Frecuencia y calidad de la relación de los hijos con los

Casing wear decreases the performance properties of casing. The burst and collapse resistance of worn casing is in direct proportion to its remaining wall thickness.

Figure 4.G - Casing Wear

A major contributing factor to reducing the life of a casing string is poor handling throughout the supply chain. All personnel in this chain must adopt the proper handling procedures.

The major factors affecting casing wear are:

• Rotary speed

• Tool joint lateral load and diameter

• Drilling rate

• Inclination of the hole

• Severity of dog legs

• Wear factor.

The location and magnitude of volumetric wear in the casing string can be estimated by calculating the energy imparted from the rotating tool joints to the casing at different casing points and dividing this by the amount of energy required to wear away a unit volume of the casing. The percentage casing wear at each point along the casing is then calculated from the volumetric wear.

Eni-Agip acceptable casing wear limit is </= 7%.

Volumetric wear is proportional to an empirical ‘wear factor’ which is defined as the coefficient of friction divided by the volume of casing material removed per unit of energy input.

The wear factor depends upon several variables including :

• Mud properties

• Lubricants

• Drill solids

• Tool-joint roughness.

Note: The chemical action of gases such as H2, CO2 and O2 tends to reduce the surface hardness of steel and, thus, contributes significantly to the rate of wear.

4.11.2. Volumetric Wear Rate

The volume of casing worn away by the rotating tool joint equals:

Wear Volume Per Foot(V) =

Energy

Specific Energy = The amount of energy required to wear away a unit volume of casing material.

The frictional energy imparted to the casing by the rotating tool joint equals:

Energy Input Per Foot = Friction Force Per Foot x Sliding Distance where:

Friction Force Per Foot = Friction Factor x Tool Joint Lateral Load Per Foot Sliding Distance = n x TJ Diameter x Rotary Speed x Contact Time and:

Tool Joint Contact Time

DPJL TJL = Tool Joint Length P = Rate of Penetration DPJL. = Drill Pipe Joint Length The lateral load on the drill pipe equals:

Drill Pipe Lateral load per Foot (L) =

DPJL TJL x TJLLPF

where:

TJLLPF = Tool Joint Lateral Load Per Foot TJL = Tool Joint Length

DPJL. = Drill Pipe Joint Length

The Wear Factor controlling the wear efficiency is defined as:

Wear Factor = Friction Factor/Specific Energy

Combining the above equations. shows that the Wear Volume, V, equals:

P

L = Lateral Load on Drill Pipe Per Foot (lbs/ft) D = Tool Joint Diameter (ins)

N = Rotary Speed (RPM)

S = Drilling Distance (ft) P = Penetration Rate (ft/hr)

The tool joint and drill pipe lengths do not appear in Equation 6 because they do not effect the amount of casing wear in the linear model.

Note: Wear volume increases non-linearly with wear depth, because grooves become wider as the wear depth increases.

4.11.3. Wear Factors

Wear Factor (F)

Drilling Fluid Tool Joint (10^-1 psi^-l)

Water+Betonite+Barite Smooth 0.5

-Water+Betonite+Lubricant (2%) Smooth 0.5 - 5

Water+Betonite+Drill Solids Smooth 5 - 10

Water Smooth 10 - 30

Water+Betonite Smooth 10 - 30

Water+Betonite+Barite Slightly Rough 20 - 50

Water+Betonite+Barite Rough 50 - 150

Water+Betonite+Barite Very Rough 200 - 400

Table 4.A - Typical Casing Wear Factors

Wear Factor

Drilling Fluid Tool Joint (10^-1 psi^-l)

Water+Betonite+Barite Rubber Protector 1 - 2

Water Rubber Protector 4 - 10

Table 4.B - Typical Casing Wear Factors (Shell-Bradley, 1975)

Mud Weight Tool Weighting Wear Factor

Drilling Fluid (lbs/al) Joint Material (10^-l0psi^-1)

Oil+Bentonite 10 Smooth Barite 0.9 - 1.2

Water+Bentonite 10 Smooth Barite 0.8 - 1.6

Water+Bentonite 10 Smooth Iron Oxide 3 - 4

Water+Betontite 10 Smooth Drill Solids 5 - 11

Water+Betontite 10 Smooth Sand 11 - 13

Water+Betontite 8.8 Smooth None 22 - 27

Table 4.C - Effect of Weighting Material on Casing Wear Factor (Bol, 1985)

4.11.4. Wear Allowance In Casing Design

With the design loads recommended it is highly unlikely that a reduction in collapse resistance due to wear will be critical at shallow depths or similarly that the reduction in burst resistance will be critical at the lower end of the casing string.

The most likely wear points in a deviated wells are at the kick-off point and near surface in the vertical portion where buckling may occur (particularly at the top of cement).

In the vertical wells, wear points may also develop at the top of cement if buckling occurs but unless there are known sudden changes in formation dip, which could cause a large

‘drilled dogleg’, wear is likely to be small and uniformly spread over the entire length of the string.

For most purposes, consideration of wear allowances can be restricted to deviated wells, with the most likely wear point at the kick-off point where burst reduction will be the prime consideration.

Since wear estimates are order-of-magnitude calculations, it is recommended that wear allowances be considered only in cases where the burst (or collapse) resistance of the casing at the wear point will be approached during the anticipated operating time in the string.

In marginal cases, it may well prove cost effective to run a base caliper survey to re-survey the casing prior to entering a hydrocarbon bearing zone (or pressure test the casing to the equivalent of the burst pressures anticipated from the zone) than to run heavy walled casing through all the anticipated wear sections.

The recommended procedure is therefore:

1) Conduct the casing design.

2) At the wear points, calculate the allowable reduction in wall-thickness so that the burst (or collapse) resistance of the casing just equals the burst (or collapse) load, including the appropriate Design Factor applied.

3) Estimate the wear rate in terms of loss of wall thickness per operating day.

4) Calculate, from the allowable loss in wall thickness and the rate of wear, the allowable operating time in the string.

If the allowable operating time is less than the anticipated operating time, use heavier casing (or increases the grade) 100m above and to 60m below the wear point until the allowable operating time exceeds the anticipated operating time.

If the allowable operating time is greater than the anticipated operating time (say estimated 50 days allowable versus estimated 20 days operating) do not include a wear allowance. If the allowable operating time and the anticipated operating time are about the same, either:

a) include a wear allowance or

b) monitor casing wear during drilling, and commission an intermediate string if the worn casing strength approaches the design loads.

In any given situation whether option a) or b) is exercised will be dependent upon a number of factors, many of which are beyond the scope of routine casing design.

Option a)

Is the conservative approach, but it may be too high, given the gross uncertainties inherent in wear estimations. However, in rank wildcats, particularly in remote locations, it may be justified.

Option b)

Requires a base caliper survey to be run immediately after installing the casing string, followed by runs at discrete intervals during the drilling phase.

If wear is proven to have occurred, and an intermediate string has to be commissioned early, the deeper objectives of the well may not be reached. However, conditions as drilling proceeds may indicate that the design loads assumed are not going to be encountered and the reduction in casing strength is acceptable.

In any event, valuable data on casing wear in the area will be obtained and field practices may be improved as result of the attention paid to wear, eventually leading to a reduction in overall wear rates.

In most cases, option b) is preferred.

4.11.5. Company Design Procedure

There is no reliable method of predicting casing wear and defining the corresponding reduction in casing performance. Because the reduction in burst and collapse rating is directly proportional to wall thickness the revised theoretical value may be calculated.

The normal procedure to cater for possible wear when designing casing is to select the next casing grade or wall thickness, therefore, in a vertical well, casing wear is usually in the first few joints below the wellhead or intervals with a high dogleg severity.

Consideration should be given to increasing the grade or wall thickness of the first few joints below the wellhead.

In deviated wells, wear will be over the build-up and drop-off sections. Again the casing over these depths can be of a higher grade or heavier wall thickness.