Abstract

In a potential gas-bearing Woodford shale reservoir (Ro% = 2.62) with a standard logging suite [Array Induction – Neutron-Lithodensity] it is possible to calculate the following log parameters:

Reservoir pressure (Pr)

Weight Percent Total Organic Carbon (TOC)

Volumes of Clay (Vcl), Quartz (Vqtz), and Kerogen (Vke) plus Total Porosity (Φtotal) using simultaneous equations plus ρb and Φnls data

Effective Porosity (Φe) = Φtotal – CBW (CBW = Clay-Bound Water)

Water Saturation using the Simandoux Shaly Sand Equation (Swsim) Gas-Filled Porosity (Φgas) = Φe * (1.0-Swsim) Permeability (k) in nannodarcies (nD) = [(0.0108*Φgas) – 0.000256] * 10^6

Adsorbed Gas (gc) and Free Gas (Gfcm) in scf/ton

In addition to log data laboratory analyses was done on samples from three depths [9480’, 9500’, and 9520’]. A comparison of Vcl, Vqtz, and Vke values determined in the laboratory with Vcl, Vqtz, and Vke values determined from log data are listed below:

Depth	Vcl(lab)	Vqtz(lab)	Vke(lab)	Vcl(log)	Vqtz(log)	Vke(log)
9480	24.3	65.1	10.8	21.6	65.6	12.8
9500	24.4	62.6	13.0	16.7	67.0	16.3
9520	18.7	68.5	12.7	17.7	69.5	12.8

The data listed above reveals that the Simultaneous Equation Method using only ρb and Φnls data (Lewis, 2009; see below) predicts the laboratory results fairly accurately. However, if large amounts of calcite or dolomite were present in the Woodford, the results would be much less reliable. In the Woodford Shale in this well the amount of calcite was 0.0% and the amount of dolomite ranged from 1.8% to 2.5%.

MINERAL VOLUMES and TOTAL POROSITY [Lewis, 2009]

Vcl + Vqtz + Vke + Φtotal = 1.0

Vke = (TOC*Kvr*ρb)/rkerogen

Vcl*ρclay + Vqtz*ρqtz + Vke*ρke + Φtotal*ρf = ρb

Vcl*Φnclay + Vqtz*Φnqtz + Φke*Φnke + Φtotal*Φnf = Φnls

Of the entire gross organic shale interval (9448’ to 9548’) with high gamma ray (GR > 200 API) and high resistivity (AT90 > 15ohm-m up to AT90 = 320ohm-m) only 52 feet is net pay as defined by the following cutoffs: 1.) TOC > 2%, 2.) Φe > 4%, 3.) Φgas > 2%, 4.) Swsim < 45%, and 5.) permeability (k) > 100 nD. In the net pay intervals the total gas (adsorbed+free gas) in scf/ton was converted to BCF/sec using the equation below:

BCF/sec = {[(h*5280^2)*(ρb*62.43)]/2000}*Total Gas

BCF/sec = 84 (in 7 net pay intervals)

The well was a horizontal completion with an IPF of 4.36 MMCFPD and an EUR of 5.2 BCF. The horizontal was placed in the thickest net pay interval (34 feet) with calculated permeabilities from 108 nD to 445 nD which contains 19 BCF/sec. Based on the calculated reserves of 19 BCF/sec for and an EUR of 5.2 BCF the recovery would be 27%.

A flow chart for the analysis of gas-bearing shales that was utilized in the analysis of the Woodford is illustrated below:

Three conclusions maybe reached from this study:

1. Log analysis is possible on Gas-Bearing shales using only Deep Resistivity, Bulk Density, and Neutron Porosity.

2. The determination of Vcl, Vqtz, Vke, and Total Porosity using Bulk Density and Neutron Porosity data is reliable if only small amounts of calcite and/or dolomite are present.

3. The Shaly Sand Producible Plot (Q-PLOT) may be useable in Gas-Bearing shales as an indicator of reservoir versus non-reservoir shale.