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A ''workover'' is any operation done on, within, or through the wellbore after the initial completion. Although proper drilling, cementing, and completion practices minimize the need, virtually every well will need several workovers during its lifetime to satisfactorily fulfill its purpose.

A ''workover'' is any operation done on, within, or through the wellbore after the initial completion. Although proper drilling, cementing, and completion practices minimize the need, virtually every well will need several workovers during its lifetime to satisfactorily fulfill its purpose.

To help understand why workovers are required, it is useful to visualize a wellbore. A typical wellbore schematic is shown in Figure 1. The production casing is only 5 to [[length::9 in.]] in diameter, and it has been run into a corkscrew-shaped hole that is not entirely circular and may be substantially deviated from vertical. An attempt has been made to cement the casing centrally, at least across the productive intervals, to obtain a hydraulic seal and casing support. Inside the production casing, even smaller equipment is run for completion and production or injection purposes. Figure 2 illustrates the tight fit of these concentric strings. Although the diameters are small, the surface area of the equipment is significant and may be exposed to corrosive fluids and pressure for long periods of time.

To help understand why workovers are required, it is useful to visualize a wellbore. A typical wellbore schematic is shown in [[:file:workovers_fig1.png|Figure 1]]. The production casing is only 5 to [[length::9 in.]] in diameter, and it has been run into a corkscrew-shaped hole that is not entirely circular and may be substantially deviated from vertical. An attempt has been made to cement the casing centrally, at least across the productive intervals, to obtain a hydraulic seal and casing support. Inside the production casing, even smaller equipment is run for completion and production or injection purposes. [[:file:workovers_fig2.png|Figure 2]] illustrates the tight fit of these concentric strings. Although the diameters are small, the surface area of the equipment is significant and may be exposed to corrosive fluids and pressure for long periods of time.

 

 

Another visualization to keep in mind is the very small wellbore size relative to the reservoir area to be drained by that well. A minor restriction (such as scale, fines, paraffin, or pipe dope) at the wellbore has a significant effect on production rate and ultimate recovery.

Another visualization to keep in mind is the very small wellbore size relative to the reservoir area to be drained by that well. A minor restriction (such as scale, fines, paraffin, or pipe dope) at the wellbore has a significant effect on production rate and ultimate recovery.

==Reasons for workovers==

==Reasons for workovers==

Workovers may be required for one or more of the following reasons:

Workovers may be required for one or more of the following reasons:

The majority of workovers are done because the well is not performing up to expectations (see [[Production problems]]). Careful analysis will determine if the decline is abnormal or the result of normal reservoir depletion. If deemed abnormal, it must be determined if the problem is in the reservoir inflow system, the wellbore outflow system, or both. Inflow problems can be corrected with [[stimulation]] procedures such as acidizing, fracturing, scale, or paraffin treatments (see [[Stimulation]]) or by reperforating or additional perforating. Outflow problem resolution may require equipment changes, cleanouts, or chemical treatments.

The majority of workovers are done because the well is not performing up to expectations (see [[Production problems]]). Careful analysis will determine if the decline is abnormal or the result of normal reservoir depletion. If deemed abnormal, it must be determined if the problem is in the reservoir inflow system, the wellbore outflow system, or both. Inflow problems can be corrected with [[stimulation]] procedures such as acidizing, fracturing, scale, or paraffin treatments (see [[Stimulation]]) or by reperforating or additional perforating. Outflow problem resolution may require equipment changes, cleanouts, or chemical treatments.

Mechanical problems such as failures of the cement, tubulars, packers, wireline components, safety valves, or [[artificial lift]] equipment are also reflected in the well performance. Cement and casing problems are most often corrected by squeezing cement into the damaged areas (Figure 3). Very serious problems of this nature occasionally require a sidetracking operation. Depending on the initial wellbore configuration, some mechanical problems can be corrected by applying wireline methods. More often, however, the equipment must be pulled using a rig. This may be a routine operation, but it may be a major task if the equipment is stuck or has parted downhole. To recover parted or stuck downhole equipment, a [[fishing]] operation may be required (see [[Fishing]]).

Mechanical problems such as failures of the cement, tubulars, packers, wireline components, safety valves, or [[artificial lift]] equipment are also reflected in the well performance. Cement and casing problems are most often corrected by squeezing cement into the damaged areas ([[:file:workovers_fig3.png|Figure 3]]). Very serious problems of this nature occasionally require a sidetracking operation. Depending on the initial wellbore configuration, some mechanical problems can be corrected by applying wireline methods. More often, however, the equipment must be pulled using a rig. This may be a routine operation, but it may be a major task if the equipment is stuck or has parted downhole. To recover parted or stuck downhole equipment, a [[fishing]] operation may be required (see [[Fishing]]).

 

Initiation of supplemental recovery projects causes workovers because many wells must be converted to injectors, observation wells, or disposal wells. Also, all wells need to be completed in correlative zones, thus a significant number of wells need perforating and stimulation and/or zones shutoff via squeeze cementing or plugs (cement or mechanical).

Initiation of supplemental recovery projects causes workovers because many wells must be converted to injectors, observation wells, or disposal wells. Also, all wells need to be completed in correlative zones, thus a significant number of wells need perforating and stimulation and/or zones shutoff via squeeze cementing or plugs (cement or mechanical).

* In thermal projects, the elevated temperatures create special needs for doing a workover safely.

* In thermal projects, the elevated temperatures create special needs for doing a workover safely.

* The condition of the well equipment may be a major factor in what and how much is done in a particular workover. The toll taken by corrosion, erosion, and mechanical wear may be significant, particularly if the casing is involved. Extra funds and time may need to be allocated to cover potential squeeze cement jobs, fishing operations, and equipment replacement.

* The condition of the well equipment may be a major factor in what and how much is done in a particular workover. The toll taken by corrosion, erosion, and mechanical wear may be significant, particularly if the casing is involved. Extra funds and time may need to be allocated to cover potential squeeze cement jobs, fishing operations, and equipment replacement.

==Types of workovers==

==Types of workovers==

Operations typically done in an existing zone may include stimulation, reperforating, perforating additional intervals, and plugging off unwanted perforations (because of high gas or water production).

Operations typically done in an existing zone may include stimulation, reperforating, perforating additional intervals, and plugging off unwanted perforations (because of high gas or water production).

Recompletion work varies depending on whether the new zone is above or below the currently open zone. If above, the lower zone will be abandoned via squeeze cementing (Figure 4), a cement plug, or a mechanical bridge plug, and the zone of interest will be perforated and stimulated. Appropriate outflow equipment will then be installed. If the new zone is below, the existing zone will probably be squeeze cemented (in the same manner as repairing casing, as shown in Figure 3). The cement in the wellbore is drilled out, the lower zone perforated and stimulated, and production equipment run. Some wells have had sufficient well work done upon initial completion to permit recompletion by simply using a wireline to run plugs and open sleeves.

Recompletion work varies depending on whether the new zone is above or below the currently open zone. If above, the lower zone will be abandoned via squeeze cementing ([[:file:workovers_fig4.png|Figure 4]]), a cement plug, or a mechanical bridge plug, and the zone of interest will be perforated and stimulated. Appropriate outflow equipment will then be installed. If the new zone is below, the existing zone will probably be squeeze cemented (in the same manner as repairing casing, as shown in [[:file:workovers_fig3.png|Figure 3]]). The cement in the wellbore is drilled out, the lower zone perforated and stimulated, and production equipment run. Some wells have had sufficient well work done upon initial completion to permit recompletion by simply using a wireline to run plugs and open sleeves.

 

Wellbore workovers can include casing or equipment repair, but may also simply involve cleaning out fill over the producing zone or circulating chemicals to remove scale or paraffin.

Wellbore workovers can include casing or equipment repair, but may also simply involve cleaning out fill over the producing zone or circulating chemicals to remove scale or paraffin.