Optimum primary development and secondary recovery of a low permeability reservoir

Continental Oil Co., has successfully developed the low permeability San Miguel-1 Sand Oil reservoir in the Sacatosa field, Maverick County, Texas. The average air permeability of this sand is only 3.6 md and fracturing is necessary to economically produce the wells in this field. Producing depths range from 1,140 to 1,775 ft. Initial development was on 40-acre spacing and included 275 producing wells and 4 infill pressure observation wells on closer spacing. After 5 yr of production and pressure history had been obtained, these data were used in a computer study, which showed that 20-acre spacing was economically feasible. This resulted in the drilling of an additional 193 infill wells. At the time of the 20-acre drilling, a pilot waterflood was initiated which consisted of 16 injection wells and 9 producing wells. All injection wells were fractured with gelled water. After 2-1/2 yr of injection, no breakthrough has occurred and oil production has increased significantly. Injection rates average 24 bwpd per well with an input-output ratio near unity. Secondary oil now accounts for 53% of pilot production.