Abstract

Unconventional gas-bearing shale has a two pore system made up of Organoporosity [ɸom] and Mineral Matrix Porosity [ɸmm]. The organoporosity is hydrophobic therefore [Sw = 0.0], however the mineral matrix porosity is water-wet [Sw>0.0]. Therefore in calculating OGIPscf it is necessary to separate the two pore types. Outlined below is one method for separating ɸom and ɸmm:

ɸe =ɸtotal - CBW

CBW = Vcl*ɸclay

[ɸclay = 0.10: Illite]

om = VkeɸOM

<ɸ>mm = ɸe - ɸom

Where:

ɸtotal = total porosity [simultaneous equations using ɸNls and RHOb logs]

ɸe = effective porosity

Vke = volume of kerogen from TOC

Vke = {[TOC(wt%)*RHOb*Kvr]/RHOkeogen}/100

OM = intra-kerogen porosity

The most critical parameter in the above method is the determination of the intra-kerogen porosity [OM]. The method that is commonly used is based on the relationship between vitrinite reflection [Ro] and the density of the kerogen.

RHOfinal = (0.32*Ro) + 0.98

[URTEC, 2015: Paper # 1921752]

Then:

OM = 1 – (RHOinitial/RHOfinal)

Assuming RHOinitial = 1.1g/cc

Woodford Shale Ro = 2.62 RHOfinal = 1.82g/cc

OM = 1 – (1.1/1.82) = 0.40

Using the above methods plus a TOC versus adsorbed gas (gc: scf/ton) chart (Hartman, 2012) the following OGIPscf was calculated:

ka > 100nD [ka = [(0.0108*ɸgas)-0.000256]*10^6]

Gs Adsorbed Gas [OGIPscf]/Area

Gs = 1,359.7*gc*RHOb*A*h*(/Bg) = 14BCF/sec

Gf Free Gas [OGIPscf]/Area

Gf = 43,560*ɸom*h*A*(1/Bg) = 19BCF/sec

Gf = 43,569*[ɸmm*(1-Sw)]*h*A*(1/Bg) = 2BCF/sec

However is there Quick Method for calculating OGIPscf with a limited logging suite? Thanks to Rick Lewis with SCHLUM-BERGER there is (Lewis, 2014). Lewis (2014) presented a slide illustrating the relationship between TOC and gas-filled porosity (ɸgas). An approximate fix to the data with the origin a zero gave [ɸgas = 0.911 TOC]. Using [ɸgas = 0.911*TOC] plus the TOC versus adsorbed gas (gc: scf/ton) chart (Hartman, 2012) the following OGIPscf was calculated:

ka > 100nD [ka = [(0.0108*ɸgas)-0.000256]*10^6]

Gs Adsorbed Gas [OGIPscf]/Area

Gs = 1,359.7*gc*RHOb*A*h*(/Bg) = 13BCF/sec

Gf Free Gas [OGIPscf]/Area

Gf = 43,569*[ɸgas*h*A*(1/Bg) = 17BCF/sec

The Quick Method has the following advantages in calculating OGIPscf: 1.) limited log suite [RHOb], 2.) no Rw needed, 3.) no porosity calculation from well logs, and 4.) no Archie Parameters a,m, & n needed.