Potential Infill Reserve Additions by Interval--San Juan Basin

Robert H. Hugman, E. Harry Vidas, Thomas J. Woods

A detailed analysis of gas-well spacing and ultimate recovery per completion in the New Mexico portion of the San Juan basin indicates that a substantial incremental gas potential, on the order of 6 trillion ft³ or more, remains to be developed in the currently producing sections. The majority of this potential is associated with the Dakota and Mesaverde intervals and would be economically producible at a resource cost of $2/million Btu (1987 dollars) or less.

Each major productive interval in the basin was evaluated for completion density, areal extent, and ultimate recovery by section vintage and completion year. In addition, ultimate recovery of current infill completions was evaluated separately from initial spacing wells. Primary spacing for all reservoir intervals is assumed to be 320 ac and infill spacing to be 160 ac. The resource appraisal involved the assignment of ultimate recoveries to each potential 160-ac infill site for each formation. Through 1985, over 7,000 infill sites remained to be completed in the basin.

Historical infill drilling has been concentrated in sweet spots of each reservoir where the primary spacing wells have higher-than-average ultimate recoveries. Maps of ultimate recovery of the first completion per section clearly correlate with drilling density. Remaining infill potential exists largely in extensive fringe areas characterized by lower initial well recoveries (generally < 2 billion ft³ of gas per completion). Many of the fringe areas currently produce from one-completion sections. Thus, assumptions about subsequent completions in these sections have a large impact on the resource estimate. Our study indicates that infill well recoveries in these fringe sections should be similar to initial well recoveries.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.