Abstract

Using a standard logging suite (Фn, ρb, and AIT90) plus the Smoker Equation to calculate the total organic carbon (TOC) the following was calculated in a Devonian Woodford gas-bearing shale in Oklahoma: 1.) Total porosity (Фtotal), 2.) Volume of Clay (Vcl), 3.) Volume of Quartz (Vqtz), and 4.) Volume of Kerogen (Vke). Next effective porosity (Фe), effective water saturation (Swe), and gas-filled porosity (Ф gas) were calculated:

Фe* = Фtotal – CBW

CBW = Vcl*Фclay

Swe = (Ro/RAIT90)^0.5

Ro = (1/Фtotal^2)*Rw

Фgas = Фe*(1-Swe)

The above data along with a calculated gas formation volume factor (Bg) and a adsorbed gas content (gc in scf/ton) determined from the TOC content were used to calculate OGIPscf/sec with a permeability cut-off of greater than 100nD.

Adsorbed Gas = 8.2BCF/sec

Free Gas = 11.5BCF/sec

Total Gas = 19.7BCF/sec

The effective porosity used in the above OGIPscf includes both organoprosity (Ф om) and mineral matrix (Фmm) porosity. What would the OGIPscf be if the only effective porosity is the organoporosity (Ф om).

Фe* = (Vke*OM) OM = 0.30 (porosity within the kerogen)

Фgas = Фe*(1-Swe) Swe = 0.0 [Фom is hydrophobic]

Adsorbed Gas = 7.9BCF/sec

Free Gas = 8.4BCF/sec

Total Gas = 16.3BCF/sec

So which is correct?

The Maxwell-Garnett Equation which was designed model large spherical pores in bedded in finer spherical matrix pores (i.e. a vuggy carbonate) may provide an answer. The fragments of kerogen containing the organoporosity (Фom) have no water (i.e. Swe = 0.0), and therefore the kerogen fragments act much like a vug that contains very little water (Swe = 0.0). The resistivity measured by the electromagnetic wave can only travel through the porosity that contains water (Swe > 0.0) which includes CBW and Ф mm. The CBW and Фmm are similar to the matrix porosity in a vuggy reservoir. In order to apply the Maxwell-Garnett Equation we need the following:

Фmm = Фe* – Фom

Ф = Фmm + CBW

[water-bearing porosities: Sw > 0.0]

Sw = [(1/Ф^2)*(Rw/RAIT90)]^0.5

Фom

Sw = 0.0 [i.e. the kerogen is an insulator]

MAXWELL – GARNETT EQUATION

Ct = Cm {1+{2V[(Cv-Cm)/(Cv+2Cm)]}/1-{V

[Cv-Cm)/(Cv+2Cm)]}}

Rt = 1/Ct

Where:

Ct = total conductivity

Rt = total resistivity

Cm = matrix conductivity

[I.e. CBW + Фmm]

Cm = [Ф^2 * Sw^2]/Rw

Ф = CBW + Фmm

Sw = [(1/Ф^2)*(Rw/RAIT90)]^0.5

Cv = vug conductivity

Cv = Sw(vug)/Rw

Sw(vug) = 0.0

Cv = 0.0

V = percent kerogen

[percent vugs both are insulators]

When these data are input into the Maxwell-Garnett Equation, and a resistivity is calculated (Rcalc) there is a very close agreement with the measured resistivity (RAIT90). However, if it is assumed that the mineral matrix porosity (Фmm) is full of water (Sw = 1.0), there is very poor agreement within some zones with the measured resistivity (RAIT90).

The Woodford example presented here would suggest that within some zones both organoporosity (Фom) and mineral matrix (Фmm) porosity are gas-bearing. However, of the two porosities the organoporosity (Фom) is the most important.

Фe* - the effective porosities have been corrected for adsorbed gas in Фom.