Completion techniques

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As noted earlier, it is possible tо complete horizontal wells as open hole, with slotted liners, liners wilh external casing packers (ECPS), and cemented and perforated liners. The choice of completion method can have a significant influence on well performance. The various completion options and their advantages and disadvantages are summarized below. Ad­ditionally, various issues that need to be considered before choosing com­pletions are also briefly discussed.

Сompletion options

Figure 1-11 shows a schematic diagram of various completion options for horizontal wells.

Open-hole completion is inexpensive but is limited to competent rock formations. Additionally, it is difficult to stimulate open-hole wells and to control either injection or production along the well length. A few early horizontal wells have been completed open hole but the present trend is away from using open hole completions, except in formations such asAustin Chalk.

It is important to note that many horizontal wells, especially in offshore Europe and Asia, have been successfullycompleted using slotted liners. In many of these wells, flow rates exceed a few hundred to thousands of barrels per day. At a high flow me the well has a better chance of self cleanup than at low flow rates.

A cemented horizontal well is preferred if the well is to be fractured. The well can be isolated in several zones along its length by using bridge plugs and each zone can be fractured inde­pendently. Recently, several wells have been completed by inducing multiple fractures along the well length. From a mechanical slandpoint, it is preferable to fracture various zones along the well length in stages. It is prudent to use reservoir engineering criteria to design the number of fractures required along the well length, to maximize recovery and minimize fracturing cost.

Recently, several papers have been published, which discuss the orienta­tion of induced fractures with respect to drilled direction of a horizontal well. In principal, for primary production one would like to obtain fractures perpendicular to the drilled wellbore direction and to enhance drainage. However, such a configuration is not desirable for secondary recovery ap­plications. In a developed pattern, induced fractures perpendicular to the well direction would cause short-circuiting of the injection fluid, resulting in an early breakthrough and poor sweep efficiency. Thus, if secondary recovery is contemplated, inducing a long fracture perpendicular to a horizontal well's direction should be avoided. For secondary recovery an induced fracture paralled to the horizontal well's direction may be de­sirable.

It is difficult to fracture wells completed as an open hole or with a slotted liner. This is because of large leak-off occurring along the well length. Similarly, uniform acidization along the length of a well completed as open hole or with slotted liner is also difficult. The difficulty in uniform acidization along the well length can be reduced by using coiled tubing. To ensure uniform acid distribution along the well length, the coiled tubing may have to be moved up and down the hole while spraying the acid. Another alter­native is to use a staged acidization treatment where each stage is separated by a chemical diverter.

In some wells, especially those drilled in a fractured reservoir with a bottom water drive, water may break through in acertain portion of the long horizontal well. Similarly, in an enhanced recovery application, the injected fluid, such as water, may show a premature break through along a small portion of a long producing horizontal well. In such cases, one may have to plug off a certain portion of a long well.

The effective way to plug off well length is to isolate the zone where un­desirable fluids are entering the well, squeeze that zone off using cement. A completion plan should include design considerations for such contin­gencies.

In reservoirs with gas caps, it is important to obtain effective isolation well from the gas cap. One can either use packers or cemented liners to isolate production tubing from the gas cap. Literature indicates that some horizontal wells were not able to meet their expectations due to premature gas breakthrough in the well portion, located in the proximity of the gas cap.

Rarely are horizontal wells truly horizontal; rather, they wander up and down in the vertical plane. In low rate wells, well shapes can have significant impact on well productivity, especially when multiphase flow is involved. For example, water may accumulate in a low portion of the well, and it may be difficult to displace it. Similarly, there is a possibility of gas lock occurring near hook-shaped well portions. In such situations, gas anchors can be used to mitigate the problem. However, the best way to handle this problem is to design a well path slightly up-dipping or down-dipping, depending upon the reservoir mechanism. This will fa­cilitate fluid segregation along the well length and reduce problems due to gas blocking in oil wells and liquid loading in gas and condensate wells.

Before selecting a completion option, workover requirement must be considered, but they are also difficult to anticipate. For example:

a) One can insert a slotted liner

b) One can cement the well and perforate it.

c) One can complete the well as an open hole and wait until water breakthrough occurs to design a course of action.