6.0WELL SERVICING AND WORK OVER6.1 INTRODUCTION

Work over operations is basically remedial measures that are carried out in a well to add/ restore the production from the well.

The work over performance is gauged through Work Over Index which is defined as average number of wells worked over per rig per year.

6.2 NEED FOR WORK OVER

Several problems contribute to a decrease in productivity from a well such as:

1. Well bore and reservoir problems:

a. Reservoir problems Low reservoir pressure & reservoir pressure, small pay zone thickness

b. Fluid related problems water & gas coning, formation of precipitates & scale, flow of heavy viscous oil

c. Around well bore problems accumulation of formation fines in well bore & plugging of perforations

2. Casing damage/ leak or wellhead component/ seal failures

3. Production of sand, paraffin and scale deposition in flow conduit

4. Failure of completion or A/L equipment.

5. Bad cementation and channeling behind casing resulting in production of undesired water and gas from other layers

6. Loss of hydrocarbon of present layer to other layer.

All the above problems cause a loss in production. In addition, the casing damage and well head or completion equipment failures lead to unsafe well conditions that need earliest possible mitigation. Work over jobs is necessary to restore/ increase production in a safe manner.

6.3 WORK OVER RIG SELECTION:

6.3.1 Selection criteria is basically depends upon their use and selection methodology based on specific job requirement (Major and Miner)

The term workover is used to refer to any kind of oil well intervention involving invasive techniques, such as wireline, coiled tubing or snubbing. More specifically though, it will refer to the expensive process of pulling and replacing a completion.

Walkovers rank among the most complex, difficult and expensive types of well work i.e. they are performed only if the completion of a well is terminally unsuitable for the job at hand. The production tubing may have become damaged due to operational factors like corrosion to the point where well integrity is threatened. Downhole components such as tubing retrievable downhole safety valves or electrical submersible pumps may have malfunctioned, needing replacement.

In other circumstances, the reason for a workover may not that the completion itself is in a bad condition, but that changing reservoir conditions make it unsuitable. For example, a high productivity well may have been completed with 5" tubing to allow high flow rates (narrower tubing would have unnecessarily choked the flow). Some years on, declining productivity means the reservoir can no longer support stable flow through this wide bore. This may lead to a workover to replace the 5" tubing with 4" tubing. The narrower bore makes for a more stable flow.

To perform one or more of a variety of remedial operations on a producing oil well to try to increase production. Examples of workover operations are deepening, plugging back, pulling and resetting liners, squeeze cementing, and so on.

A remedial cementing operation designed to force cement into leak paths in wellbore tubulars. The required squeeze pressure is achieved by carefully controlling pump pressure. Squeeze cementing operations may be performed to repair poor primary cement jobs, isolate perforations or repair damaged casing or liner.

Squeeze - The careful application of pump pressure to force a treatment fluid or slurry into a planned treatment zone. in most cases, a squeeze treatment will be performed at down hole injection pressure below that of the formation fracture pressure. in high-pressure squeeze operations, performed above the formation fracture pressure, the response of the formation and the injection of treatment fluid may be difficult to predict.

The selection of a rig depends mainly on the following factors:-

The depth of wells.

Size of the well.

Type of job to be performed

6.3.2Identification of problems to decide for deployment of RIG

To arrest High Water Cut

To arrest Sand Influx

To rectify Tubing Choke

Need Repairs / replacement of the well equipment

Have Poor Reservoir Potential to improve performance

Repairing Casing Damage

Installation / Opt. A/Lift

To carry out Fishing jobs

Well Needs further Testing

Well requires to Be Abandoned.

To carry out Well Recompletion / Zone Transfer plan

To repair Well Bore Damage

To prepare well for conversion to Water Injector / Effluent Disposal wells

To Side Track the well

6.3.4

Identification of RIG based on type of problems to decide for deployment of RIG

Based on these factors the load capacity of the determined and the rig available suitable to handle desired hook load and having facilities as per requirement of well is selected. For example workover is to be performed in a well of 3000 meter depth having string of 2-7/8 and the job is fishing. In this case we may have to use 3-1/2 drill pipes for fishing purposes and the string load for 3-1/2, 13.3 ppf 3500 meter drill pipes comes out to 70 Tonnes and for fishing operations you need to pull higher loads therefore you would require rig of capacity more than 100 Tons. The rigs available are of 120T & 150T and taking care of unforeseen situations you may select a rig of 150T capacity.

6.3.4

Job planning for specific well problem (Examples)

(I) Job SRP Servicing

Drilled depth-1875m, casing 5-1/2 upto 1874m, Interval -1633-1629 & 1625-1617, Tubing shoe 1599.59

The sequence of operations is as under:-

(i) Displace the well volume with water. Subdue the well with brine of sp gr 1.01.

(ii) Pull out pump on S- rods.

(iii) Nipple down x-mas tree. Nipple up BOP and carry out function & pressure test.

(iv) Add tubing to string and lower upto 1660 meter and try to clear. If not clear, circulate.

(v) Pull out pump seat on tubing.

(vi) Test integrity at 80 ksc of tubing above P/seat.

(vii) Run in pump seat on tubings with BB shoe. Keep P/ seat at 1150m & BB shoe at 1600m.

(viii) Nipple down BOP. Nipple up x-mas tree.

(ix) Run in pump on s rods & commission. Test pump build up at 50 ksc.

(II) Job Re-perforation, Stimulation, SRP Servicing

Drilled depth-935m, casing 9-5/8 upto 196m, 7 upto 840m. Interval -830-824, Tubing shoe 810

The sequence of operations is as under:-

(i) Displace the well volume with water. Subdue the well with brine of sp gr 1.02.

(ii) Pull out pump on S- rods.

(iii) Nipple down x-mas tree. Nipple up BOP and carry out function & pressure test.

(iv) Add tubing to string and lower upto 831 meter and try to clear. If not clear, circulate.

(v) Pull out pump seat on tubings.

(vi) Scrap the 7 casing upto 831m, circulate & P/O scrapper.

(vii) Perforate the interval 830-824m @ 18SPM.

(viii) Run in tubings with BB shoe upto 829m.

(ix) Nipple down BOP. Nipple up x-mas tree. Change over well brine to water.

(x) Carry out mud acid job.

(xi) Reverse out the well with water.

(xii) Change over well water with brine of sp gr 1.02.

(xiii) Nipple down x-mas tree & adjust BB shoe at 810m.

(xiv) Nipple up x-mas tree. Displace the well volume with water.

(xv) Activate the well conventionally.

(xvi) Carry out BHS.

(xvii) Complete the well with SRP; keep BB at 810m & P/seat at 800m. Test SR pump build up at 50KSC.

(III) Job cement repair.

Drilled depth-740m, casing 9-5/8 upto 90m, 7 upto 680m. Interval -622-614, Tubing shoe 603.30

The sequence of operations is as under:-

(i) Perforate interval 610.5-611.5 @ 6SPM.

(ii) Run in 2-7/8 tubings on open end up to 614m.

(iii) Squeeze cement in open interval 610.5-611.5m. WOC. Meanwhile P/O Tubings.

(iv) Run in TCR bit on 2-3/8 Drill pipes and tag cement top. Circulate the well with brine and test cement plug at 80KSC. Drill down cement plug upto to 635m. P/O drill bit on drill pipes.

(v) Run in scrapper and scrap 7 casing upto 635m. Circulate the well with brine. Change over brine to water. Test well hermetically at 80KSC. If OK change over water to bring of sp gr 1.13.

(vi) Record CCL, CBL-VDL log. If OK, perforate 617-614m @ 18SPM.

(vii) Run in 2-7/8 tubings with BB shoe upto 595m.

(viii) Nipple down BOP. Nipple up x-mas tree. Change over well brine to water.

(ix) Activate the well conventionally.

(x) Carry out BHS.

6.4 WORK OVER OPERATIONS

Works over operations are carried out for several reasons. Some of them are:

6.4.1 WATER SHUT OFF (WSO)

During the producing life of a field, a well may start producing oil along with high percentage of water. This causes loss of revenue in terms of less amount of oil produced and requires larger process facilities for removing water. The high water cut has been a major problem in Mumbai offshore due to preferential depletion of high permeability layers in a set of high and low permeability layers perforated together and subsequent preferential flooding of theses layers for enhanced recovery.

Various techniques are used for WSO such as:

Cement Squeeze through the water-producing layer. In most cases this requires the retrieval of completion string.

Use of through tubing run bridge plug that is set on top of water producing layer to isolate it. Cement is then dumped on top of bridge plug using wire line dump bailer. This method can be successfully used in case the bottom most layer in a well is contributing water and can be resorted to without pulling out the string.

In case any other layer is producing water then a bridge plug and cement retainer are used in conjunction to isolate the water-producing zone.

Increased water production through channeling behind casing is remedied through block cementation jobs

WSO using gels wherein gel polymers are pumped into the well. The gel, after coming in contact with water, forms a pancake type barrier to the flow of water owing to cross-linking of polymer molecules and thus stops water production. The well is then re-completed in the layers of interest.

6.4.2 GAS SHUT OFF (GSO)

The excess and unwanted flow of gas from the reservoir causes a loss of reservoir energy that may ultimately affect the reservoir recovery.

GSO is carried out in wells producing with high Gas to Oil ratios by:

Squeezing cement between oil and gas producing layers.

Gel application as in the case of WSO

6.4.3 LAYER TRANSFER

A layer transfer job is done to change the layer of production or injection based on requirement.

6.4.4 RECOMPLETION

Such operations involve change in completion type such as from single to dual completion or vice versa, recompleting a well with gas lift etc.

The single completions are recompleted dually either to avoid cross flow from one layer to other owing to unequal pressure depletions over the producing life or to control injection rates in individual layers in an injection well.

In dual completions with gas lift, gas injection optimization is most essential to avoid one of the strings starving for gas with the resultant decreased production.6.4.5 PROFILE MODIFICATION IN A WATER INJECTION WELL

An injection well is used to inject water into the reservoir for maintaining its pressure and ensure better recovery of oil. Work over job for profile modification is carried out in water injection wells when some of the layers are taking either more or less amount of water than desirable.

A water flow log is used to record the injection profile of all the layers. The layers taking more than desired amount are squeezed off with cement. The layers taking less water are re-perforated or some more layers added to them.

6.4.6 SERVICING

6.4.6.1 Servicing jobs are generally safety related and involve operations such as replacement of gas lift valves, Sub-surface safety valves, tubing and packer etc.

6.4.6.2Removal of tubing blocked due to sand, scales, wax & paraffin depositions and stimulation jobs also form a part of servicing work over jobs.

6.4.6.3Well deepening

A well is deepened to include additional producing layers in the well. If the well has been on production before deepening, the existing layers are subdued and isolated through cement squeeze job. The well is then deepened, logged, cased, cemented and completed in the deepened horizon for production. Such jobs are carried out by work over rig when the rig is equipped with killing fluid conditioning facilities at site and all safety and control equipment as per Drilling Rig norms. In some instances, production from shallow well may be affected adversely by offset production from nearby deeper wells. The shallow well is then deepened to prevent offset drainage.

6.4.7 SIDE TRACKING

Side tracking is a very commonly used method to abandon or bypass the lower part of an existing well and add new layers. Many reasons exist for side tracking a well such as damage or collapsed casing, irretrievable junk in the hole, a damaged production zone in the old well, access to remaining/ new drainage area, drilling of drain holes/ laterals and horizontal wells etc. The well is side tracked by cutting a window in the casing and drilling a new hole through it. The conventional method of sidetracking involves placement of a cement plug immediately below the desired window depth to facilitate the section milling of the casing. This method requires several cement plug jobs before a window can be cut successfully. Also, it requires the cutting and retrieval of several casings to enable sidetracked hole to reach the targeted depth.

The side tracking is of two types:

6.4.7.1Long Drift Side Track (LDST)

In LDST, the 9 5/8 casing is retrieved first. A window is cut in 13 3/8 casing below 20 casing shoe and a new hole is drilled. The LDST enables drilling a well with a horizontal drift in excess of 1000 m. The well is generally completed in 5 liner against the pay zone.

6.4.7.2Short Drift Side Track (SDST)

In SDST, a window is cut in 9 5/8 casing below 13 3/8 casing shoe and a new hole is drilled. The SDST is used when the horizontal drift required is less than 1000 m. The well is generally completed in 7 liner against the pay zone.

A present method for side tracking uses a kickoff tool or a whip stock packer that is set at the proper depth. Drilling is then directed out of the hole and to the desired location by setting the tapered whip stock at different points to change the route of the new hole. When the desired depth and target are reached, the new hole is logged and a liner is run and cemented in place. Completion is then carried out in the normal manner with a packer and tubing. The use of whip stock does not require the cutting & retrieval of casing thereby saves costly rig time. Side tracking using whip stock has been extensively employed for drilling horizontal drain holes in Mumbai offshore.

6.4.8 FISHING

Fishing refers to the application of tools, equipment and techniques for the removal of junk, debris or fish from a well bore. The fishing operations are carried out to remove unwanted material from the well for the purposes of completion. Fishing forms a major part of work over operations and has been discussed in detail in a separate section.

6.4.9 PLUGGING & ABANDONMENT

Plugging & abandoning job involves abandonment of wells that have outlived their economic producing lives for ensuring safety and environmental protection in future. 6.5 COMMON EQUIPMENT USED DURING WORK OVER

Most of the work over jobs involve operations like perforating, cement squeeze jobs, cement clearing, scraping of casing/ liner, milling etc.

The completion string, X-mas tree and well head assembly are same as discussed in Chapter-3 under Well Completion. Some of the other frequently used tools in work over are:

6.5.1 Work string

The string of pipe used during work over is called the work string. In offshore drill pipes of sizes 2 3/8 to 5 are used as work string.

6.5.2 Casing Scraper

A casing scraper is used to remove foreign substances such as scale, perforating burs and cement from inside the casing wall. The scraper is usually run above a bit and is reciprocated to scrape the walls of the casing. Spring-tensioned blades provide the scraping action against the casing wall.

6.5.3 Junk and Boot baskets

Junk and boot baskets remove milled or drilled material from a well. A junk basket is run at the bottom of the work string along with TCR bit below junk basket. Through application of reverse/ direct circulation, the junk is swept into an inner chamber of basket and recovered once the basket is pulled to surface.

6.5.4 Cement retainer

A cement retainer is similar to permanent packer but has a check valve inside the bore. A stinger / seal assembly is run during cementation job. The valve is opened when the stinger assembly is stabbed into packer during the job but closes as soon as the stinger is picked out of the retainer bore. The closed valve holds final squeeze pressure as the excess cement is circulated out.

6.5.5 Casing roller

A casing roller consists of several rugged, heavy-duty rollers mounted on different centerline on a mandrel so that as the tool rotates only one roller at a time contacts the wall of the casing. This eccentric motion restores collapsed, dented or buckled casing to its normal diameter and roundness.

Figure 6.1 Casing ScraperFigure 6.2 Junk and Boot Basket

Fig6.3 Cement retainerFig 6.4 Casing RollerFig 6.5 Bridge plug

6.6 WORK OVER PROCEDURE

The procedure for working over an offshore well, in general, involves the following steps:

1. Lock opens SSSV.

2. Stop gas injection and bleed off annulus gas through burner in case of gas injection wells.

3. Bulldoze string volume (twice) into formation.

4. Fill annulus with seawater.

5. Perforate 5 m above packer.

6. Circulate and kill well. Under loss conditions a LCM pill is placed and in case activity is observed then brine of sufficient specific gravity is circulated.

7. The tubing condition is assessed using Multiple Imaging Tool (MIT).

In case the tubing condition is very bad then cut tubing and fish out packer separately.

In case tubing condition permits over pull then try to release the packer and string directly.

8. POOH old completion string.

9. Make bit & scraper trip to required depth.

10. Re-perforate / add layers and re-complete the well.

11. Activate through gas injection in case of gas injection well.

12. Stimulation job, if required.

13. Hand over the well to platform. 6.7 FISHING OPERATIONS IN WORK OVER6.7.1 DEFINITION OF FISH In oil field parlance, a fish is anything that is left in a well bore. Once the component is lost, it is referred to as "the fish."

In open hole during the course of drilling, the fish may be anything from a part of or all of the drill or tubing (work) string, to smaller pieces of equipment such as bit cones, pieces of tools or any material accidentally dropped into the well. Similarly, in cased hole, there are various types and kinds of problems that occur which create the fishing jobs, such as objects being dropped

into the well, packers to be retrieved, parted tubing, collapsed casing, dropped pipe and wire line tools either parted or stuck.

Fishing refers to the application of tools, equipment and techniques for removal of lost or stuck objects from the well bore. The term fishing is taken from the times of the earlier cable tool drilling when the crew simply put a hook on a line and attempted to catch the wire line when it would break so that the tool could be retrieved. Over the years, with advancement in drilling, completion and work over, fishing has also evolved greatly as an art and science of removing broken or stuck equipment or small non-drillable materials from the well bore.

6.7.2 CAUSES LEADING TO FISHING OPERATIONSSome of the common causes that result in fishing operations are:

Human error in a majority of cases

Corroded tubing and equipments

Stuck packers. The stuck-up can be due to either differential or mechanical reasons.

Damaged casings above the packers that result in packer stuck-up during pulling out the string.

Work string stuck-up in open hole-Differential, Mechanical or key seating

Logging tool stuck-up in tubing/casing due to scale deposition or damage

Premature setting of cement during cementation

6.7.3 CONSIDERATIONS FOR FISHINGThe key considerations for a fishing operation include:

1Knowledge of down hole tool configuration

An understanding of the dimensions and nature of the fish to be removed is essential for designing a successful fishing operation. Typically, anything that is lowered into the hole is accurately measured and sketched so that appropriate fishing tools can be used if required to fish any item out of the hole.

2Well bore conditions

The well bore conditions need to be understood clearly to determine the cause of stuck-up. Based on Hookes law that stretch is proportional to strain, the free point/stuck point in the string are determined.

3Well profile

Successful fishing is much easier in a relatively straight well than in a highly deviated well bore. However, it is still quite possible to perform a successful fishing job in a highly deviated or horizontal well bore if the proper approach is taken. When a highly deviated well does require a fishing job, most of the tools used in straight hole fishing can be successfully run. Even wash pipe with specialized connections can be run in highly deviated wells. Because the pipe is still large and not very flexible, sections still have to be short in order to pass through high-angle doglegs. Jars, over shots, magnets and junk baskets can also be used successfully.

There are several special considerations that should be taken into account when planning a fishing job in a high-angle deviated or horizontal well:

When a high-angle hole has been drilled by rotating drill pipe, a trough usually forms on the low side of the hole that is smaller in diameter than the drilled portion of the hole. This is a factor when fishing with an overshot or similar tool, as the fish will lie in the trough or smaller section.

Broken or twisted-off pipe can fall under a ledge of a dogleg.

Hole drag in horizontal or highly deviated wells inhibits good jarring action.

Adding weight to the string for milling operations can be a problem. Drill collars are similar to wash pipe in that they cannot flex around high-angle doglegs and bends. They have to be run higher up in the more vertical section of the hole to be effective. (In a horizontal section, the drill collar is on the low side of the well bore, not adding any weight on mill).

It is difficult to get the required torque down and around deviations when attempting to back off pipe in a highly deviated well.

4Cost-Benefit analysis of fishing vis--vis side trackingFishing should be an economical solution to a problem in the well. The cost of the fishing job must be less than the cost to re-drill or sidetrack the well for it to make economic sense. The larger the capital investment in the well, the more time and expense can be devoted to a fishing solution.

In the end, only experience, good judgment, a careful analysis of the problem and effective communication among all parties generally leads to a solution that will allow a return to normal drilling, completion or work over operations with the least amount of lost time and money.

6.7.4 FISHING CATEGORIESThe fishing operations can mainly be categorized into:

1Open hole operations are those in which the fishing tools are run in open hole to retrieve a fish present in open hole.

2 Cased hole wherein the fishing operations are carried out in cased hole.

Through tubing: Through-tubing fishing takes place through the restriction of the smaller-sized tubing. Through-tubing fishing applications have grown dramatically with the increased use of coiled tubing as a conveyance method.

6.7.5 FISHING TOOLSThe key enabling technologies for successful fishing operations are cutting, milling, catching/engaging and pulling.

Based on the kind of fishing operation, a variety of fishing tools are available. The use of appropriate fishing tool for any particular job will largely depend on the type of fish in the hole, whether the fish is stuck or free, whether it is in an open hole or in a cased hole, the condition of the hole at the site of break and the condition of the top of the fish. Each fishing job is unique but there are some basic tools such as safety joint, bumper sub, hydraulic jar and heavy weight drill pipes that are used in most jobs along with appropriate fishing tool and drill pipe work string. Based on intended application, the fishing tools can be classified into:

A. EXTERNAL CATCH TOOLS

The external catch tools (Figure 6.6) engage a fish on its outside body. Some of the commonly used external catches fishing tools are:

1OVERSHOT

The overshot is one of the most widely used fishing tools. It is a highly versatile and efficient tool. There are several different types of over shots; however, each overshot is designed to engage a specific size of tubing, pipe, coupling tool joint, drill collar or smooth O.D tool. The over shots are designated by a series number that indicates their application for fishing certain types of fish e.g. Series 70. Over shots are used to release a fish with short space to engage the tool on it. Similarly, Series150 over shots are used to release fish with sufficient neck length and allow for circulation too.

Over shots may be identified by one of the following strength types also:

Full Strength (F.S.) that is engineered to withstand all pulling, jarring and torsional strain.

Extra Full Strength (X.F.S.) that is engineered for extreme abuse.

Semi-Full Strength (S.F.S) that is engineered to withstand all pulling strain only.

Slim Hole (S.H) that is engineered to withstand heavy pulling strain only.

Extra Slim Hole (E.S.H) that is engineered for pick-up job only.

DESCRIPTION OF TOOL: The basic overshot (from top down) consists of a top sub, a bowl, grapple, control and a guide. In addition to the basic components, some over shots can be dressed with either:

Spiral Grapple used if the fish diameter is near the maximum catch of the overshot.

Basket Grapple used if the fish diameter is considerably below maximum catch size (usually )

However, in operation the overshot functions in the same manner, whether dressed with spiral grapple or basket grapple. When the circulating pack off is not used, the fluid circulates down the drill pipe, around the top outside of the fish, thorough the slip or grapple assembly around the guide shoe and up the annulus.

Figure 6.6 External Catch Fishing Tools

When the circulating pack off is used the annular space between the top outside of the fish and the inside of the lower part of the overshot is packed off, diverting the fluid flow down into the fish. If circulation can be diverted through the fish, it is easier to release and recover the fish.

Pack offs usually are not high-pressure devices but will often withstand sufficient pressure to establish circulation through the fish.

Both the grapples or slips and the pack-off can be easily damaged if the top of the fish is ragged, out of round, bent or damage.

Some times over shots are used with extension subs that are installed between the top sub and the bowl of the overshot and extends the overshot bowl. Extension subs are used to either establish a longer hold on a fish that may be undersize at the top by having been pulled in two/ an overshot released several times or cover a bad section of pipe so that a tool joint can be caught. Extension subs will only cover a fish O.D. equal to the maximum catch of the overshot using a basket grapple and still remain full strength.

2DIE COLLAR

The Die Collar is designed to retrieve tubular members from the well bore. The Die Collar is manufactured from high-grade alloy and specially heat-treated. The hardened cutting teeth (wickers) are machined on a shallow taper (approximately 3/4 inch per foot) to provide an excellent grip and positive engagement. For operation, the tool is run to the fish top and minimum weight and sufficient rotation is applied to allow the wicker threads to become embedded in the exterior surface of the fish.

A major disadvantage of die collar is that the dis-engagement of die collar, in case the stuck-up is not released, is extremely difficult and may further complicate the fishing operations. This is one of the reasons for which the die collar is typically used for mechanically backing off the string after just engaging the fish with die collar.

3CJ MILLING TOOL FOR PLUG & PACKER

The CJ milling tool enables retrieval of packers and bridge plugs used in the well. The tool, after milling the top slips of packer/ bridge plug, latches on to their collet for pulling them out. Mill out extension may additionally be required for packer.

B. INTERNAL CATCH TOOLS

The internal catch tools (Figure 6.7, 6.8, and 6.9) engage a fish on its inside body. Some of the commonly used internal catches fishing tools are:

Figure 6.7 Taper Tap

An Internal Catch Fishing ToolFigure 6.8 Die collar

1TAPER TAP

The Taper Tap operates in an exactly opposite manner to a die collar and is basically designed to retrieve tubular members from the well bore. It is the most economical tool of its kind for freeing fish. The Taper Tap is also manufactured from high-grade alloy and specially heat-treated. The basic Taper Tap is a single piece construction. The hardened cutting teeth (wickers) are carbo-nitrided and machined on a shallow taper (approximately 3/4 inch per foot) to provide an excellent grip for light duty pick-up jobs. For operation, the taper tap is run to the top of fish and rotated sufficiently to allow the wicker threads to get embedded into the interior surface of the fish.

2SKIRTED TAPER TAP

Skirted taper tap is used when the Taper Tap has been threaded for a skirt. The skirt is assembled to the Taper Tap and assists in guiding the fish for proper engagement and retrieval. Several types of skirts can be ordered; plain end, lipped end, or threaded end with a thread protector for adding standard oversized guide.

3GRAPPLE RELEASING SPEARS

The grapple releasing spears are rugged, dependable and inexpensive tools used to retrieve casings for side tracking purposes or tubing left due to free fall. The simple design assures positive engagement throughout the fishing operation, is easy to release and re-engage if necessary and may be run in conjunction with other equipment such as pack-off attachments and internal cutting tools. The basic tool consists of a mandrel, a grapple, a bull nose nut and a release ring.

Various types of grapple releasing spears are in use as indicated below:

TypeApplication

Inside hydraulic casing cutterTo cut single casing at precision depths

Inside mechanical casing cutterTo cut casing & drill pipe and can be converted to alternate cutting sizes

Inside mechanical tubing cutterTo cut tubing without using any typeof hydraulics

Multi-string cutterTo cut multiple strings of casing cemented together using hydraulic actuated cutters

Outside cutterTo cut & remove stuck pipe in a long undamaged section

Fig 6.9 Grapple releasing spears

C. JUNK CATCHER TOOLS

The junk catcher tools are used to remove junk/ debris from the well bore prior to/during fishing/milling operations. Some of the commonly used such tools are:

1BOOT BASKET

Boot baskets are used to catch cuttings that are too heavy to be carried out of the hole by normal circulation. They can be used when drilling up bridge plugs, retainer production packers, cement retainers or other drillable material such as iron, bronze or aluminum.

The basket is normally positioned in the drilling string just above the bit or mill. In operation the basket gathers cuttings into the cylinder over its upper lip as circulation slows, allowing the denser materials to fall out. Circulation must sufficient to raise the heavier cuttings from the bottom so that they can be trapped by the basket. Sometimes, depending on the severity of the operation, baskets are run in tandem. This increases the stability of the milling tool and increases the capacity for cutting.

2JET BASKET

Jet basket is used to recover small objects such as bit cones, bearings, mill cuttings, broken slips, hand tools and fragments of steel. Objects are forced into the basket by high-pressure jets forcing fluid down the outside of the tool. The fluid exits through the tool I.D. moving the object into the basket allowing the spring loaded catch fingers to catch the object.

The jet basket is usually made up in the fishing string below the drill collars in place of the bit. The basket is lowered into the hole to a few feet off bottom and full circulation started while slowly rotating the string to flush the hole of cuttings.

3JUNK BASKET

The junk basket is a highly successful fishing tool that incorporates a mill with double sets of free finger type catchers. It will catch most small objects that may be dropped into the hole including irregular objects. It is used to catch bit cones, slip fragments, wire line, hand tools and other similar objects. Additional uses are to take core samples, drill full gauge holes or ream. The manganese bronze catching fingers can be field dressed.

4REVERSE CIRCULATING JUNK BASKET

The conventional junk catchers employ forward/direct circulation at the bottom to lift the cuttings and trap them. The distance to which the junk is to be lifted is normally more and hence such tools are less effective.

In the Reverse Circulating Junk Basket also forward circulation is applied in the string that gets converted to reverse circulation at the tool due to a ball. The RCJB is a better junk catcher tool since the junk needs to travel a shirt distance only before it gets trapped in the catcher sub. RCJB is used to remove objects such as slips, hand tools, bit cones and any other small pieces of junk from bottom of well bore.

Fig 6.10

Boot BasketFig 6.11

Jet junk basketFig 6.12

RCJB

D. ACCESSORY TOOLS

Various accessories are used in the fishing string (Figure 6.13). Some of them are:

1WIRE CATCHER / WIRE LINE SPEAR

The wire line catcher is used to retrieve wire line.

2FISHING MAGNET

The fishing magnet is used to retrieve all types of small objects having magnetic attraction from the borehole bottom. Objects such as bit cones, bearings, slips, tong pins and mill cuttings can often be retrieved only by magnetic attraction.

1 2 3 4 5 6 7

Figure 6.13 Accessory tools 1. Wire line spear, 2. Fishing magnet, 3. Impression block, 4. Fishing jar. 5. Jet impact amplifier, 6. Bumper sub, 7. Safety joint

3LEAD IMPRESSION BLOCK (LIB)

The LIB is used to determine the configuration of fish top and to locate its position in the well bore.

The impression block is lowered on the end of the fishing string to approximately 5 feet above the fish. Circulation is used to clean the top of the fish and the string is then rapidly slacked and set on fish with 15,000 to 20,000 pounds of weight on the fish to get a good impression of fish top.

4HYDRAULIC FISHING JAR

The hydraulic fishing jar is used when a powerful upward blow is required to release the stuck fish.The hydraulic jar is placed directly below the drill collars in the fishing string. The intensity of each blow is controlled by the amount of stretch placed in the drill string. More the pull harder is the blow. The jarring effect is enhanced by placement of drill collars above the jar.

5BUMPER SUB

Bumper sub is used below hydraulic jar to prevent transmission of impact generated by jar to tubing.

6SAFETY JOINT

The safety joint provides a simple means of releasing and re-engaging fishing tools during many fishing operations. It is especially useful in operations requiring a fishing tool that is not normally easily releasable such as a tap or die collar.

The safety joint can be released from the fish by applying a tension load to the string to shear the screws. Once the screws are sheared and the tension on the line is released, the string is picked up while applying slight left hand torque. The safety joint lugs will then shift into the release slot and the fishing sting is released from safety joint.

E. MILLING & WASHOVER TOOLS

1JUNK MILL

Junk mills are used to mill up almost everything that falls or becomes stuck in the hole. Pipe that has become cemented both inside and outside can only be milled with this tool. Loose or rotating junk can be pounded down to break it into smaller pieces and hold it in place so that the mill can cut it. Most people run too much weight on junk mills. A good rule of thumb is 1000 pounds per O.D. inch of the mill.

For use inside a casing, a mill with stabilizer pads and a smooth O.D. is used so that the casing will not be damaged. The O.D. of the mill should be same or slightly smaller than the drift diameter of the casing.

In open holes, a mill without stabilizer pads in used. The O.D. of the mill should be about inch less than the hole diameter with rough O.D. that will cut a small amount of side clearance.

2TAPER MILL

Taper mills are used to ream partially collapsed or damaged casing to clean up ragged holes or windows and generally to enlarge and smooth rough and jagged surfaces.

Milling out collapsed casing is done in increments of about i.e. a mill with an O.D. of about larger than the minimum collapsed pipe I.D. is run into the hole and the collapsed interval is then milled to about larger. Next, a second mill is run and the pipe I.D. increased to about again. This procedure is repeated until the pipe I.D. is back to full size.

3WASHOVER PIPE

In cases where the fish is a stuck pipe, the fish may be surrounded with settled mud, cement and other debris. This will not allow proper engagement of the fish with tool. The wash over pipe is used to clear the outside area of fish.

The wash over pipe is run at the bottom of work string. The cleaning is done using circulation as the wash over pipe is lowered over the fish.

Junk mill W/O pipe Taper mill

Figure 6.14 Milling & Wash over tools

F. REMEDIAL & REPAIR TOOLS

1CASING ROLLER

The casing roller is used to restore buckled, collapsed and dented casing to its Original diameter.

Figure 6.15 Remedial & repair tools casing rollar

2CASING PATCH

The casing patch is used to make fast, economical repairs to damaged casing string without much reduction in its I.D. The casing is removed from a point below the damaged section. The top of the down-hole casing is then milled and the casing patch is run over the casing remaining in the well to sufficient depth to engage the slip. The string is then raised to engage the slip by compressing the lead packing ring until a permanent seal is accomplished between the patch and the casing. After squeezing the packing, the casing is suspended in the well to keep it under tension.

6.7.7 SAFETY REQUIREMENTS DURING WORK OVER JOBS6.7.7.1 PRINCIPLES OF WELL CONTROL (Well Killing)

A kick is an intrusion of formation fluids into the well bore that, if not controlled, will result in a blow out. The objective of well control procedures is to safely prevent or handle kicks and re-establish primary well control.

During normal drilling operations, the primary well control is maintained through use of drilling fluid normally called as killing fluid also that provides sufficient hydrostatic head to overcome the formation pressures.

Also, owing to the inherent resistance of liquids to flow, an additional pressure is applied to circulate the fluid during drilling to lift the formation cuttings to the surface. The pressure, so applied at surface, alters pressures in the pipe and well bore bottom. Thus fluid circulation creates additional over balance to contain the formation pressures. The density equivalent to the sum of friction pressure in the annulus and the drilling fluid density in the hole is often expressed in terms of Equivalent Circulating Density (ECD).

The total pressure at the bottom of the hole while circulating will be the sum of the hydrostatic head of the drilling fluid, the annulus friction circulating pressure and imposed surface backpressure. During drilling, this pressure must not be less than the formation pressure to avoid a kick and must not be greatly in excess of formation pressure. This is required to be calculated to prevent fluid losses.Subsequent to drilling of a well, it is completed using clear completion fluids/ brines in the annulus to prevent formation damage prior to production stage and also to avoid subsequent complications that may arise out of solids settlement on top of down hole packer. The solids may hamper the retrieval of string during work over.

6.7.7.2 RECOMMENDED SAFE PROCEDURES & GUIDELINES FOR WORK OVER OPERATIONS

The recommended safe procedures & guidelines for work over & well stimulation operations have been issued by Oil Industry safety Directorate (OISD) vide OISD-GDN-182.

The purpose and scope of these procedures and guidelines are:

1. To provide recommended procedures and guidelines to safeguard the life, limb and health of employees and employers in the oil and gas well servicing industry.

2. To provide recommended procedures and guidelines for safe transportation, rigging-up/ rigging-down, operation of well stimulation and work over and related equipment.

3. To provide safety norms for mobilization/ demobilization of WSS units, their placement at site, laying down/ rigging-up equipment and pumping of stimulation/ well servicing fluids.

The OISD-GDN-182 lists out general safe operating practices and also recommended procedures & guidelines for Personnel individual responsibilities, various safety equipments such as Personal Protective Equipment, Life saving equipment, Fire fighting equipment etc. Some of them are given below:

General Safe Operating practices for work over jobs are:

Good housekeeping on the job is essential for successful accident control and fire prevention. Keeping everything in its place promotes efficiency, quality and good work. Tools, equipment and working areas should be kept clean, neat and orderly.

Thorough safety inspection should be made of all well servicing equipment before starting the job.

Men on the rig floor should stand clear when rigging up or repair work is in progress overhead.

Riding cat line is prohibited except in a n emergency.

Unsafe or otherwise dangerous conditions, no matter how small, should be immediately reported to supervisor in charge for corrective action. Correction of unsafe condition may take one of the three forms-remove, guard or warn.

Never leave well unattended unless it is safely shut-in.

Upon completion of job, clean equipment and keep them neatly and safely in toolbox.

All occupational injuries, no matter how small, will be reported promptly to the supervisor in charge.

In the event of equipment, guy wires, lines etc. being present in a location frequented by personnel and thus creating a hazard, proper signs must be placed to warn the people.

Control must be maintained over leaks and spills. However, if they occur, they should be cleaned up promptly to eliminate slipping hazard to personnel as well as fire hazard.

Well Subduing

Well plan should be prepared well in advance and circulated to all concerned including driller in charge. It should contain the following information but not limited to:

Brief of earlier work over done

Complications encountered during drilling/ work over

Mud parameters during drilling/previous work over jobs

Details of down hole equipment, tubing and X-mas tree

Casing and cementation details

Details of perforation

Bottom hole pressure of existing well and expected BHP after work over job

Some of the checks that are essential during well subduing operations are:

The kill line must be tested to 1 times the maximum expected shut-in pressure or its rated working pressure whichever is less.

No hammering must be done on pressurized line to tighten the joints.

The parameters of kill fluid must be regularly monitored.

The parameters of return fluid must be monitored regularly during circulation to ensure that the well is killed and the fluid inside the well is properly conditioned.

Improper returns or quick pressure build-ups must also be monitored to assess the conditions of loss / activity.

Before the X-mas tree is nippled down, Back pressure Valve (BPV) must be installed in the tubing hanger. The SCSSV must also be kept closed. The BPV must be removed and SCSSV opened only after installing the BOP.Following requirements apply during work over operations with X-mas tree removed:

Well control fluids, equipment and operations shall be designed, utilized, maintained and/or tested as necessary to control the well in foreseeable conditions and circumstances, The well shall be continuously monitored during well work over operations and shall not be left unattended at any time unless the well is shut-in and secured.

When coming out of hole with drill pipe or work over or well completion string, the annulus shall be filled with well control fluid.

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