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Abstract
Formation damage created during drilling or workover operations significantly reduces the performance of many wells. Long, horizontal and multilateral wells crossing heterogeneous, possibly multiple, reservoirs often show greater formation damage than conventional wells. This is partly due to the longer exposure of the formation to the drilling and completion fluid due to the well geometry as well as to the greater overbalance pressure often applied during drilling such wells and poorer cleanup.

The typical well clean up process involves flowing the well naturally or aided by artificial lift to remove the external and internal mudcake and flow-back the mud filtrate. This process can be effective in conventional wells but is not adequate in long horizontal and multilateral wells suffering from increased frictional pressure drop along the wellbore and heterogeneity. The cleanup efficiency is improved by employing Advanced Well completions. Inflow Control Valves (ICVs) control the contribution from individual laterals or a specific zone along the extended horizontal wellbore. Inflow Control Devices (ICDs) equalise the contribution along the (long) completion length. In addition, Autonomous ICDs can manage the influx of unwanted fluids.

This paper studies the cleanup performance of such wells completed with these advanced, downhole flow control technologies. It provides valuable insights into how these completions improve the well cleanup process and compares the ability of (A)ICD and ICV technologies to provide the optimum:

• Drawdown to lift off the filter cake formed by different mud systems (without causing sand production).
  
• Recovery rate of the invaded mud filtrate.

Guidelines for Advanced Well Completion cleanup along with simulated results of synthetic and real field cases are included.

1 Introduction
Formation damage is a deterioration of the near wellbore, reservoir formation characteristics. It has been described as: “The impairment of the invisible, by the inevitable and uncontrollable, resulting in an indeterminate reduction of the unquantifiable” [1]. Its causes include: “physico-chemical, chemical, biological, hydrodynamic, and thermal interactions of porous formation, particles and fluids and mechanical deformation of formation under stress and fluid shear” [2]. These processes can be triggered at all stages of the well or field’s life: drilling, workover, completion, gravel packing, production, injection, stimulation, etc. Formation damage reduces the absolute formation permeability and/or causes an unfavourable relative permeability change; both of these will adversely impact the well and reservoir performance.

Increasing the well-reservoir contact has become an increasingly popular well construction option. It brings a number of potential advantages - increases in the well productivity, drainage area and sweep efficiency plus delayed water or gas breakthrough. Drilling, workover and (re)completion are all major interventions that result in severe formation damage in Extended Reservoir Contact (ERC) wells. External and internal mudcakes are often formed at the sandface in addition to mud filtrate invasion into the near wellbore area during these interventions. Increased levels of formation damage is to be expected in ERC wells compared to conventional wells due to the increased exposure to the reservoir, use of a higher overbalance pressure and the increased time required to drill and complete these wells.

Both water and oil based mud are used to drill ERC wells. Polymers are added to these mud systems to enhance their ability to suspend drill cuttings within the long and tortuous wellbores so that they can be circulated to surface. These polymers will absorb on water wet, formations; altering the irreducible water saturation around the wellbore and complicating the water based filtrate’s flow back during the cleanup process.