G.B. Asquith
A 5187ft. well drilled in the Texas Panhandle had some bad news [primary zone non-productive], and some good news a drilling break [3.6MPF to 1.2MPF} and mudlog oil and gas show up hole in the Pennsylvanian Kansas City Limestone shown below:
4560ft.- 4572ft. Light Tan Oil Stain w/ 20% Bright Gold Fluorescence w/ Weak Milky Cut
4574ft.- 4600ft. Trace Dull Mineral Fluorescence no Cut
The well logs revealed that the Kansas City Limestone has a 38ft. porosity zone, but only the top 12ft. appeared to be potentially productive, the remaining porosity appeared to be wet with very low resistivity. The first step in the log analysis of the Kansas City is to obtain a value for Rw. Using the Rmf from the log header [Rmf = 1.10 at Tf] and the SP Log gave a Rw of 0.07 at Tf. Using the 0.07 to calculated Sw in the wet zone [4574’- 4596’] resulted in Sw significantly greater the 100%; which is impossible, because a clay free [no CBW] reservoir cannot hold more water than the storage capacity. Using the resistivity and porosity data from the wet zone resulted in Rw = 0.031 [Rwa = Ro/F] and Rmf = 0.66 [Rza = Rxo/F]. In order to calculate these values for Rwa and Rza, the sonic porosity [PHISls] was used to calculate the formation factor [F = 1/PHISls^2], because the Kansas City Limestone has vuggy porosity [PHIDls > PHISls]. The sonic log MUST be used because resistivity logs are insensitive to water in the vugs, only the water in the matrix porosity [PHISls] (Guyod, 1945; Nugent and others, 1978; and Rasmus, 1987).
Petrophysical Analysis 4560ft. to 4572ft. [Average Values]:
PHIDls PHISls PHIrxo Sw(archie) BVW Sw/Sxo Sw(ratio) KmD
0.21 0.15 0.14 0.46 0.089 0.58 0.51 5mD Kozeny-Carman Eq.
Because the Kansas City Limestone includes both Oil and Wet zones plus PHIDls and PHISls porosities, it is possible to approximate Special Core Analysis with well logs that include Cementation [m] and Saturation [n] Exponents plus Relative Permeability Curves for the matrix porosity [PHISls]. The results are listed below:
Cementation Exponent [m] = 2.37 Saturation Exponent [n] = 1.47
The Relative Permeability Curves [matrix porosity] indicated only 6ft. with Kro > Krw.
The conclusions from the above analysis are that the Kansas City Limestone contains vugs plus intergranular porosity with very high BVW, a low degree of moved hydrocarbons and only 6ft. with Kro > Krw. Using a Dew Plot [Rt/Rw versus Rxo/Rmf] as a potential water-cut chart indicates that the potential water-cut in the Kansas City Limestone would be 75%. In addition, the 12ft. oil bearing zone is sitting above a 26ft. thick water bearing zone so that the potential to frack into water is very high. Therefore, it was recommended not to complete the Kansas City Limestone.
There are four important lesson in this petrophysical analysis:
1.) ALWAYS RUN SONIC LOGS.
2.) WET zones can be critical to an analysis [PHIrxo, Rwa and Rza].
3.) Local knowledge and sample information is also important.
4.) On Modern Array Induction and Laterologs the Continuous Dynamic Resistivity Mud Measurement [RM] would have been an important check of the header Rm.
