Abstract This paper focuses on a new application of underreaming a well bore utilizing an air lift reverse circulation drilling system to increase reservoir production. The theory used and the equipment built thus far is presented and discussed. presented and discussed. Underreaming is the process used to enlarge a bore hole beneath a restriction, such as casing, below the surface. They do this by expandable cutting arms which are collapsed in the tool body while running in the hole. They are usually expanded either by hydraulic, pneumatic, or mechanical means into the formation. Air lift reverse circulation underreaming utilizes relatively small circulation rates to remove the cuttings when enlarging a bore hole. This is due to the fact that the cuttings are returned through the I.D. of the underreamer and drill string which is tremendously smaller in cross sectional area than the area of the bore hole annulus. Utilizing the system presented, the hole wall of the producing formation is subjected only to the hydrostatic pressure of the drilling fluid in the hole annulus and minimal erosion of the formation due to the decreased fluid velocity and absence of cuttings. "Virgin" producing zones are entirely exposed to the well bore prior to gravel pack completions. Introduction In late 1975, a program was initiated by Chevron, U.S.A. to attempt to develop a more efficient, economical method to drill the numerous shallow wells being drilled and projected to be drilled in the San Joaquin projected to be drilled in the San Joaquin Valley area of California. Some success had previously been achieved utilizing air lift previously been achieved utilizing air lift reverse circulation for drilling water wells in this area. This technique was investigated and is thus presented. During this investigation, it was found that a "dual-tube" concentric drill string was manufactured for air lift reverse circulation for big hole drilling as well as to overcome adverse drilling conditions encountered in smaller holes. Throughout the rest of this paper, the term "dual-tube" will be used as a prefix to identify the drill string components; i.e. dual-tube drill pipe. Also, air lift reverse circulation drilling offered advantages in areas of critical sampling, lost circulation zones, and economical circulating volumes. Initially, two attempts were made using air lift reverse circulation on a conventional drilling rig working in the area. The major problems were:Excessive drill pipe connection time.Inner tube vent plugging while making connections.Difficulty in unloading the innertube annulus and establishing circulation after each connection was made and hole depth increased. Drilled solids plugging production zones has historically, been a problem in the industry utilizing conventional drilling procedures. In conventional circulation, procedures. In conventional circulation, all cuttings generated by the bit are circulated back through the penetrated production zone as they are circulated out production zone as they are circulated out of the hole.