Offshore reservoirs requiring sand control pose a major completion challenge because of extremely high cost and risk involved in remedial treatments, particularly in sub-sea completion and/or deep-water environments. It is therefore of utmost importance to ensure sand control without sacrificing flow conformance, recoverable reserves and well deliverability throughout the expected life of the completion. A major trend in these environments is towards open-hole, horizontal, gravel-packed completions. Although gravel packing stabilizes the wellbore, it can also entrap the filter-cake formed by the reservoir drilling fluid, potentially resulting in high drawdown requirements (flow initiation pressures) and/or low production rates (retained permeabilities). The cleanup procedures in the industry have varied significantly from no cleanup at all to complicated two-stage breaker treatments involving post-completion coiled tubing intervention, with no guidelines existing in the literature. In this paper, we present experimental results and field cases involving filter-cake flow-back through gravel packs with and without cleanup. Effects of various parameters, including gravel size (40/60, 20/40, and 12/20), formation permeability, drill-solids type (clays, quartz) and concentration, and the type of cleanup fluid have been investigated. Flow initiation pressure and retained permeabilities during flow back are reported as a function of these parameters. The experimental results show that the flow initiation pressure is a strong function of gravel size and the type of drill solids. It is concluded that, in clean (low-to-no clay content) formations of large grains and high permeabilities (~ several darcies) requiring large gravel sizes (e.g., 12/20), an enzyme or an oxidizer treatment is sufficient based on laboratory results and productivity predictions. This conclusion is also supported by several field applications as shown. In lower permeability (~ 100–250 md) formations of small sand sizes requiring smaller gravel (e.g., 40/60) elimination of both the fluid loss control agent (starch) and bridging agent (CaCO3) is necessary based on high flow initiation pressures and low retained permeabilities. In intermediate permeability (~ 500–800 md) formations of medium size sand-grains typically requiring 20/40 gravel, the results depend strongly on the type of drill solids: in clean formations (no clays in drilling fluid), an enzyme or an oxidizer treatment is sufficient, while in dirty formations removal of both CaCO3 and starch is necessary. These results are also supported by field case histories presented in the paper. Introduction Gravel packing has been gaining wider popularity in open-hole horizontal completions where sand control is required, particularly in sub-sea completion and/or deep-water environment. The cost of intervention in such cases makes risk mitigation a much more pronounced task. Until recently, a large majority of horizontal sand control completions have utilized standalone screens. However, because a substantial fraction of these wells have failed prematurely (either productivity loss due to screen plugging or loss of sand control due to screen erosion),1 many operators have changed their primary completion technique in these wells from standalone screens to gravel packing. This is particularly true in formations containing a large fraction of non-pay (shale, mudstone/siltstone) and/or have a wide particle size distribution.2

Wellbore cleanup in horizontal, open hole sand control completions has been the subject of many publications in recent years. Although a large majority of horizontal wells have been standalone screen completions, an increasing number of these wells are being gravel-packed, particularly in deep water, sub sea environment where reliability of the sand face completion is of utmost importance due to prohibitively high cost of intervention. In such wells, increased significance of "doing it right the first time" further necessitates an emphasized consideration of wellbore displacement and filter cake removal treatments. Although a substantial amount of laboratory data on filter cake cleanup are available in the literature, a great majority of these data are relevant to non-gravel-pack completions. In field practice to date, cake cleanup in GP completions has almost exclusively been done after gravel packing and typically involved coiled tubing. Although several new methods have recently been proposed and successfully practiced in several applications (e.g., inclusion of cake-breaking chemicals into gravel-pack carrier fluids, post-GP breaker treatments immediately after GP w/o requiring coiled tubing), laboratory data directly applicable to such conditions have been scarce. In this paper, we present laboratory data relevant to gravel-packed completions. We show that the cake removal time scales in the presence of a gravel-pack are longer compared to absence of a gravel-pack on top of the filter cake. The degree of delay is shown to depend primarily on carrier fluid viscosity and whether the breakers are included in the carrier fluid or introduced as a post-GP treatment. It is further shown that including breakers in brine during water packing must be exercised with extreme caution since even slow-reacting breakers can yield premature screen out due to increased losses should external cake erosion occur. Introduction A large fraction of wells drilled in reservoirs requiring sand-control are being completed as horizontal open holes. Although a large fraction of these wells have been completed with screens-only, an increasingly higher number of these wells are being gravel packed, particularly in deep water and sub-sea completion environment where reliability of the sand face completion is of utmost importance due to prohibitively high cost of intervention. Furthermore, because the costs associated with filter cake cleanup treatments are typically marginal compared to potential intervention costs, a thorough cleanup treatment is considered an integral part of the completion in such wells, in order to maximize well productivity and longevity, and provide more uniform production profile and avoid premature water or gas breakthrough, although the actual cleanup methodology, including the type of chemicals and the placement techniques may vary significantly. Cake cleanup in gravel pack completions has traditionally been done after gravel packing, and typically with coiled tubing.1 Several new techniques have recently been proposed.2,3 These involve incorporating breakers into the gravel-pack carrier fluid to place the cake-breaking chemicals into the wellbore during gravel packing, as well as a modified service tool that can be used to displace wellbore fluids and spot breaker solutions by allowing circulation down through the wash pipe and up through the wash-pipe/base-pipe annulus.2

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractOffshore reservoirs requiring sand control pose a major completion challenge because of extremely high cost and risk involved in remedial treatments, particularly in sub-sea completion and/or deep-water environments. It is therefore of utmost importance to ensure sand control without sacrificing flow conformance, recoverable reserves and well deliverability throughout the expected life of the completion. A major trend in these environments is towards open-hole, horizontal, gravel-packed completions. Although gravel packing stabilizes the wellbore, it can also entrap the filtercake formed by the reservoir drilling fluid, potentially resulting in high drawdown requirements (flow initiation pressures) and/or low production rates (retained permeabilities). The cleanup procedures in the industry have varied significantly from no cleanup at all to complicated twostage breaker treatments involving post-completion coiled tubing intervention, with no guidelines existing in the literature.