Simple Approach to the Cleanup of Horizontal Wells With Prepacked Screen Completions
Authors: Browne S.V. , Ryan D.F. , Chambers B.D. , Gilchrist J.M. , Bamforth S.A. 
Published: December 2007

Original SPE manuscript received for review May 15, 1995. Paper peer approved June 14, 1995. Paper (SPE 30116) first presented at the 1995 SPE European Formation Damage Conference held in The Hague, The Netherlands, May 15-16. Journal of Petroleum Technology, September 1995.

Summary
Openhole completions with prepacked screens are increasingly the completion of choice for horizontal wells requiring sand control. For maximum productivity from these wells, preventing mud damage to the formation and the screens or removing it before bringing the well onto production is vital. A common industry approach to this problem is to displace the drilling mud to an aqueous clear fluid before production, typically followed by a breaker fluid. This paper details an alternative approach: bringing the well on without cleanup. The paper outlines the decision-making process, shows how the drill-in-fluid and cleanup operations are designed, explains how laboratory testing can be used to support the design philosophy, and highlights that quality control of the mud system while drilling is critical to the success of such a well. Case histories in two very different reservoirs are presented that illustrate the success of this approach.

Introduction
The use of high-angle and horizontal wells is increasingly widespread, primarily because of the enhanced productivity they provide from many types of reservoirs. These wells often use uncemented predrilled liner completions or, for wells requiring sand control, prepacked screens. Because the wells are not perforated, mud damage cannot be bypassed. Therefore, to maximize productivity, preventing or removing mud damage to the formation and the screens before bringing the well onto production is vital. Mud damage can affect well productivity in a number of ways.Formation damage to the rock matrix owing to mud-filtrate invasion. This is almost impossible to remove but can be minimized by appropriate mud design.Internal and external mud filter cake. Oil or gas must be produced through the cake in uncemented completions. Considerable pressure drop may be required to initiate production.Plugging of sand-control screens by insoluble drilling solids or weighting material [mud or mud filter cake (from the wall of the hole)] is backflowed into the well.
The most common industry approach to this problem is to displace water- or oil-based drilling mud to a clear aqueous fluid before production. In addition, a breaker system usually is placed in the open hole to remove or destabilize filter cake, thus reducing the drawdown required to initiate production. However, this approach can lead to problems on many wells.Breakers will often cause significant loss of fluid to the reservoir. This can result in formation damage from the breaker system itself and indirectly from subsequent kill pills. Severe losses also can compromise well control.Available pit space limits the number and volume of pills that can be pumped at many offshore locations.For certain oil-based muds (OBM's), displacement to completion brines can result in the formation of thick damaging sludges owing to poor displacement and fluid incompatibilities.Despite use of corrosion inhibitors, acid breakers can cause corrosion problems for metals commonly used in screen design, such as 316L stainless steel.
Novel laboratory testing has lead to questioning many of the assumptions used in horizontal well cleanup. Depending on reservoir characteristics and inflow performance, the use of breakers is not al-ways necessary and can actually decrease well deliverability. Also, mud systems with fluid-loss control can be formulated that will flow through screens, eliminating the need to displace to clear fluids.
On the basis of these findings, a simpler approach has been taken on two recent horizontal wells drilled with OBM systems. These wells were successfully brought on production with low-solids drilling fluid in the openhole section. Even though no conventional cleanup operation was carried out on these wells, comprehensive well-testing and production-logging programs showed that these wells were producing with minimal productivity impairment. In our opinion, this approach was not only operationally simpler but also carried a lower risk of productivity impairment for these wells.

Cleanup Design
System Design Parameters. Determining how the well should be completed and cleaned up requires an integrated understanding of a number of factors.Wellbore geometry (vertical, slanted, or horizontal well).Interaction between the drilling fluids and the formation to be completed.Reservoir quality (permeability, heterogeneity, etc.).Well/reservoir production management (sandface drawdowns available/permissible).
For example, optimal completion of a horizontal well in a very-high-permeability oil sand where sandface drawdowns are on the order of a few psi will be very different from that of a slant well in a tight gas sand where drawdowns may be in the region of thousands of psi. Laboratory core testing must be integrated with inflow modeling and a clear definition of well objectives to quantify the effect of mud damage on well performance.
Fluid-System Design. To determine whether the mud system proposed for use on a openhole completed well is likely to reduce well productivity, a number of different laboratory tests should be carried out.
Reservoir-Condition Coreflood Test. A reservoir-condition core flood test is a standard way of assessing the level of damage that a mud filtrate might induce in reservoir core. In this test, the level of permanent formation damage can be determined by comparing the permeability of the core before and after its exposure to mud filtrate. Mud-filtrate damage levels ranging from 0% to 99% permeability reduction have been recorded. Note that damage levels of less than 50% have minimal effect on the productivity of horizontal wells. When interpreting the results from this type of experiment, care must be taken to ensure that the test really does simulate field events.
Mudcake-Cleanup Test. A mudcake-cleanup test is a novel test that can be conducted to establish a "lift-off pressure." In this test, the core permeability at reservoir temperature and pressure is first established. A mud filter cake is then laid down on the face of the core. Finally kerosene is produced in the reverse direction, simulating production. The lift-off pressure, which can be attributed to the partial removal of internal and external cake required to allow hydrocarbon production, is calculated from the initial pressure peak (Fig. 1). Flow is maintained until a steady-state pressure is recorded from which a final (damaged-core) permeability is obtained.
In well-consolidated reservoirs, this testing procedure is fairly representative of initial production conditions.
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