Successful application of acid prepacking requires that good diversion be obtained while not imposing an additional damage mechanism. One of the most commonly applied diverters for acid-prepacking is gravel pack sand carried in a gel led fluid. In addition, low pump rates are often utilized to ensure long contact times between the acid and the formation. Although this combination provides excellent diversion, field data indicate that inconsistent productivity results are also obtained. Several non-damaging diverter systems for acid-prepacks are available. This paper highlights three of these techniques, foam, rate diversion, and gravel pack sand carried in brine. Treating data are evaluated to judge the ability of each of these techniques to provide the needed diversion. In addition, reported well test data are reviewed to assess the formation damage aspects of each diverter. Finally, the range of applicability of each technique is outlined.
A successful cased-hole gravel pack completion requires an unobstructed flow path from the native reservoir, through the perforation tunnels, and into the wellbore There are several keys to obtaining this unrestricted flow path. First, the screen/casing annulus must be completely packed with gravel to ensure that gravel pack sand remains in the perforation tunnels, and that no formation material is produced into the wellbore. Second the perforation tunnels must be completely packed with gravel pack sand. Very high pressure drops can be added to the system, if even a very small portion of the perforation tunnel is filled with formation material rather than gravel pack sand. Finally, the effects of any near-wellbore damaged zone must be eliminated. Much work has already been published concerning techniques to optimize perforation filling, and to provide complete packing of the screen/casing annulus. It is the purpose of this paper to discuss procedures for eliminating the effects of near-wellbore formation damage.
As we seek to remove the effects of formation damage it must be kept in mind that all three of the concerns listed above must be addressed as a system. For example, when seeking to eliminate the effects of a near-wellbore damaged zone, we must take care not to sacrifice the ability to pack perforation tunnels, or the quality of the annular pack.
The effects of a damaged zone can be addressed in either one of two ways. The first is damage bypass, and the second damage removal. The primary damage bypass technique is to prepack a well at pressures that exceed the formation's fracture pressure. By exceeding the fracturing pressure of the formation, high permeability gravel or proppant can be forced out into the formation, and provide a highly conductive flow path through the damaged zone. Several techniques are available for accomplishing this, with brine fracturing and large-scale frac-packs using gel led carrier fluids being the most commonly applied. Both of these techniques have proven themselves to be very successful with nearly equivalent completion efficiencies (typical skins for both techniques in the 0 to 3 range).
The high-level of success achieved through damage bypass treatments lead to these techniques being the most commonly recommended. However, even with this high frequency of success there are times when it may not be required or desired to fracture a formation. Fracturing is typically not desired when the perforated interval is close to an oil/water or gas/oil contact. Even though the risk of fracturing out of zone can be reduced by using low viscosity fracturing fluids and restricting total job size, situations still exist where there are virtually no barriers to fracture growth. On the other hand, fracturing is probably not required for high permeability formations with only a moderate to light amount of damage. In these situations a damage removal technique can prove to be very effective.
Acid prepacking is the most commonly applied damage removal technique.