CEMENTING JOB EVALUTION:

CEMENTING JOB EVALUTION:

                                       CEMENTING JOB EVALUATION:

                                                               HOW, WHEN AND WHY?

                                                                                      INTRODUCTION

At any point in time that the top plug bumps on the bottom plug it is assumed that a successful cementing operation has been carried out. To ensure that one of the most primary goal of cementing operation is not defeated, namely: ‘to achieve zonal isolation ‘it is necessary to evaluate each cementing job so as to ascertain whether it is actual a successful one.

The process of carrying out this evaluation is known as cementing job evaluation. Since we cannot see what the cement sheath actually looks like we need to employ the services of some devices that can help us to infer what the wellbore condition looks like , such tool are known as cement evaluation tools .

                                                   METHODS OF CEMENT EVALUATION

There are various methods of carrying out a cement evaluation job and they include the following:

  1. Hydraulic testing.
  2. Temperature logging.
  • Nuclear logging.
  1. Acoustic logging.

 HYDRAULIC TESTING

This is a post remedial operation carried out to confirm if perforations have adequately been sealed off or not. Sometimes it is also conducted after primary cement especially where water bearing sand is close to the pay zone.  Hydraulic testing is of two types:

  1. Dry testing
  2. Pressure testing

PRESSURE TESTING

This is a type of test carried out to determine the integrity of surface and intermediate casing after cementing it in place.

PROCEDURE

The shoe track is first drilled off alongside the plugs {the top and bottom plugs} then pressure is applied within the casing. The pressure is increased beyond the expected hydrostatic pressure of the next phase of formation to be drilled if the casing maintains it integrity without leakage then a good cementing job has been carried out but if there are leakages within the shoe, that means a secondary cementing operation is needed to correct the problems.

DRY TESTING

This appears to be the opposite of ‘pressure testing’. This test is also similar to Drill Stem Test {DST}. The objective of this test is to ascertain whether fluids will flow within the cemented casing.

PROCEDURE

A pressure differential is created within the casing in a quest to ascertain whether fluids will flow within the casings if fluid should flow then, the casing is poorly cemented and if remedial cementing is not carried out the casing will corrode in no distant time.

TEMPERATURE SURVEY

Because setting cement release energy in form of heat, hence cement is said to be exothermic. A bird can only bleed according to the amount of blood in its vein, they say, likewise it is worthy of note that the amount of heat given off or generated by a setting cement is a function of the amount of slurry behind the casing hence a small volume of slurry behind the casing will generate less heat, vice versa.

Hence a temperature log is run to determine the top of the cement usually deployed for primary cement job.

a sample temperature log.

     FACTORS AFFECTING TEMPERATURE LOGS

  1. Amount of slurry behind casing
  2. Type and amount of additives in a slurry
  • Cement type
  1. Time used for testing

PROCEDURE

A temperature measuring sonde is run in hole at a time that the cement has attained its highest temperature between 4 to 12 hours after displacing the slurry into the annular space

The temperature is recorded on the bottom of the chart and depth is recorded on the scale on the left vertical side of the chart .starting from the top where the curve shows a sudden temperature increase .this location is the top of the cement because, as cement sets, it gives off heat which is transmitted through the casing.

 

POSSIBLE INFERENCES DRAWN FROM A TEMPERATURE LOG

CEMENT BELOW EXPECTED TOP:

if the actual top of the cement is actually below expected top then it can be inferred that perhaps:

  1. The cement slurry was underestimated.
  2. A large washout section has taken a lot of the cement.

IMPLICATIONS

If cement level is too low formation fluid may easily enter the annulus above the cement hence corroding the casing.

CORRECTIVE MEASURE

A reverse cementing should be carried out that is cement should be pumped directly through the annulus to fill up the void space

 

TOP HIGHER THAN ANTICIPATED:

In this case cement top is higher than anticipated. The possible cause of this maybe:

  1. Cement might have channelled through around drilling mud or un­-removed cake which will make the cement slurry to rise higher and leaving some area uncemented.
  2. It could also be that the volume of the slurry was over estimated.

IMPLICATIONS

One of the main purposes of cementing has been defeated which is avoiding cross flow of formation fluid.

CORRECTIVE MEASURE

The cement bound log should be run to identify the zones without cement and perform squeeze job as appropriate.

 

NUCLEAR LOGGING

This is also used to determine the top of the cement just like temperature log.

PROCEDURE

Radioactive material is added to the cement slurry and gamma will later be run to determine its location after cement slurry has been placed in the annulus; soluble radioactive tracers with a very short life like  is added to mix water pre-hydration of slurry . It is expected that any trace of radioactive contamination should disappear within few months hence preserving cement sheath integrity.

CEMENT BOND LOG {CBL}

This is one of the most frequently used and a very efficient method of cementing job evaluation. It is used to determine cement to casings bound quality. This is measurement is based on the acoustic properties of the casing.

PRINCIPLE OF OPERATION

This logging tool has a transmitters and receivers which are of definite distance from each other. These transmitters generate sound waves inform of compression as well as shear waves. The shear waves can pass through solids but not through liquids. The velocity at which these waves move depends on the elasticity of the materials they are passing through.

Loss of sound energy is experienced and is very high in a porous and unconsolidated materials and the reverse is the case for solid rocks.

The receiver measures the transit time, the time used by the sound waves to move from the source {transmitter} along the casing through the mud, measured in micro-seconds.

Ultrasonic tools provide the most beneficial data when evaluating cement placement and bonding. Instead of a separate source and receiver, the ultrasonic source and receiver are packaged together as a transducer. When a signal emitted by a transducer encounters an acoustic interface (for example, between casing and annular material outside casing), some of the signal energy is reflected at the interface, and some is transmitted across the interface. The fractional amounts of reflected and transmitted energy depend on the acoustic impedances of the materials at the interface. The signal strength received at the tool, the acoustic impedance, or Z, is a function of the bulk density of the material through which the sonic waves travel (Frisch, et.al., 2000).

 

CBL INTEPRETATION

When the transit time is slightly long {more than 15 micro-seconds} with low amplitude, and high acoustic impedance. This indicates good cement bounding with the casing and the formation.

But when the transit time is very long excess of about 30 micro-seconds, this implies that the casing was poorly cemented.

Sometimes the variable density log is used alongside CBL to get the best qualitative interpretation.

SOME ILLUSTRATIONS

 

  1. GOOD CEMENT JOB

Low amplitude                        

VDL formation signals are strong

No need for squeeze job.

 

  1. POOR CEMENTING JOB

Amplitude high

VDL straight no formation signals

Squeeze job needed

  1. PARTIAL CEMENTING

Amplitude is low and moderate

VDL shows wiggly formation signals and straight casing signals

Squeeze job may be applied.

 

CONCLUSION

Cement job is said to have been accomplished when the top plug rest on the bottom plug.

Cement is a long-lived and versatile isolation material when used within the bounds of sound engineering judgment.  Proper modifications of the cementing slurry can stretch the ability of cement to handle shrinkage, cyclic pressure applications, hotter, deeper, colder and acid gas environments. The first few cement isolation jobs in an area may need to be logged to determine bond, fill, gas channelling, micro annuli extent and other placement objectives. Once the cement recipe is determined and the proper well set-up and placement details are worked out, there is little need for continuous logging unless a problem is indicated by the cementing pump chart.

When a well is evaluated it helps to ensure that all the objectives of cementing a well are not defeated.

 

BY SOSTHENES UGOCHUKWU

PETROLEUM ENGINEERING AND GEO-SCIENCE DEPARTMENT  OF PETROLEUM TRAINING INSTITUTE NIGERIA.

Sosthenes

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