Cotton Valley Lime (Oolitic Shoal) Play
Production Decline Curves
This play has experienced very successful re-frac of old wells, as well as tight-spaced infill drilling. Gilmer Field serves as the de facto example of how to approach the play. Figure 4 shows the decline curve for the Indian Rock Gas Unit 8–1 in Gilmer Field. The well shows an initial decline curve, with an acid frac later, which added reserves and improved rate. Then a gelled-sand frac brought the well new life, with a higher Initial Potential than any historical rate, and additional reserve adds. Water production did increase with the gelled-sand frac, as some wet rock was tapped, but reserves offset this factor.
A decline at the same field, the No. 1 Hefner Unit, shows similar results after a gelled-sand frac (Figure 5, a Rate-Time curve, and Figure 6, a Rate-Cum curve). The unit was formerly operated by Exxon, and later assumed by Apache. A Pickett Plot (Figure 7) for the same well, derived from core and log data, shows why the frac was so successful. The low resistivity, or even wet rock, at the bottom of the Cotton Valley Lime interval has only minimal perms, and the rock with higher water saturations is too tite to move much fluid, although it should be noted that if this wet rock is naturally fractured, fluid is mobile.
Figure 8 demonstrates what Apache has done with four adjoining Exxon units at Gilmer Field by adding rate, reserves, and present value.