RELATION OF MUD CONDITIONING AND SCREENS

Parameters affecting shaker Screen Impairment

While a great deal of effort had focused on the formation damage potential of drill-in fluids, this effort is now slowly shifting towards a better understanding and controlling of shaker screen impairment by these fluids. In 1997, two comprehensive laboratory studies specifically investigated plugging of shaker screens with drill-in fluids (DIF). A number of parameters were investigated and showed varying impact on final shaker screen productivity. According to these studies, several critical factors affecting shaker screen impairment were identified:

• the relationship between the shaker screen ‘slot size’ and the particle size distribution of the solids present in the DIF (weighting agent and drill solids);
• the amount of solids in the DIF (a high density and/or dirty mud plugs a shaker screen faster than a light and/or clean one);
• the type of mud and LCM material used (the conditioning and particle-particle interactions affect mud dispersion and its plugging tendency).

Another parameter playing a role in enhancing (or reducing) shaker screen impairment is the amount of open pores available to mud flow. For a given sand retention characteristics, a screen medium with a higher void volume resist impairment better than another one. Figure 2 compares the amount of mud required to plug two shaker screens with the same sand retention characteristics (90% at 110-120 μm) but different void volume (as measured by helium pycnometry). This result is confirmed by sand retention tests where high void volume or high open area shaker screen leads to a significant increase in plugging resistance compared to the woven mesh.

Preventing Impairment with Effective Mud Conditioning

A light mud weight generally minimizes shaker screen plugging. Even in this case, DIF conditioning is required to prevent shaker screen impairment (ie use of fine shaker screens to remove drill solids and large mud aggregates). The study of Marken et. al. suggests that a 200 mesh shaker provides adequate protection for a 100-150μm shaker screen. Actual field data reported by Browne et. al. demonstrated the use of 230 and 300 mesh shaker screens to protect 20/40 prepack shaker screens.

The latter report provided some other useful recommendations. It indicated that significant mud crossflow through the shaker screen may be taking place during the actual running of the screen caused by surge pressure. Deploying the screen slowly reduced but did not eliminate crossflow. Impairment could be minimized by displacing the top hole to a solids-free fluid before running the shaker screen. The solids-free fluid filled up the void between the shaker screen and the washpipe, thus reducing the impact of mud contamination during deployment. Additional tests by Hodge et. al. compared the plugging performance of sized salt, calcium carbonate, and a clear mud systems without clean-up. Although the different mud systems exhibited significantly different shaker screen plugging tendencies, the introduction of drill solids into the mud systems resulted in virtually identical and unacceptably poor performance. In all tests, the return permeability of the 40/60 consolidated shaker screen was less than 1% of the initial permeability, a permeability reduction of enough magnitude to affect well productivity. Acid cleanups did not necessarily guarantee a clean screen, and the breaker systems were very sensitive to corrosion inhibitors.

The study by Lau and Davies suggested a solution to this potential source of impairment: acceptable screen permeability returns were achieved on shaker screens impaired with DIF and drill solids once a ‘backwash’ step was implemented (this was effective only on surface filter media as depth filters such as prepacked shaker screen could not be cleaned effectively in any case). By pulling a washpipe equipped with washcups (6ft apart) at 6 ft/min while circulating at 3/4 bbl/min a solidsfree fluid (through the shaker screen, inside the annulus and back inside the tubing through a sliding sleeve), the filtercake deposited on the outer shaker screen surface could be removed.

In a recent paper, Price-Smith et. al. provided a State-of-the- Art review of open hole horizontal well clean-up practices.12 In the field cases described, completions designed to produce mud directly through the shaker screen are limited to low density muds (< 1.3 s.g.) and a washpipe is always recommended to allow proper fluid circulation around the shaker screen.

Oil based mud systems are often not displaced and are typically produced at the onset of oil production. The best method to ensure good return permeability in this case is to condition the mud through fine shaker screens so that most of the insoluble solids remaining in the mud will be able to pass through the sand control screen upon production. Recommended mud conditioning guidelines is to use shakers mesh sizes on the order of 1/5 to 1/4th of the shaker screen openings.

Mud quality and the integrity of the solids control system on the rig must be carefully monitored as a small amount of contamination by a coarse mud fraction (5-10%) may be enough to induce significant screen impairment when producing the mud through the shaker screen at the start of production.23 In appendix, a mud test procedure is described that can be used both in the lab and on the rig to determine the quality of the mud for flowback and its screen damage potential. It is critical that such a technique be implemented operationally, especially when it is planned to produce the mud through the shaker screen.

From the results shown in Figure 1 and Table 1, reducing contact between the shaker screen surface and the wellbore walls considerably limit the onset of flow convergence associated with partial screen blockage. Using centralizers to maintain a standoff between the shaker screen and the wellbore walls is strongly recommended for screen only completions.

Table 1. Effect of screen clean-up and gravel packing on flow convergence and well productivity: Case of a 1000 ft well producing 5000 bpd).
	Reservoir	PI/PIo 100 % Clean-up	PI/PIo 10% Clean-up 10 cm Openings	PI/PIo 10% Clean-up 1 cm Openings
No Gravel Pack	Homogeneous 100 mD	0.96	0.88	0.96
	Homogeneous 1000 mD	0.94	0.86	0.93
	Heterogeneous 90 % 100 mD 10% 1000 mD	0.86	0.70	0.83
Gravel Pack	Homogeneous 100 mD	0.97	0.97	0.97
	Homogeneous 1000 mD	0.95	0.95	0.95
	Heterogeneous 90 % 100 mD 10% 1000 mD	0.88	0.87	0.88