The primary objectives of a gravel pack are preventing formation sand production, achieving high productivity, and providing completion longevity. For cased hole completions, inadequately filling the perforation tunnels with gravel is probably the main cause of formation damage which ultimately translates into a well that is completion limited since it produces below the capacity of the reservoir. Hence, performing prepacking operations that fill the perforations with undamaged gravel will usually enhance productivity. Aside from prepacking, effectively packing the casing-tubing annulus produces a stable, void-free gravel pack that will not settle or deteriorate with time.

Gravel pack procedures were developed in the mid-1970's and subsequently optimized to address the above concerns. They consisted of preacidizing (if required) the formation followed by a prepack performed with lightly-gelled water pumped at matrix rates that filled the casing with gravel to the top of the perforations. After washing the prepack gravel from the casing, the gravel pack assembly was run and an annular gravel pack was performed using water as the transport fluid. This procedure provided two opportunities for gravel to prepack the perforations. Acidizing was subsequently performed to achieve the desired productivity. Well test data show that over 70% of the wells tested had skin factors that were less than 10 and many were in the zero range. Gravel pack failures were rare and over 95% of these completions produced to depletion without requiring a workover.

More recently, enhanced prepacking has been performed by pumping at high rates and using water as the transport fluid. These treatments were designed to fracture the formation a distance of 5-10 ft from the well to bypass formation damage. The high rate prepacks have been performed with either the screen in place (single step) or prior to running the screen (two-step) in the well. In either case, the annular pack continued to be performed with water. The high rate completions have been performed at little or no incremental cost compared to those conducted at matrix rates since they were performed with platform-based, moderate horsepower equipment. The performance of these completions has been excellent. The need for postcompletion acidizing and associated flowback concerns has been virtually eliminated. Wells clean up more rapidly, consistently have higher productivities, and appear to have improved completion success as compared to those performed at matrix rates.

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