Considerations in Gravel Pack Design

Tests with physical models have shown that sand production and pack impairment are minimized when the ratio of pack median grain size to formation median grain size is between 5 and 6. In a study of the inertia and viscosity effects of flow in gravel packed wells it was found that increasing the size and the density of the perforations should increase productivity. Introduction Gravel packs have been used extensively along the Louisiana Gulf Coast in an effort to reduce or avoid sand production from unconsolidated formations. Statistics show, however, that through 1966, gravel packs were only about 70 percent successful.*The early literature on gravel pack design is based primarily on the work of Coberly and Wagner and of Hill. Coberly's work in essence suggested that a gravel pack having granular particles of diameter 10 times the formation grain size at the 10-percent-coarse point on a cumulative sieve analysis would provide effective sand control. Numerous failures of this criterion were noted, especially in the Gulf Coast sands. Hill suggested that the ratio of 10 be reduced to 8. Failures were still noted in many applications, At least one writer suggested concentrating on the "fines" end of a cumulative sieve analysis. Winter-burn states that "actual experience in the field has shown that sand entry can virtually be eliminated by the use of gravel which is approximately 10 times the grain size of the 10 percentile of the finest sand to be screened." Clearly a finer gravel will be more effective in screening formation particles. However, it must be evaluated in the light of how the finer gravel affects permeability and reduces production.Depending upon the writer, recommended ratios of gravel to the 10-percent-coarse point may range from 4 to 13. Other suggestions appear in the literature; see, for example, the paper by Tausch and Corley for a summary of earlier gravel pack investigations.In more recent literature, Sparlin has discussed gravel placement rate and fluid viscosity in his recommendations for a "slurry pack." Schwartz recommends a size ratio of 6 at the 10-percent-coarse point and at the 40-percent point for uniform and nonuniform sands, respectively. Williams uses Schwartz's grain-size ratio of 6 and finer and discusses well productivity as a function of perforation size and density.In addition to the apparent disagreement on the geometrical basis for gravel pack design, a more subtle lack seems to exist. Few writers have explored the influence of flow parameters on the functioning of gravel packs. Sage and Lacy appear to be the first authors who attempt to take hydrodynamic (and other) factors into consideration. We supposed that under certain conditions, hydrodynamic factors could possibly outweigh geometric factors in the functioning of gravel packs. This investigation, begun in 1967, proceeds from that basic premise. Laboratory Test and Evaluation Program A large number of variables are involved both directly and indirectly in gravel pack behavior. Five were considered to be most significant and fundamental to the study. JPT P. 205^