The Orito field in the south of Colombia was initially put on production in 1969 and has produced continuously since then. The most prolific reservoir, is the Caballos Formation, a thick (250 ft avg.) laminated sandstone located at a depth of 6100 to 7500 ft that has produced (30 to 45 °API crude) for over 35 years, with production peaking at 66,000 BOPD. The permeability varies from 20 to 200 mD with streaks exceeding one Darcy. At different times in the past, attempts were made to hydraulically fracture one or more of the sands, using a variety of different (water- and oil-based) fluids. However, many of the wells indicated positive skin factors following the fracture treatments, irrespective of the fluid system used. In at least one case, a well stopped producing after being treated.
A core study revealed that despite the relatively low clay content in the formation the critical velocity was less than one cc/min. Moreover, the retained matrix permeability after performing a static leakoff test (500-psi differential for 30 minutes) was less than 5%, regardless of the fluid used. From this testing it was concluded that the reduction in the permeability was due to the mechanical plugging of the kaolinite or disrupted mica in the pore throats. This reduction in the matrix permeability creating a very high fracture face skin that would account for the higher skin factors following fracture treatments.
To eliminate the fracture face skin created during the fracture treatment a new treatment incorporating a pre-pad of acid viscosified with a solids free visco-elastic surfactant was developed. By incorporating this stage into the fracturing treatments, the retained matrix permeability was increased to +/- 30%, resulting in a negligible fracture face skin. The productivity of fracturing treatments performed using this technique resulted in negative skin factors and production ratios that exceeded expectations.