A Geostatistical Study of a Pilot Area in the Griffithsville Oil Field
Authors: Damayanti Maria Caecilia , Hicks Paul J. 
Published: December 2007

Abstract
A geostatistical study of the Berea Sandstone reservoir was conducted for a ninety acre pilot area in the Griffithsville oil field in West Virginia. The effect on the predicted oil recovery of different techniques to estimate interwell reservoir properties is presented. Semivariograms were constructed for several reservoir properties including permeability and porosity. Despite the relatively tight well spacing, a large apparent nugget effect was observed.
A parametric study examined the effect on predicted oil recovery of using polygonal weighting, linear interpolation, kriging and conditional sequential Gaussian simulation (SGS) to estimate key reservoir properties for grid blocks without wells. History matching of the pilot waterflood provided estimates for some other reservoir parameters. Due to the large apparent nugget effect and the absence of data at small lags, two semivariogram models were used for the kriging and SGS cases. One was a pure nugget model which assumed reservoir properties were randomly distributed spatially. The second was a spherical semivariogram model with no nugget effect and the range roughly equal to the well spacing.
Oil recovery from the pilot area was calculated using a black oil reservoir simulator with selected input reservoir properties calculated using the above methods. Recovery was calculated for primary depletion and waterflooding assuming two scenarios for each case; one in which the wells were not stimulated and another in which the wells were hydraulically fractured. Fracture stimulation usually increased the calculated producing rates more when the input data was estimated using the pure nugget semivariogram model than when using the spherical model. Simulations using SGS generated input data tended to have the highest recoveries followed by simulations with data generated using kriging, linear interpolation and polygonal weighting. The reservoir simulation results illustrate the need to use the best reservoir description in reservoir engineering analysis.

Introduction
A pilot project in the Griffithsville oil field, West Virginia was the object of this study. Production and injection records, core data from six wells, and log data from 24 of 25 wells in the pilot area were available. Several bottom hole pressure and water cut records were also available. These data and a previous reservoir study formed the data base. In the first stage of this work a geostatistical study of the reservoir properties was performed. Properties examined include permeability, porosity, net pay and water saturation. A large apparent nugget effect was observed for all properties except net pay.
The purpose of the second stage was to develop a consistent set of reservoir properties to use in the geostatistical parametric study of the third stage. A history matching study integrated the log, core and production data into a consistent description of the reservoir. In this stage, a series of simulation runs were performed using three 2-phase, 2-layer models and a 2-phase, 5-layer model.
In the third stage, a parametric study on the reservoir description was performed using different conditional geostatistical techniques to model reservoir properties. Four methods for assigning for grid block properties were examined: polygonal weighting, linear interpolation, kriging and sequential Gaussian simulation (SGS). Fracture stimulation usually increased the predicted producing rates more when using a pure nugget semivariogram model than when using the spherical model. Simulations using SGS generated input data tended to have the highest recoveries which is the opposite of what is usually observed.

Field History, Geology and Reservoir Properties

Field History.
The Griffithsville oil field covers about 20 square miles and is located about 18 miles southwest of Charlestown in Lincoln County, West Virginia. Discovered in 1908, this field was rapidly developed with both oil and gas production. About 550 wells were drilled in the field and about 90% of the wells have been plugged and abandoned.
This study area was developed in 1976 as a CO2 pilot project (a waterflood was conducted prior to the CO2 flood). Production from the 90-acre pilot area was 2,250 to 3,175 barrels oil per acre. P. 93