Incorporating Geologic and Data Uncertainties Into Hugoton Field Pore Volume Estimates
Kvk Prasad, Terri M. Olson, and Steve D. Boughton
Estimating volumetric original gas in place (OGIP) for a large gas field is a process involving several uncertainties. In Hugoton Field, chief among these are uncertainties inherent in estimating porosity from well logs and in the spatial distribution of porosity. This paper will discuss the results of a reservoir characterization study to estimate a plausible range of pore volumes for Hugoton reflecting these uncertainties.
Hugoton Field is a mixed-lithology reservoir (limestones/dolomites and sandstones/siltstones) consisting of 5 main reservoir intervals with intervening non-reservoir zones. As a first step in the study, a 3-D geological framework was established using data from 420 wells in the field with modern wireline logs. Artificial neural networks (ANN) were used to estimate transform functions to calculate porosity from well logs. These functions were based on 15 wells in the field with core porosity data. Due to problems with log data such as thin beds, residual gas etc., the log-derived vs. core porosity correlation was only about 70%, even with the ANN method.
Estimates of spatial continuity of porosity showed significant differences in spatial variation within the reservoir formations, whereas the non-reservoir formations showed no systematic variation. The Sequential Gaussian Co-simulation method was used to estimate the porosities in each formation by including both log-derived and core porosity values and specifying a 70% certainty factor for the log-derived porosity values.
The OGIP results from geostatistics were more realistic both in terms of the spatial distribution of gas volumes as well as the range of total OGIP. In addition, the geostatistical approach resulted in a range of pore volumes, thus quantifying the uncertainty in the calculations, unlike the deterministic approach which produced a single number.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California