# Casing Wear Series – 3: Prevention

Computer casing wear modeling reduces risks and can identify potential problems prior to its occurring. Necessary modifications on casing designs and drilling parameters could be made before the pumping starts once we can predict the location and magnitude of wear.

Figure 1 shows the 3D visualization of magnitude and location of wear in a previously set casing.

Figure 1. 3D Visualization of Casing Wear

The knowledge we have acquired through decades of studies, lab testing, post-job analyses and computer modeling provides a good foundation for the following casing wear preventive measures:

• Minimize dogleg severity and expect real dogleg at least 1.5 times higher than the planned value.
• Use casing friendly tool joint materials.
• Reduce rotor speed and use downhole motor.
• Increase ROP.
• Select proper mud type and add lubricants to reduce wear and friction.
• Use drill pipe protectors.
• Use thick wall casing in the anticipated wear section area.
• Use software to reduce risks.

# Casing Wear Series – 2: Prediction

1. Wear Mechanism

The casing wear model applied in CWPRO (casing wear prediction software developed by PVI) assumes that the metal volume worn away in a wear groove section is proportional to the frictional energy transmitted to the casing by a rotating tool joint as shown in Figure 2 in the Casing Wear Series – 1: Causes.

The transmitted frictional energy is defined in this formula:

Where:

E  = Frictional Energy, lb-ft

μ  = Friction factor, dimensionless

SF= Side force on tool joint per foot, lbf/ft

SD = Sliding distance traveled by the tool joint against casing wall, in

The volume of casing wall removed per foot in time t hours is mathematically expressed in the equation:

Where:

WV = Casing wear volume per foot, in3/ft

WF = Wear factor, E-10psi

SFdp = Side force on drill pipe per foot, lbf/ft

Dtj = Tool joint OD, in

N = Rotary speed, rpm

t = Rotating time, hr

The definition of wear factor is the ratio of friction factor to specific energy, which is the amount of energy required to remove a unit of steel. The unit for wear factor is E-10psi-1; therefore, when a wear factor is reported as 8, the actual value used in casing wear calculation is 8E-10psi-1.

Quite a few experiments were conducted to find the casing wear factors under different mud systems, tool joint materials, casing interior and drill string protectors. Among them, Maurer Engineering Inc. conducted joint-industry project DEA-42. It was reported that more than 300 laboratory tests were performed under DEA-42 to determine the wear factors for various drilling conditions.

For a typical water-based mud, WF can vary as follows:

Normal or low: 3 – 7

Medium: 8 – 13

High: 14 – 20

WF above 20 can be considered as very high and may cause severe casing damage.

2. Wear Geometry

A typical wear groove is shown in the following figure.

Figure 1. Casing Wear Groove

The relationship between wear depth and casing wear volume is:

Where:

WV = casing wear volume per foot, in3/ft

h = wear depth, in

r = tool joint outer radius, in

R = casing inner radius, in

S = R - (r - h), in

P = (R + r + S) / 2, in

3. Software

Based on the R & D results from the past two decades, PVI developed CWPRO, software that enables us to understand the casing wear phenomenon and accurately predict casing wear before the drilling operation or perform a post-drilling analysis.

CWPRO is a comprehensive casing wear prediction software with built-in torque and drag function. For every incremental drilling interval, the amount of energy transferred from drill pipe to casing is calculated. Accumulative wear and wear depth are first obtained and then the burst and collapse strength of the worn casing can be assessed.

Being a time-dependent incident, casing wear deepens as we drill deeper. Figure 2 shows the sequence of drilling and corresponding wear profile along the previously set casing.

Figure 2. Time-dependent Casing Wear

# Flying Among The Clouds

Louis D. Brandies once said:

“Most of the things worth doing in the world had been declared impossible before they were done. Impossible means that you haven’t found the solution yet.”

A little over 100 years ago there were things that were considered impossible to do and that there was no way they could ever be achieved. For instance, to be able to fly among the clouds, but was it really an impossibility? Time proved that it wasn’t.

Just like flying among the clouds was impossible to do once, there are many things that thanks to the advancement of technology now are possible. For instance, a few decades ago horizontal or extended-reach drilling was considered impossible as well as casing wear prediction. In these environments, casing design is critical to a safe and successful drilling operations and well production, and unexpected casing wear can result in significant costs or even the loss of the well itself. This is the problem that drilling companies want to prevent.

So the question is: Is there any tool or software to calculate and predict casing wear severity? Yes there is! It’s called CWPRO.

This casing wear model uses the number of drill string rotations and contact force between the drill pipe and casing to calculate wear. The contact force is calculated using the dogleg severity inside the well. The maximum dogleg severity frequently determines the location and extent of the most severe casing wear. CWPRO helps operators and service companies identify, control and prevent potential problems. In overall the goal of CWPRO is to more accurately quantify casing wear risks and to ensure that the integrity of the casing is maintained during drilling operations.

Like mentioned before, there are many things that were considered an impossibility not too long ago like for instance, flying among the clouds. Likewise, thanks to software like CWPRO, predicting casing wear is no longer impossible; it is a fact.