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September 1999 Issue 2 Section C10 Casing Wear 10-1

BP Amoco Casing Design Manual BPA-D-003

The information in this section of the casing design manual has beenassembled from practical experience, experimental tests on casing, tool joints, mud, and common sense. There are few ‘theoretical’ solutions or rulesto solving casing wear, but by understanding its cause, we can reduce itspotential impact through well design and drillpipe specifications.

Casing wear takes the form of a localised, crescent-shaped groove cut by arotating drillstring forced against a casing’s internal surface. Figure 10.1shows where and how casing wear develops. The combination of highsidewall forces and extended drillstring to casing contact around the kick-offsection of a well profile puts this area at greatest risk but localised doglegs,eg from buckling, can cause severe wear wherever they occur. Casing wearultimately leads to failure. However, before actual failure there are foureffects.

• The wear groove reduces the casing string’s pressure integrity which willaffect well control, leakoff, testing and production procedures

• Casing and drillstring tool joints both wear, the latter leads to expensiverebuild or replacement charges

• Rotational friction and hence surface torque can be high (a secondaryeffect not always observed)

• The wear groove may act as a preferential start point for future corrosion

10.1

Parameters forCasing Wear

Casing wear is the result of a complicated system involving the relationshipbetween casing, tool joint and mud. Minimum casing wear can only beachieved by balancing the effect of each component. The best lubricant inthe world will not perform with rough hardbanding. There are threedifferent wear mechanisms (types of wear) which can occur, depending onthe operating conditions.

FIGURE 10.1WHAT IS CASING

WEAR?

A

Section

A'

Kickoff

Casing Wear

Dogleg

Casing

Drill string Tension

Tension

RPM

Tooljoint

Wear Groove

Cross Section AA

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September 1999 Issue 2 10-2 Casing Wear Section C10

BP Amoco BPA-D-003 Casing Design Manual

ADHESIVE WEARAlso known as galling adhesive wear involves the transfer of materialfrom one surface to another by solid-phase welding, see Figure 10.2A.Transfer commonly occurs from the casing to the tool joint. This process

may result in the production of loose wear particles which can be flake-likewear debris.

FIGURE 10.2AADHESIVE WEAR

Adhesion Transport Flake Formation

Tool

Joint

Flake

Steel asingSteel Casing

ABRASIVE WEAR – MACHININGThe removal of material from one surface due to the cutting action of hardprojections on another, see Figure 10.2B. These hard projections willcommonly be the tungsten carbide particles of tool joint hardbandingexposed as the softer binding material holding the particles in place wearsaway. Alternatively hard particles may become embedded in one surfaceand machine the other. The resulting wear debris resembles long chipsor ‘steel wool’.

ABRASIVE WEAR – GRINDING/POLISHING

Occurs when, hard particles become trapped between the tooljoint andcasing surfaces and abrade one or both. This can either cause ploughingand the formation of grooves or grinding producing a honed surface andfine powdery wear debris. See Figure 10.2C.

FIGURE 10.2CGRINDING WEAR

Chip

Weld

Matrix

Tool Joint

Tungsten arbide

Steel asing

Weld

Matrix

Tool JointTungsten Carbide

Steel Casing

Hardmetal Tool

Steel asing Powder

Hardmetal Tool

Steel Casing Powder

FIGURE 10.2BMACHINING WEAR

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September 1999 Issue 2 Section C10 Casing Wear 10-3

BP Amoco Casing Design Manual BPA-D-003

There are two important points to note about the wear mechanisms:

• The wear rate increases by two to three orders of magnitude throughthe transition from grinding to machining wear. (Note: most casing

wear is a type of machining and is generally caused by poorhardbanding specifications.)

• The wear process determines the condition of the two sliding surfaceswhich in turn governs the friction. It is interesting to note, however,that high friction and low wear can occur simultaneously, especiallywhere the applied force is evenly distributed, eg a car brake shoe.

Controlling the wear mechanism is the key to minimising casing wear.However, it is impossible to rank individual parameters as each contributes

to balancing the whole system.

Practical tests have also shown that tripping does not normally contributemore than five percent of the wear depth. This is because contact timeis low, except at casing crossovers in high tension wireline operations,and the wear mechanism is due to ploughing, hence the vertical scoremarks in the wear groove typically seen on recovered casing.

The Maurer CWEAR program is the recommended BP Amoco softwaretool for casing wear prediction.

Figure 10.3 shows that as the wear groove deepens, the area of contactbetween tool joint and casing increases. For a constant sideload, thisequates to a decreasing contact pressure. It has been experimentallyproven that the choice of wear mechanism is governed by the contactpressure between the two surfaces.

10.2

Predicting CasingWear

FIGURE 10.3CONSTANT LOAD BUT

DECREASING PRESSURE

10.2.1Contact Pressure vs.

Load

1 2

Casing

Rotation

RPM

Tool

Joint

Tool

Joint

Point Line Contact

RPM

Increased Area

Decrease Pressure

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September 1999 Issue 2 10-4 Casing Wear Section C10

BP Amoco BPA-D-003 Casing Design Manual

Specifically the transition between machining and grinding appears tooccur at approximately 250psi (see Figure 10.4). In practice this meansthat the high initial contact pressure caused when tool joint and casing firsttouch, causes severe machine wear which, as the load is distributed,

decreases quickly to a steady state of grinding wear. Figure 10.5 showsthe empirical results of full-scale hardbanding/casing wear tests undertwo loads which validate this statement. Note how the wear rate flattensoff when the wear groove depth reaches 0.15in.

The Drillstring Simulator can be used to calculate the sideloading requiredto keep beneath the transition pressure of 250psi at various groovedepths.

Before designing a well, consider ‘what-if’ scenarios. What casing wearallowance will cover an unexpected change of plan at minimum initialdesign cost, eg a sidetrack? The higher the risk of drilling extra footage,the greater the wall thickness allowance or precautionary measuresrequired. It is important to note that extra rotating time can alsooccur because of stuck pipe, fishing operations, etc. If the drillingprogram is prone to such problems, then additional allowancesshould be made.

FIGURE 10.4TRANSITION PRESSURE

W e a r R a t e

Contact Pressure (psi)

0 100 200 300 400 500 600

GrindingWear

Machining / Adhesive Wear

Threshold Pressure:

≈ 250 psi

10.2.2Well Design

Guidelines

Documents

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