Analysis of Pore Throat Size and Use of the Waxman-Smits Equation to Determine OOIP in Spindle Field, Colorado

ABSTRACT This paper describes the development of a method used to define pay and calculate OOIP in the Spindle Field. Traditional methods of calculation do not yield reliable results for pay or OOIP due to the shaly sand nature of the reservoir rock. The Spindle Field contains more than 1,150 wells and covers parts of seven townships in northeast Colorado (Figure 1). In this study, a minimum pore throat size of 0.5 microns was selected as a pay cut off. Methods were developed to estimate from logs the porosity and permeability data necessary for input into an equation to calculate pore throat size. Water saturations were calculated using a form of the Waxman-Smits equation. Cation exchange capacity values were estimated from the SP log for input into the Waxman-Smits equation. The net pay intervals were combined with the results of the Waxman-Smits equation to give values of hydrocarbon pore thickness (HCPT) in individual wells. The HCPT values of the field wells were spotted on a map and a volumetric analysis made to give an OOIP figure. Confirmation of the pay definition was made by the correlation between calculated HCPT and ultimate primary recovery of twenty individual wells. Confirmation of the OOIP calculation results was made by the nearness of the OOIP calculated using this method to an OOIP estimate made from ultimate primary recovery of the Amoco wells in the field.