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 claim 1, 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 important 2. 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.

Introduction
Field Background and History

The El-Agreb field (see Figure 1) is located approximately 100 km southwest of Hassi Messaoud and about 1000 km from the Algerian coastal ports. It is currently operated by SonaHess (a joint venture between Sonatrach and Hess Corporation). The JV operates three fields, El-Gassi, Zotti, and El-Agreb. El-Gassi was the first field discovered in the Gassi El Agreb (GEA) area in 1958, then Zotti in 1959, and finally El-Agreb in 1960. The El-Agreb field has produced some 303 MMstb of crude. The production performance and pressure data to date indicates that the fault which splits the field into East and West areas is sealing, and this understanding has affected how the fields have been developed. The primary recovery mechanisms influencing production in El-Agreb reservoir are a combination of natural depletion and waterflooding in West Agreb, and aquifer influx in East Agreb.

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