Laboratory Determined ‘A’, ‘M’ & ‘N’ Parameters Have a Significant Impact from Computation of Fluid Saturations in Shaly Reservoirs

Satwant S. Pabla, Mohan Lal, and Kalika Prasad
Logging Group, KDMIPE, ONGC, Dehradun, India

Shaly reservoirs of Tripura fields have always posed problems while dealing with the computations of fluid saturations. More often than not, the computed hydrocarbon saturations have been significantly low using conventional values of electrical parameters ‘a’, ‘m’ & ‘n’. The aspect has been observed during the initial development phase of the field when laboratory determined parameters are normally not available and conventional parameters are used for the evaluation of logs. This has resulted in un-realistic estimation of in-place hydrocarbons, normally on lower side, as proved through the testing and production from these reservoirs. The observation prompted to the laboratory determination of electrical parameters on available rock samples to validate the conventional values.

The study was carried-out on 1” diameter core plugs at a limiting salinity of 200gm/lit so that the effect of shaliness is eliminated. The porosity and resistivity of saturated core plugs were measured and the parameters determined at ambient conditions using the Archie’s relationships:

Formation factor F =Ro/Rw = a/ø^m
Resistivity Index RI =Rt/Ro = Sw^(-n)
Where, Ro & Rw are the resistivities of fully saturated plug and saturating brine respectively.

Thus the ‘a’, ‘m’ & ‘n’ parameters so established for pay zone B-VI-I of Well No.-A; MP-21 of Well No.-B & KP-60 of Well No.-C have been found to be 1.00, 1.74 & 1.83; 1.00, 1.72 & 1.79 and 1.00, 1.77 & 1.61 respectively.
It has been found that the gas saturation ‘Sg’ values increase by about 10% to 12% using these laboratory determined parameters as compared to those calculated with conventional parameters. This has brought out the significant impact of laboratory determined parameters on estimation of hydrocarbon saturations as in the present case the GIIP of these fields will go up.

Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery