimated by the method me are in-place reserves, by the method md - producible reserves. Both options of correlation methods estimate matrix cutoffs for carbonate fractured reservoirs.

 

 

Any estimation of hydrocarbons in-place after initial acquisition and analysis of required input data begins from substantiation and acceptance of producer lower or cutoff reservoir properties allowing making boundary between reservoir-nonreservoir of oil-and-gas rocks.

Mostly the same procedural concepts, methods and techniques are used to estimate reservoir properties cutoff of various reservoirs (clean and clayey sandstone, fractures and non-fractured carbonates). However, currently we are realizing that more stringent requirements are raised for carbonate fractured reservoirs (CFR) with respect to information support of cutoffs estimation procedure. While for sandstone reservoirs it is sufficient to measure core permeability with accuracy 1-10 mD to set boundary between reservoir-nonreservoir for CFR, as practice shows, permeability shall be measured with accuracy up to 0.001-0.0001 mD [1]. Boundary between reservoir-nonreservoir problem for CFR has distinctive features; and it seems reasonable to isolate its study in the individual research trend. Especially with the development of mathematical simulation of fractured reservoirs the aim of estimating oil reserves in matrix and fractures and their interference becomes more vital.

Both regional and global practice often uses "core-core?? correlation between porosity m and permeability k: m=f(k). Using this relation one randomly selects certain k as cutoff value (usually 1-10 mD) and then from m=f(k) find cutoff of porosity and other parameters.

Alongside with the said, Russian specialists use correlation method to define reservoir property cutoffs, which is free from voluntarism of method m=f(k).

In 1972 Kotyakhov F. proposed to use a "core-core?? correlation of connate water saturation awc versus porosity awc=f(m) and permeability awc=f(k) for porosity cutoff estimation. While developing this idea in 1972 Yatsenko G. and Ruchkin V. [2] converted awc=f(m) correlation into two versions of relationship between effective porosity and porosity m. Later on this proposal [2] was included into recommended practice [3] approved by the State Reserves Committee Rosnedra (SRC) specifying to use petrophysical correlation of effective porosity me  and dynamic porosity md versus porosity to substantiate reservoir/nonreservoir rock quantitative criteria. Effective porosity is defined from function me=m(1-awc) and dynamic one - from  function md=m(1- awc- aor), where awc and aor - connate water and residual oil saturation.

Judging by foreign publications, correlations me=f(m) and md=f(m) are used only in domestic practice of estimating hydrocarbon reserves and thus this method can be considered as Russian know-how.