In order to properly design a completion, reservoir rock and fluid properties must be carefully taken into account because they directly influence on equipment selection. Reservoir properties (rock and fluid properties) which must be considered are as follows;
Low permeability formation may require fracturing operation to enhance production. The completion for tight formations must be able to withstand pumping pressure and allow fracking fluid and proppant to flow through.
Unconsolidated formations are required to complete a well with a sand control completion; thus, a well can be produced without any damage to downhole and surface equipment.
Reservoir pressure directly affects the pressure rating on all completions because all components must be able to work under reservoir condition. What’s more, formation pressure will affect how much flow of the well can produce.
High reservoir temperature will quickly degrade some components, especially elastomer, and this will result in well integrity issues due to pressure leakage. This is one of the critical concerns in selecting the right equipment to work under high temperature conditions.
Type Reservoir Fluid
Reservoir fluid directly affects the completion strategy. For gas reservoirs, there are some aspects for which oil reservoirs do not have the same level of concerns, such as velocity corrosion and rate dependent skin. Furthermore, Water Gas Ratio (WGR) is another critical part of tubing size selection because water has much higher density than gas and it will reduce the vertical lift performance (VLP) of the well. However, most of artificial lift methods as gas lift or down hole pumps cannot be used in gas wells.
For oil reservoirs, two key parameters, water cut and gas oil ratio, must be carefully accounted for when selecting the proper size of completion tubing. Another important part is an artificial lift planned to use in a well. Oil production can be enhanced by several artificial lift methods for instant down hole pumps, a beam pump, gas lift, etc. Hence, the artificial lift tool will affect size the completion string and production tubing.
PVT of Reservoir Fluid
A phase envelope of reservoir fluid analyzed from PVT data is imperative to completion selection because it informs about the condition of reservoir fluid at a down hole and a surface condition. For example, fluid at a reservoir condition is liquid, but when it flows to surface, it may become both liquid and gas. The mixture of the produced fluid influences the completion size selection, completion strategy and available recovery enhancement methods.
H2S content will accelerate corrosion of all components and potentially harm human life. Consequently, for reservoirs with a high level of H2S, all completion components both metal and non-metal must be sour service material. Example of sour service grade metal is VM SS grade (Vallourec Oil & Gas, 2015). Internal plastic coating is also a good method to prevent H2S corrosion. Furthermore, a completion string must be designed to facilitate corrosion a inhibitor injection.
CO2 reacting with water causes acid which will increase the corrosion rate on steel components. Special steel for instant Nickel-and Cobalt-Based Alloys, 13%Cr steel or Duplex Stainless Steel must be utilized to ensure well integrity. Elastomer in CO2 service is recommended to use nitrile sealing materials (Stone, et al., 1989). Additionally, completion must provide access to inject any corrosion inhibitor.
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