Publication Rights Reserved
Numerous low permeability reservoirs, most of which cover long vertical sections, have been developed commercially in the last few years as a result of advancements in the field of stimulation technology. Many of these had been tested previously and unsuccessful completion attempts made. Lithologically, these formations include cherty limestones of Devonian age, silty limestones from the Mississippian and limey sands of Canyon age, as well as others. Measured formation permeabilities range from 0.01 to about 0.4 md. with some naturally fractured sections indicating greater values as fracture permeability. Gross vertical thicknesses vary from 100 feet in some more homogeneous lime sections to 1,000 feet or more in highly broken sand and shale sections. Most of these occur at depths below 6,000 feet, and some in excess of 10,000 feet. For some time, stimulation techniques which were then considered applicable failed to provide flow systems necessary for commercial production. Based upon past experience, stimulation programs were designed and completion practices altered where practical to meet treatment demands. Industry's willingness, even in an atmosphere of increasing cost consciousness, to spend greater amounts than ever before for stimulation has contributed to the development of such programs. Research provided through field operations, supported by mathmatical analyses and laboratory experimentation, has contributed to improved techniques. To date, hydraulic fracturing appears to be the only method which will affect the desired results in these low permeability formations.
The elements of treatment design are numerous and difficult to define unless associated with plans for stimulation of a specific well. Consideration must be given to both materials and technique of application in light of individual well conditions and formation characteristics. Design of treatment for low permeability reservoirs is essentially the same as for any other, with some modifications providing greater vertical coverage and radial extent of fracture system. This is usually reflected in greatly increased fracture volumes and/or injection rates. A brief general discussion of several major factors in treatment design is presented.