EVALUATION OF THE WINDALIA SAND, BARROW ISLAND AN EMPIRICAL APPROACH

Abstract

The Barrow Island oilfield derives 97 percent of its 46,000 barrels per day production from the Lower Cretaceous Windalia Sand. The lithology of the sand, which is 110' + 20' thick across the field, is very finegrained, glauconitic sandstone, shaly and silty in parts and varying from moderately unconsolidated to firm. Thin, hard beds of dolomitic and calcareous, sandstone occur throughout. The sand has high porosity and low permeability.

The argillaceous and unconsolidated nature of the formation precludes the use of log interpretation methods based on standard parameters, and it was decided to develop an empirical log evaluation method. In order to calibrate the logs, sixteen of the early wells were fully cored and logged, and the data compared using the Holgate method, which allows two parameters to be correlated to determine their relationship. In the example which is the subjert of this paper, core porosity was correlated against both sonic transit time and bulk density and hence calibration of these log parameters was obtained.

The best fit straight line relating porosity and sonic transit time has its origin at 76 microseconds per foot and extrapolates to 246 microseconds per foot at 100 percent porosity. The bulk density — porosity cross plot gives a grain density of 2.71 grams per cubic centimetre and fluid density of 1.16 grans/ cc. The deviations from the standard parameters of delta-t matrix = 56. delta-t fluid = 189, grain density = 1.65, fluid density = 1.0 are explained by the shaliness and lack of compaction of the formation. Using charts for the calculation of water saturation and porosity from induction conductivity and sonic transit time (or bulk density) at 2' intervals through the sand, backed up with traced SP and caliper curves, an evaluation plot of standard format is developed. Intervals of nett effective pay are then chosen.

Other evaluation techniques used during the development of the Windalia Poo! include a modified movable oil plot, used in the water injection wells where a saturated saline drilling fluid was employed, and a Sonic-Neutron log comparison for the identification of suspected gas columns in the Windalia.

440 wells have now been drilled at Barrow Island, and the empirical evaluation methods evolved have enabled the definition of beds of producible hydrocarbons in all cases.