Quantitative Analysis of Infill Performance: Robertson Clearfork Unit

Abstract

Infill drilling has significantly increased the cumulative production, conventional remaining reserves, and EOR prospects in the Robertson Clearfork Unit, Gaines County, Texas. This paper documents the successful infill program and the techniques used to quantify reservoir performance under varying well spacings and injection patterns. Robertson is a highly stratified, lenticular dolomite reservoir. Continuity of pay between wells is unusually poor, and the reservoir was only partially drained by wells on 40-acre spacing. Ultimately 10-acre wells and 40-acre inverted ninespot patterns were required for an effective waterflood. Expected recovery from the unit has been increased about 24 million barrels by drilling 218 infill wells and increasing the number of water injection wells to form the 40-acre inverted nine-spot patterns. Three approaches were used to quantify reservoir continuity as a function of horizontal distance: geologic correlation, pressure transient behaviour, and regression analysis of infill performance. All three techniques indicate that 10-acre/well spacing can effectively drain 80%-85% of the reservoir volume by primary production (solution gas drive). However, on this spacing, reservoir discontinuities limit the floodable volume to only 60%-65% of the total reservoir. These studies have permitted the operator to quantitatively project the results of further infill drilling. They also permit amore » realistic assessment of EOR potential based on actual floodable reservoir volume calculations.« less