Design and Productivity of Gravel-Packed Completions

Abstract

Field-scale gravel-pack studies have shown that openhole gravel packs have higher productivity than cased-hole gravel packs; however, in the latter, well productivity can be improved by increasing the size and number of perforations. Results also show that cased-hole gravel-pack productivity can be enhanced by prepacking gravel outside the perforations. Introduction Gravel packing to exclude formation sand from produced fluids has been used as a completion technique for oil and gas wells for the past 40 to 50 years. However, the technology used in early gravelpack work was borrowed from the water-well industry, which for many years had been applying several types of mechanical sand retention devices to prevent sand production from produced groundwater.Early oil and gas completions, similar to those used in water wells, were what normally is referred to as openhole gravel packs (Fig. 1). Accurately sized gravel was placed around a slotted liner to act as a filter for formation sand. Subsequent completions sometimes consisted of cementing casing through the formation and perforating to communicate with the oil reservoir. This technique became common with the advent of gun and jet perforating, because the exclusion of water or gas was sometimes difficult to achieve in openhole gravel packs.If fluid production also resulted in sand production, cased-hole gravel packs were performed inside the well's casing (Fig. 1). The well productivity of these packs, however, quickly was recognized as being far below that experienced with the openhole packs. Field experience showed that, in general, the productivity of prepacked cased-hole completions was sometimes only 25 to 33% of that of the openhole pack, whereas the lack of prepacking could cause productivity losses of greater than 95%, as shown in Table 1.As a result of these problems, a whole new dimension in gravel packing arose with the cased-hole gravel-pack completion, since substantially different techniques and procedures are required to perform these completions properly in comparison with openhole gravel packs.The productivity limitations encountered in cased-hole gravel packs prompted numerous engineers and researchers to begin work to define and solve problems related to them. Early work on the proper gravel size required for effective sand control indicated that gravel/sand ratios of from 5:1 to 13:1 should be used. Formation sand design points anywhere from a cumulative of 10 to 70% based on a representative formation sand sample were recommended.Most of the early research was conducted in linear-flow packs operated under single-phase flow conditions. Additional work indicated that the main restriction to flow was sand-filled perforations and that for maximum productivity, plugging in the perforation with formation sand must be avoided. The remedy for this situation has been to pressure-pack the gravel through the perforation so that the gravel in the perforation is of the highest permeability capable of preventing the production of formation sand. JPT P. 1679＾