--> Depth Derived Estimates of Porosity and Water Saturation, Valhall Field, by J. G. Patchett and G. W. Gunter; #90986 (1994).

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Abstract: Depth Derived Estimates of Porosity and Water Saturation, Valhall Field

Jay G. Patchett, Gary W. Gunter

Statistical relationships have been developed for the Valhall Field, Chalk Reservoir, Tor Zones One and Two which provide valid estimates of porosity, water saturation, and oil-in-place from subsea TVD, formation tops, and formation thickness. The objective of this work was to use these relationships to compute reliable reservoir parameters in the flank areas of the field where drilling density is low. These parameters are then ready for input into a reservoir simulator.

Log-calculated values of porosity and water saturation were used as input to a least squares fitting program so that these parameters could be estimated from depth-derived data. This results in porosity predictions with a standard error of about 3.0+ p.u. and water saturation estimates within 10 percent water saturation units. The overall predictions of water saturation and porosity are believed to be unbiased estimates of their true values. The height above free water was generally a more significant parameter in predicting porosity than true vertical depth. This observation is believed to have geologic significance.

Statistical predictions were made for only the Tor Zones One and Two. Slightly better estimates were possible by considering these reservoirs separately; however, the zones could be merged for analysis if desirable for other reasons.

This Valhall chalk evaluation was conducted in steps, including:

1. The statistical evaluation of mercury capillary pressure data to gain insight into fitting log data;

2. The construction of a multiwell data set by formation and well (14 wells); and

3. The development of depth-related transforms to use with regression analysis to predict porosity and water saturation.

Regression statistics provided the basis for model selection. Also, computed porosity and water saturations derived from the depth medel were validated by comparing with log-derived values of these parameters on a meter-by-meter basis (depth increment by depth increment, or 0.25 meters), by comparing the depth profiles of the model data with depth profiles using log data, and finally by comparing reservoir summary parameters by zone and well, such as average water saturation or hydrocarbon pore volume.

The algorithms developed through this process are being used to estimate porosity and water saturation on a meter-by-meter basis (depth increment by depth increment) at grid locations in Valhall field. These are then used in a reservoir modeling program.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994