The gravel pack forms the basis of formation sand control in low consolidation strength formations. An extension of the gravel pack is the frac pack technique using water, referred to here as a Water Frac, which is designed to bypass wellbore skins and thus increase well productivities. The increased productivity is accomplished by a length-limited, properly designed, hydraulic fracture that provides a highly conductive flow path through a wellbore damaged zone in conjunction with a proper gravel packed completion. The process is also applicable to intervals of several hundred feet comprised of multiple pay zones by using a Multi-Stage Water Frac technique. The entire process of creating and packing the fracture(s) and gravel packing is accomplished using a properly defined brine.
The Water Frac (WF) process has been applied and evaluated in a worldwide arena. Industry Frac Packs placed using gels (FP) have been similarly applied. Evaluations based upon field performance indicate that in many areas, the performance of WF and FP cannot be distinguished. Selection processes support this observation as well as indicating conditions where FP and non fractured water packs (WP) are recommended.
In the following, the processes of a WF and FP are described and typical field pumping techniques are provided. Results of field applications of WF and FP are presented. A completion selection process is outlined to assist in defining the proper completion technique.
Gravel packing forms the basis of completions requiring sand control. Both perforation and annular packing are recognized as critical elements of the gravel packing process. Mullen et. al., as well as many other authors, have demonstrated the need to completely pack perforation tunnels with gravel pack sand. (Figure 1). To insure complete perforation packing, injectivity to the perforations must be maintained. Because brines do not act as a fluid loss control material (as can gels) perforation packing is best done using brine. Continued effective packing of the screen/casing annulus is also recognized as best done using brine, especially as hole angle increases above about 60.
By prepacking and gravel packing using low viscosity fluids high quality completions are often obtained. However, Figure 2 illustrates that even though water-pack completions performed below the formation's fracture pressure usually produce low-skin completions, high-skin completions can also result. This indicates that there is a damaged zone surrounding some wells that still reduces well inflow in spite of best efforts at perforation prepacking and gravel packing.
In addition, as a result of the increased pressure drop associated with flow within a packed perforation tunnel, it can also be shown that attempts to shoot through such damage would assure significant damage to a well even if perfect perforation packing was achieved. These findings indicate that fractures are required to extend the gravel pack beyond the damaged zone, thus bypassing its effects and assuring high well productivity.
The following describes the process of the WF and the FP including typical pumping techniques. Comparison of these two processes is accomplished using field performance data. A completion technique selection process is also discussed.
Bypassing near wellbore damage in relatively high permeability formations using a propped hydraulic fracture is an increasingly well known and commonly applied practice. However, wellbore damage typically extends a short distance (e.g. about 2 to 3 ft) from the wellbore.
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