ABSTRACT: A New Method of Mapping Reserves in Heterogeneous Gas Reservoirs

MORTON, J. G. G., and M. MALAVAZOS, South Australian Department of Mines and Energy, Adelaide, South Australia, Australia

Gas reserves of the Cooper basin in South Australia are produced from fields that are characterized by stacked meandering fluvial sandstone reservoirs, commonly produced as a co-mingled completion. Conventional reserve mapping techniques, using deterministic methods (proved, probable, and possible), and using the concept of porosity pay cutoffs, have demonstrated some limitations. The reservoirs have variable permeability between production wells that is not obviously related to geological paleoenvironment. In addition, although these reservoirs are primarily structural four-way dip closures, there is in most cases also a stratigraphic component. Reservoirs have unpredictable distributions, and well density is often not sufficient to reliably map reservoir limits. Using arbitrary poro ity cutoffs result in distorted original gas in place (OGIP) volumes that are really an estimate of the OGIP that will contribute to recoverable gas reserves. In addition, computer modeling of gas reservoirs using a "tank" model indicates that recoverable reserves are better related to the permeability-thickness (Kh) of the reservoir. The traditional porosity cutoff method combined with an assumed average abandonment pressure is scientifically unsustainable. The methods developed to better map reserves in these reservoirs included mapping of all, including "tight", OGIP; adopting probabilistic rather than deterministic methods to account for the high uncertainty in reservoir volume and high variability in recovery factors between production wells; Kh, the critical parameter in determinin recovery, is determined from drill stem tests or from log analysis. Kh relationships between wells are also accounted for in determining recovery factors.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)