Case History: Infill Drilling Increases Reserves in Blanco Mesaverde and Basin Dakota Reserves, San Juan Basin, New Mexico

Abstract

ABSTRACT Infill drilling, the drilling of additional wells on established spacing units will increase a field's rate of recovery. In a low permeability reservoir, where the sand quality of the producing formation varies greatly from wellbore to wellbore, infill drilling will-also increase ultimate recovery. Two such low permeability reservoirs are the Blanco Mesaverde and Basin Dakota reservoirs in the San Juan basin of northwest New Mexico. Production rates and cumulative production of the infill wells developed so far indicate that additional reserves are being developed on the established drillsites. This paper does not attempt to quantify the increase, but presents data which supports the basic conclusion that significant reserve increases can be developed in low permeability reservoirs by infill drilling. The Blanco Mesaverde reservoir- was, for the most part, fully developed when the New Mexico Oil Con-servation Commission approved infill drilling in late 1974. Since then, over 1,250 infill wells have been completed as of July, 1982. Approximately 1,500 infill wells remain to be drilled. Many of the infill wells completed have higher initial producing rates than the original wells and also appeal' to have more recoverable reserves than anticipated. Pressure versus cumulative production curves which indicate that reserves are being increased, are presented for the total Blanco Mesaverde reservoir and for three townships and ranges which have been almost completely infilled. In addition, pressure versus cumulative production plots for two sets of original and infill wells, where the infills have produced for seven years, are presented to show that the infill wells have not interfered with the original well. The Basin Dakota infill program was approved in May, 1979 by the New Mexico Oil Conservation Com-mission. As of July, 1982, over 750 infill wells had been completed. More than 2,000 infill wells remain to be drilled. Since the Basin Dakota reservoir is much less permeable than the Blanco Mesaverde reservoir, this infill program is also expected to substantially increase recoverable reserves. Production rates and cumulative production of the infill wells as compared to their original wells indicate that additional reserves are also being developed in the Basin Dakota reservoir as a result of the infill drilling pr-ogvam. A brief description of the Blanco Mesaverde and Basin Dakota reservoirs and their development on 320-acre spacing is presented. Two Dakota wells in which long term pressure build-up tests were run are presented to give evidence of the very low permeability in the reservoir. Production rates and cumulative production of three infill wells in different areas of the Basin Dakota field are compared with those of the original offset wells. These show that the original wells are not adequately draining the drillsite and additional reserves are being developed by the infill wells. Each reservoir must be evaluated individually significant additional reserves can be developed in low permeability reservoirs such as the Blanco Mesaverde and Basin Dakota in the San Juan basin.