The use of artificial barriers placed at the top and bottom of the producing zones before fracture stimulating a formation can control excessive fracture height growth. The reason for such barrier placement is to contain the fracture treatment within the producing zone when the stress barriers above or below the zone cannot contain height growth. Two case studies are included in this paper on wells fractured in the Richfield formation in the Northern Michigan Basin and in the Codell and Sussex formations in the Denver-Julesburg Basin.
Fracture height growth is typically contained by higher stress barriers above and below the zone being fractured. These barriers, while restricting fracture height growth allow deeper penetration of fractures. If adequate barriers above and below the pay zone are absent. artificial barriers can be placed above and below the pay zone to help contain the fracture. The techniques used for placement of artificial barriers are described by Mukherjee, et al. The techniques are defined as Invertafrac - barrier above the pay zone, Divertafrac - barrier below the pay zone and Bracketfrac - barriers above and below the pay zone.
The case studies presented review the placement techniques of artificial barriers and treating pressures from wells in the Richfield formation of the Northern Michigan Basin and the Codell and Sussex formations of the Denver-Julesburg Basin in northern Colorado. Production performance is compared to offset wells without artificial barrier placement.
The Beaver Creek and Garfield-Richfield fields of Lower Michigan (Figure 1) produce from several dolomite layers in the Devonian Richfield Formation at a depth between 4,200 ft and 4,600 ft. The Richfield is a heterogeneous formation with a network of natural fractures. It is a series of relatively thin (4 ft to 20 ft), dolomite reservoirs which are confined above and be- low by anhydrites. These anhydrites are comparable to the dolomite reservoirs in fracture resistance and essentially do not present an effective stress barrier to fracture growth. The dolomite zones have a porosity range of 12 to 20%, and the permeability is between 6 to 20 md. The dolomite reservoirs also contain significant amounts of free salt. which is apparent in the large quantities of produced salt in new wells. The productive zones are numbered 1 through 8 from the top with zone 6 usually containing 100%water. Oil gravity is 43 API and viscosity is one cp at the reservoir temperature of 110 F.
Previous completions in the Richfield have been plagued with unrestricted fracture height growth which caused the completions to be long and expensive. Standard practice has been to perforate and treat the zones in groups. An example well completion is shown in Table 1.