Laboratory Studies of Plugging and Clean-Up of Production Screens in Horizontal Wellbores
Authors: Lau Hon Chung , Davis Christopher L. 
Published: December 2007

Abstract
Tests were conducted to compare the capacity of various commercial production screens to tolerate solids in water-based drill-in fluids (DIF) used in drilling and completing horizontal wells. Screens tested included unconsolidated prepacks, pipe-based and rod-based wire-wrapped screens, porous metal membrane, and wire-cloth screens. For the DIFs tested, results showed that the solids tolerance of various screens ranked according to their pore opening and followed approximately the one-seventh rule. A DIF containing solids with D50 larger than one-seventh of the average pore opening of the screen would plug the screen. Results showed that medium screens (prepacks, porous metal membrane, and wire-cloth screen) were more prone to plugging by drill solids than non-medium wire-wrapped screens. Also found was that a combination of acid washes (using washcups) and acid soaks was more effective in cleaning up screen damage than acid soaks alone. Screen clean-up by this combination method was satisfactory for the screens tested except for the prepacks. Results also showed that flowback of filtercake from the formation sandface to the screen caused a reduction in screen permeability but was less than that due to DIF invasion during screen run-in. our results suggest that, if field conditions permit, the dirty DIF in the well should be replaced by a clean brine or a drill-solids-free DIF (solids-free DIF if possible) before running the screen into the well.

Introduction
Horizontal open-hole completions have gained wide acceptance in the Gulf of Mexico and worldwide. Many of these completions utilize a production screen as a stand-alone sand-control device. For well reliability purposes, it is important that these screens be undamaged during and after the completion process. However, a recent study has shown that sand-control screens exhibit various degrees of plugging by solids. There are at least three ways by which a production screen can be damaged in a horizontal well. The first way is by running the screen into a well containing the DIF. Invasion of the solids-containing DIF into the screen can plug the screen. Secondly, a screen may be damaged by flowback of filtercake or filtercake residue formed at the sandface to the screen. Thirdly, a screen can be plugged by sand/fines production during the production life of the well.
This paper was aimed at studying damage of various types of production screens by the first two mechanisms. Also studied was the effectiveness of acid soaks compared to a combination of acid washes and acid soaks in cleaning up damaged screens.
This paper is divided into three sections. The first section reports on a study to compare the solids tolerance of various screens by injecting a solids-laden DIF into the screen. The impact on plugging of the solid size relative to the pore opening of the screens was investigated. The second section reports on a study to compare the ease of clean-up of various screens damaged by DIF, using acid soaks versus a combination of acid washes and soaks. The third section reports on a study to determine the impact of flowback of DIF filtercake or filtercake residue from the formation sandface to the screen. Finally, field guidelines will be drawn from these studies.
It must be emphasized. however, that screen plugging and clean-up are not the only criteria for screen selection for horizontal wells. Other criteria include, but are not limited to, sand-control requirements, mechanical strength of the screen, erosion resistance, and corrosion resistance. All these criteria must be taken into consideration for site-specific screen selection.

Screen Plugging by Water-Based Drill-In Fluids
Drill-In Fluids Used. Three sized carbonate DWs were used in our tests. They were (1) an 11.0 lb/gal DIF used in drilling a deepwater horizontal well, (2) a pristine 11.9 lb/gal DW prepared in the plant for drilling another deepwater horizontal well, and (3) the pristine 11.9 lb/gal sized carbonate DIF with 15 lb/bbl Rev Dust added. Rev Dust is calcium montmorillonite commonly used to simulate effects of reactive drill solids. The pristine mud was hot rolled at 150 F for 48 hr before being used.
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