Optimizing Hydraulic Fracturing in a Mature Oil Reservoir in Algeria

Abstract

Abstract The El-Agreb field, located near Hassi Messaoud, Algeria, has been producing for over 40 years. Hydraulic fracturing was attempted in the mid 1990's with some success. This paper describes the results of a study of 11 stimulation treatments performed in the Agreb field from 1994-2003. Fracture modeling using net pressure history matching gave estimates of fracture dimensions and reservoir permeability. Fracture dimensions and conductivity data went into a reservoir simulator and the post-frac production was history matched by adjusting the effective proppant permeability. Analysis of the pre and post-frac productivity indexes (PI) was done to determine the productivity improvement factor (PIF). This analysis showed a clear correlation between the PIF and the dimensionless fracture conductivity (FcD). Such a correlation would only be expected in the case of finite conductivity fractures. For all the propped fractures, the "ideal" fracture conductivity (assuming 50% proppant permeability damage) gave an FcD greater than 1.5 (with an average FcD of 12). History matching the actual post-frac production, however, required proppant damage factors up to 97%, giving effective FcD's as small as 0.1. The proppant damage factor needed to match post-frac production was a strong function of the "ideal" FcD of the fracture. This result indicates that for good cleanup of the gel in the proppant pack, higher fracture conductivity is necessary than what would be considered sufficient based only on reservoir permeability and ideal values of proppant permeability. Previous work has made this claim1, and this field data strongly supports the earlier work. A large number of El Agreb fracture treatments were analyzed and a calibrated fracture model developed, allowing more accurate design of tip screenout treatments. Due to the low reservoir pressure in this mature field, a large differential pressure drives fracture leakoff, making spurt loss important2. The calibrated model has been applied successfully to 14 new fracture treatments in 2003-2007 (11 producers and 3 injectors). The benefits of this approach are improved productivity and more rapid post-fracture cleanup. New wells drilled in the field are now usually immediately fracture stimulated.