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"Probabilistic Addition of Gas Reserves Within a Major Gas Project"

SPE Paper 50113, prepared for presentation at the 1998 SPE Asia Pacific Oil & Gas Conference and Exhibition held in Perth, Australia, 12-14 Oct 1998.

by Peter J. Carter, Woodside Energy, Ltd. and
Enrique Rorales, Shell International Gas Ltd.

This excellent paper tells how one company applied the new "SPE/WPC Reserves Definitions." The authors say that the correct quantities of "proved" reserves will be at a 90% confidence at the total project level. Adding p.90's for the 25 fields produces approximately a 99% confident total. Properly adding the field size distributions resulted in an aggregate volume that is 9.3% higher than the sum of the individual fields' p.90's.

Of particular interest is the auditable process for recognizing inter-field correlations. A correlation coefficient matrix is used to represent mutual effects of:

Trap style, complexity and risk of compartmentalization
Reservoir unit
Seismic survey and interpretation; velocity model
Appraisal status
Pressure system and production method; aquifer
Source of risk factors

The process described in the paper seems to be one reasonable way to approach the complex problem of relating correlations between fields. It is importance to balance the complexity of the method with the value added by better analysis.

A more complete view of the process and issues is described in my paper, "Probabilistic Reserves Lead to More Accurate Assessments," SPE paper 49032, prepared for presentation at the hurricane-cancelled 1998 SPE Annual Technical Conference and Exhibition scheduled for New Orleans, Louisiana, 27-30 September 1998. Revised 3 Nov 98.

—John Schuyler, March 1999