An Evaluation of Waterflood Infill Drilling in West Texas Clearfork and San Andres Carbonate Reservoirs
Authors: Wu C. H. , Flores D. P. , Briens F. J. L. , Brimhall R. M. , Poston S. W. 
Published: December 2007

ABSTRACT
In this paper we present the results of a technical and an economic evaluation of waterflood infill drillings in fourteen Clearfork and eleven San Andres carbonate reservoirs in West Texas. The initial waterflood recovery efficiency and the infill drilling waterflood recovery efficiency were estimated from decline curve analyses. The economic analysis was performed to evaluate the relative profitability of the waterflood infill drilling operations.
For Clearfork units, the estimated initial waterflood recovery efficiency ranged from 9.54 to 32.19% of original oil in place (N) with a medium of 17.75%N; and the infill drilling waterflood recovery efficiency ranged from 13.09 to 52.56%N with a medium of 25.53%N. The medium increase in oil recovery by infill drilling was 7.78%N. For San Andres units, the initial waterflood recovery efficiency ranged from 10.68 to 39.03%N with a medium of 19.97%N; and the infill drilling waterflood recovery efficiency ranged from 13.73 to 44.58%N with a medium of 31.82%N. The medium increase in oil recovery by infill drilling was 11.38%N.
A multi-linear regression analysis was used to establish regression models correlating the initial waterflood and the infill drilling waterflood recovery efficiencies to reservoir and process parameters. For the Clearfork units, the mean and standard deviation for the best initial waterflood recovery efficiency correlation were 20.39%N and 3.19%N, respectively. The mean and standard deviation for the best infill drilling recovery efficiency correlation were 29.25%N and 2.48%N, respectively. The average increase in oil recovery by infill drilling was 8.86%N. For the San Andres units, the mean and standard deviation for the best initial waterflood recovery efficiency correlation were 22.09%N and 1.45%N, respectively. The mean and standard deviation for the best infill drilling recovery efficiency correlation were 32.23%N and 2.73%N, respectively. The average increase in oil recovery by infill drilling was 10.14%N.
Two cases were studied in the economic analyses. Case I economic analysis used past and projected production schedule and economic parameters. For the Clearfork units, the rate of return ranged from 18.37% to 125.2% with a mean of 53.3%. The payout ranged from 1.22 years to 6.58 years with a mean of 3.14 years. For the San Andres units, the rate of return ranged from 20.58% to 500.0% with a mean of 157.0%. The payout ranged from 0.36 to 5.43 years with a mean of 1.77 years.
Case II economic analysis used past and projected production data as though the projects were initiated in January 1988. The base crude oil price of $17.50/STB was assumed in the analyses. For the Clearfork units, the rate of return ranged from 11.41% to 98.80% with a mean of 40.4%. The payout ranged from 1.45 to 6.42 years with a mean of 3.53 years. A sensitivity analysis revealed that the average payout using $12.50/STB was 4.28 years and 3.13 years using $22.50/STB. For the San Andres units, the rate of return ranged from 20.58% to 500.00% with a mean of 157.00%. The payout ranged from 2.36 to 4.91 years with a mean of 2.3 years. A sensitivity analysis revealed that the average payout using $12.50/STB was 2.18 years and 2.00 years using $22.50/STB.