Analysis Of Pore Throat Size And Use Of The Waxman-Smits Equation To Determine Ooip In Spindle Field, Colorado

Abstract

Kolodzie Jr., Stanley; Amoco Production Company 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 part of seven townships in north-east 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. Introduction Although the Spindle Field is large and contains over 1,150 wells, it was developed so rapidly that the data necessary for a detailed analysis of the pay limits and OOIP was never gathered. In addition, the properties of the reservoir rocks have made properties of the reservoir rocks have made analysis very difficult. Consequently, estimates of the OOIP have always been relatively rough. However, even a cursory examination shows that the OOIP in this field must be very high, in the hundreds of millions of barrels. Spindle Field produces from the Sussex and Shannon sandstones. These formations are shaly sandstone members of the thick Pierre Shale. The geologic period is the Pierre Shale. The geologic period is the Upper Cretaceous and approximate depth is 5,000 feet. PROJECT APPROACH PROJECT APPROACH Problems With Standard Analysis Technique Problems With Standard Analysis Technique Log analysis of some of the first Spindle Field wells indicated that standard log analysis techniques would not give accurate pay and water saturation calculations. During pay and water saturation calculations. During production tests we found that some porous production tests we found that some porous intervals were not productive as expected. Similarly, some wells with pay intervals which were expected to be only marginally productive proved to be high quality wells. productive proved to be high quality wells. In addition, the nonproducing Sussex and Shannon intervals outside the field boundaries induction logs to the pay intervals also calculate "wet" using the log readings and the Archie equation. Even though Spindle wells produce very little if any water, log calculations most cases. Obviously, the unusual a "non-standard" pay definition and a "non-standard" water pay definition and a "non-standard" water saturation calculation method. Even though some kind of special analysis technique was apparently needed for this field, very little data was available for such a technique.