WORK OVER OPERATIONS.

OBJECTIVE-

Maintenance of wells in optimum producing condition and enhancement of production are the primary objective of work over operations.Judicious use of work over often can appreciably increase economically recoverable oil and gas reservoirs.

DEFINITION -.

Basically it is a remedial measure that is carried out in a well to add/restore production/injection in a well..

Work over are historically a most profitable business if optimum creative skill and imagination are exercised in-

1) Fact finding.2) Analysis 3) Experimentation. 4) Job execution and5) Job evaluation

PROBLEM WELLS

Once the well is drilled, completed and put on production/injection, at some stage of its operating life various problems encounter which leads to- decline of production, well become sick and ultimately cease to flow. Problem wells may usually be categorized-

– As limited producing rate– Excessive water production.– Excessive gas production in oil wells.– Mechanical problems.– Problems in injections/ disposal wells may be related to high injection pr. and low volume.

The well problem is due to either-

A. Formation related problem. B. Mechanical Problem.

a. Formation Related problem-

The well bore and reservoir related problems can be broadly categorized into-

Reservoir Problems-

– Low formation permeability.– Low reservoir pressure.– Thin pay zone.

Fluid Problems-

– Improper and non-compatible injection fluid.– Gas conning.– Water conning/Preferential mobility.– Formation precipitate /scales.– Heavy viscous formation fluid.

Problem around well bore.

– Accumulation of formation fines.– Emulsion blocking.– Water blocking.– Poor Perforation.– Loose, Unconsolidated formation results in sand production.

b. Mechanical problems of sickness of well includes-

Bad Cementation and channeling behind casing which may lead to-

– Production of water, gas or undesirable materials from other layers into the producing layer.– Loss of Hydro Carbon from present layer to other layer.– Loss of injection fluid.

Casing and well head failures.

– Failure of sealing elements of well head.– Casing leakage /damage.– Bad casing profile.

Perforation Problem.

– Insufficient perforation.– Plugging of perforation.– No communication from well to the reservoir.

Production of sand from unconsolidated reservoir.

Paraffin, Scale and deposition of undesirable materials in the flow string/well bore.

PROBLEM ANALYSIS

Problem well analysis may be handled on-

• Reservoir basis,• An area basis, • Individual well basis.

Broadly problem analysis is done on the basis of-

• Geological data analysis.• Geophysical /Logging data analysis.• Reservoir data analysis.• Routine production testing data.

Understanding the source or potential source a problem requires thorough investigation of all aspect of well and, reservoir parameters. Factors to consider include-

• Recovery mechanism.• Reservoir permeability and porosity.• Permeability anisotropy and heterogeneity.• Relative permeability to oil, water and gas.• Net formation height.• Portion of productive interval completed.• Location of all perforations.

• Reservoir deep.• Original water –oil and gas-oil contacts.• Connate water and irreducible oil and gas saturations.• Location and continuity of shale or other low permeable layers.• Length of time before water production begins.• Oil, gas and water production rate histories.• Location of fluid entry and types of fluid entering well bore.• Cement bond Log evaluation.

The conclusion of such study will usually result in –

1. Work over.2. Continue to produce well until oil and gas production declines to a predetermined

volume or to the economic limit.3. Pressure maintenance4. Enhanced recovery operations or 5. Abandon.

Since work over can be very expensive, it is imperative that the well be thoroughly analyzed and diagnose the problem before the work over operation begins.

6. If a well suddenly goes off production, there is no doubt that some type of problem

exists.

– Pumping well (Artificial lift)-Mechanical Problem.– Unconsolidated sand-Sanded-up.

In any event, the well data must be carefully reviewed to determine the actual problem.

2. A well can have a problem without exhibiting any obvious symptoms.

• Skin damage.

The skin damage can be determined by running and analyzing a pressure build up studies.

If a well starts to produce water-

• If the well producing from a water drive reservoir, its performance is probably normal.

• Its behavior is anomalous if it is producing from a dissolved gas drive reservoir

A. FORMATION RELATED PROBLEM..

Low Reservoir permeability.

Low reservoir permeability may be an over all reservoir characteristic, or it may be limited to a specific area. Low permeability well productivity declines rapidly as fluids near the bore produced. It can be determined from

1. Pressure build up

2. Draw down

3. Production test data.

Through tubing flow meter, radioactive tracer survey or straddle packer may be used to determine formation-well bore communication for each interval.

b. Low reservoir Pressure.

If reservoir pressure measurements have been carried out on a routine basis, reservoir pressure history should be well documented.

c. Formation Damage.

Formation damage may be define as any impairment of well productivity or injectivity due to plugging within-

• Well bore.• Perforations• Formation pores adjacent to well bore or• Fractures communicating the well bore

It can be determined by-

1. Production testing. 2. Pressure build up studies. 3. Pressure draw down. 4. Comparision with offset wells. 5. Careful analysis of production history including prior completion. 6. Work over and well servicing operations.

d. Plugging of tubing, well bore and perforation.

For all types of wells the probability of tubing, well bore or perforation plugging should be evaluated. Plugging may be caused by-

– Gravel or frac sand.

– Fines,– Mud.– Formation rock.– Paraffin.– Asphalt.– Scale.– Pipe dope.– Gun debris.– Other junk.– Collapsed tubing or casing.

Analysis of the bottom hole sample is helpful in determining the cause of plugging.

e. High Viscosity oil.

If the reservoir is being produced by dissolved gas drive ,oil viscosity will increase some what as gas is liberated from the oil. Due to high viscosity water-in oil emulsion formed.

Excessive back pressure-

It can appreciably lower producing rates in wells producing from reservoirs nearing pressure depletion .Excessive formation back pressure may be due to-

1. Limited or partially plugged formation.

2. Plugged well bore.

3. Plugged tubing. For high capacity wells-

Increase the of the tubing, flow line or separator size.

In oil reservoirs having appreciable loss of reservoir pressure-

• Artificial lift • Reduction of separator, tubing or casing pressures will increase production.

If tubing, well bore, or perforations are partially plugged,

– Removal of restrictions by cleaning. – Re perforating is frequently the best approach.

g. Problem with Artificial Lift Wells.

If lift has been installed, improper design, improper application, or malfunction of equipment is a frequent cause of reduced oil production.

Typical gas lift problems are-

– Leaky valves, tubing or casing.– Improper time cycles.– Low gas pressure.– High back pressure on formation.– Scale, paraffin, or asphalt in flowing line ,gas lift valves, tubing and well bore, or

perforations

h. Water Production Problems in oil and gas wells.

Water production problem may result from-

1. Natural water drive or water flooded aggravated by fingering or conning.

2. Extraneous sources including casing leaks or primary cement failure.

3.Fracturing or acidizing into adjacent water zones.

Water encroachment is normal in water drive reservoir.

– ‘’Fingering’’ of water, defined as up-structure water movement in the more permeable zones of multi-zone completion, is rate sensitive.

– Water conning in oil or gas wells is defined as vertical movement of water across the bedding planes in a producing formation.

– Diagnosis of Emulsion blocking- Average permeability determined by injectivity test is higher than determined by production test

• Diagnosis of Water Blocking- A temporary shift of relative permeability in favor of water as the mobile fluid causes water blocking. Water blocking is usually caused by-

– Water (if use as killing fluid.)– If water is allowed to remain on producing zone for long time.

Surfactants can bring about the following –

1. Raise or lower surface and interfacial tension.

2. Make, brake, weaken. Or strengthen an emulsion.

3 Change the wettability of reservoir rocks and casing, tubing, o flow-line.

4. Disperse or flocculate clays and other fines.

Diagnosis of water problems caused by Hydraulic fracturing- Temperature surveys will often give a clue whether subsequent water production is due to-

– Primary cement failure.– Fracturing or acidizing into water.– Casing leakage.– Normal water encroachment.

i. Gas problems in oil wells- The primary source of gas in oil wells are-

1) Gas dissolved in the oil.

2) Primary or secondary gas caps, and

3) Gas flow from zones or reservoirs above or below the oil zone

In a dissolved gas drive reservoir, gas saturation increases as oil withdrawals continue and gas coning may occur in a well with continuous vertical permeability.

In stratified reservoirs, premature fingering of gas may occur with high pressure drawdown and where permeability varies appreciably. It is due to-

1. Casing leaks.

2. Cement bond failure.

3. Natural or induced fractures communicating with gas zone, or

4. Acidizing into gas zone.

Mechanical failure-

Some of the more common failures are-

1) Primary cementation.

2) Casing, tubing and Packer leaks.

3) Failure of Artificial lift equipments.

4) Well bore communication in multiple completions.

Casing leaks/well bore communication can be ascertained by-

1. Water analysis.(casing leak or normal water encroachment).

2. Temperature surveys and other production logs.

3. Packer and bridge plug.

4. Abrupt changes in producing characteristic.

5. By observing equal shut in pressure on two or more completion.

PROBLEM WELL ANALYSIS TOOL-

A number of ‘’tools’’ are used in analyzing well performance. The most commonly used ones are-

– Well Performance Curves.

– Well Status maps.

– Well histories analysis.

– Well bore sketches.

– Bottom hole pressure data.

– Fluid analysis.

– Fluid levels.

Performance curves for oil well

Fig-7 Performance curve of dissolved-gas drive oil well

Fig-8. Dissolved-gas drive oil well with extraneous water production.

Fig-9.Water-drive oil well

• For Gas well-Gas wells can be analyzed by performance curves in a manner similar to that for oil wells. In addition to well test performance plots, another useful plot for gas wells is the P/Z vs. cumulative production plot

Fig-10.Oil well with mechanical problem

• Fig-12 Rate and flowing tubing pressure added to P/Z vs. Cumulative production plot.

• . They provide a very good way to determine whether skin or well bore damage has occurred. A pressure build up test is normally run to confirm the presence of skin damage.

• Well Status maps—

• The well status map is one of the most useful tools available for sick well analysis.

Fig-13 Well status map.

Fig-14.Typical five-spot water flooded pattern.

Well histories-

A well history is the history of all operations that have been conducted on a well. The history is usually prepared in a tabular form as below-

Well name-

Location-

Date of completion-

Elevation.-

Initial potential-

Casing-

Target depth (TD)-

Present producing interval –

Initial producing interval-

Date of work overs-

Current production rate-

• Well bore sketches-

• It shows at a glance what equipment is in the well. This information is valuable in planning work over operations as well as being of help in diagnosing well problems.

Fig-15 Well bore sketch

Bottomhole Pressure Data

Bottom hole datas are very useful in evaluating well performance.From Pressure build up and draw down studies the following reservoir parameters are obtained-

– Static bottom hole pressure.

– Interwell kh.

– Flow efficiency.-Useful in analyzing well for stimulation.

– Skin effect.

The static pressure vs. time will indicate-

– Type of drive mechanism.

– Help to identify water flood response.

Flowing and Static bottom hole pressure data, in conjunction Production test , can be used to determine Productivity Index(P.I.=BOPD/Psi draw down). PI can be used to-

– Compare well performance.

– Estimate production increases from lowering back pressure.

– Evaluate water flood well.

PI is primarily applicable to water drive and gas- cap expansion drive oil wells. Inflow performance analysis for solution- gas drive reservoirs below the bubble point should be done with Vogel’s equation.

Fluid Analyses

Fluid analyses are very helpful in analyzing problem wells. It can be used to-

– Identify extraneous water production-come from casing leakage and bad cement job that causes communication behind the casing.

– Monitor water flood performance, when injection water is different from formation water.– Breakthrough of injection water.

Oil analysis are not so useful but some times it indicates communication through in multiple completions.

Other Well Analysis Tools.-

Many other well analysis tools such as Special surveys, Cross section and Maps etc are used in problem well analysis.

– Examine structure, isopachous, isobaric, water percentage, and gas-oil ratio maps available.

– Study of stratigraphic cross sections. This can be effectively used to compare basic data on problem wells and offsets. This can be quite useful for comparing well data on reservoirs with continuous horizontal permeability over some distance.

– Obtain a current static subsurface pressure.

– Consider running wire-line tools to check for well bore plugging.

– Run production logging surveys when indicated.

– Conduct diagnostic well tests, such as pressure build up, pressure draw down, and injectivity tests. Review production decline curves.

– Check for well bore communication, mechanical failures.

– For pumping wells, check fluid level with sonic devices and carry out 3dynamometer survey.

– For gas lift wells, consider checking valve operation with producing sub surface pressure surveys,

– temperature surveys, and two-pen surface pressure chart.

• For wells that have been shut-in for some time, conduct a new production test. Justification for a new test is based on the fact that a gas or water finger or cone may have dissipated, reservoir pressure may have increased or decreased, or the well bore may be plugged with scale or debris.

• Compare recent and initial water sample analysis.

• Check bottom hole sludge sample analysis for scale, paraffin or asphaltenes. Analyze scale from pump, mud anchor or tubing.

• Check corrosion and casing leak history, including tubing or casing caliper records. A casing thickness survey may be needed.

• Consult with concerned person of pump, field service man/field superintendent for sudden changes in well behavior characteristics that may not have been reported over the life of a well.