Casing Wear

Bade Olotu

Well Academy, Vienna, March 2012

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Primary Cause of Casing Wear

Primary cause of casing wear: Rotation of drillstring against casing wall.

Resultant wall loss can have detrimental effect on mechanical integrity of casings Must be included in the design premise.

Casing wear increases with:

Increasing contact force between drillstring & casing. Build & drop sections in deviated wells (particularly in shallow regions of the wellbore), localized doglegs & buckled sections of casing. Optimized wellpath selection significantly reduce casing wear.

Increasing contact time. Sections exposed to slow ROPs, long hole intervals, & multiple hole

intervals with intermediate liners.

Increasing roughness of drillpipe tool joints.

Decreasing mud weight. Barite is a very effective wear reducer.

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Effect of Wear on Wall Cross Section

Localized wall loss due to casing wear will have a significant effect on: Burst & collapse resistance of the pipe.

Reduced burst & collapse ratings should be calculated - based on minimum wall section resulting from wear.

Effect of wear on axial rating is much less: Localized wear will reduce the cross-sectional area only nominally.

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Design Process to Account for Wear

Following steps should be included in design process to account for wear:

1. Perform casing design & calculate allowable wear based on burst & collapse design loads. Allowable wear is calculated automatically in StressCheck.

2. Estimate anticipated wear based on deviation profile, mud program & drilling practices. Casing wear programs available (e.g CWear).

3. If estimated wear ≥ allowable wear, Modify the directional program or drilling practices to reduce the wear Increase wall thickness in the wear interval to increase allowable wear.

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Definition of Casing Wear

Casing wear:

Removal of material from ID of casing due to abrasive action of tool joints while drilling the next hole section.

Usually drillstring body does not contact or wear casing. Instead, drillstring rides on tool joints, & tool joints causes the wear.

Burst, collapse, & axial strengths of casing directly related to wall thickness & hence reduced by wear

Buckled casing greatly promotes wear - contact between tool joints, drillpipe body, & buckled casing.

Important to design hanging or landing casing to avoid buckling if casing will be drilled through.

Also for high-temp wells - temp increases enhances buckling. Important to cement the casing to prevent buckling or to account for drilling through buckled casing in the wear prediction.

Wear decrease collapse strength of casing proportionaly todepth of the wear.

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Wear Pattern

Casing wear tends to create a groove at one side of the ID of the casing.

Groove shape corresponds to diameter of tool joint on drillstring - does not remove material 360° around ID of casing.

Design however assume wear applies 360° for burst & collapse calculations.

Generally TJ on drillpipe can have hardfacing: protect longevity of TJ but aggressive for wear on casing.

Wear is caused by the rotating, abrasive contact of TJ - high-side loads push TJ against casing.

Wear is often found in casing joints just below the hanger. Usually due to misalignment of rotary table relative to wellhead.

Larger wear design margin (increased wall thickness or use of mitigation techniques) may be prudent for 5–10 casing joints just below the hanger.

Ensure that CHH, WB, casing hanger, & 1st few joints below hanger have common ID

Eliminates high localized contact loads.

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Wear Mechanisms

3 types of wear mechanism:

Adhesive wear Abrasive wear Grinding wear

Parameters such as contact load, surface roughness, hardness, geometry, & chemical composition of both TJ & casing + mud composition determines kind of wear mechanism.

The rate of wear depends on the wear mechanism.

Most wear depends linearly on side load pushing TJ against the casing Independent of contact pressure (contact load divided by worn contact area).

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Wear Mechanism

Adhesive Wear (galling wear):

Dominant wear mechanism for smooth steel TJ at high contact pressures. Local galling welds forming on casing & sheared off by relative motion of the mating surfaces. Produces flake-like wear debris.

Abrasive Wear (chipping wear):

Sharp particles on a heavily hardfaced TJ cut chips out of casing material. Produces fine cuttings or chips similar to machining on a lathe

Grinding Wear:

Abrasive but not as severe - heavy mud effective buffer between TJ & casing, Wear is a milling process producing very fine powder-type debris.

Wear rate expressed in terms: Volume of casing material removed per unit of time. Wear depth per unit of time (enables calculation of remaining collapse,

burst, or axial capacity of the worn casing)

Wear volume & wear depth related by geometry of the contacting surfaces

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Modeling Casing Wear Process

Aim of casing wear modeling is to predict in advance where casing wear will occur & how severe it will be.

Most common technique is to modify theoretical models with the use of wear factors that reflects field & observations.

Casing wear models have to take into account the following: Magnitude of the tool joint–casing contact pressure Geometry of the contacting surfaces Relative roughness of the contacting surfaces Material of the contacting surfaces Magnitude of relative velocity & time mating surfaces are in contact Drilling-fluid composition

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Geometry of Casing Wear

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Depth of wear related to volume of wear by means of curvature of contacting surfaces.

High contact loads can lead to concentration of bending near connections

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Tool Joint Hardfacing

Casing wear depends very strongly on the type of TJ hardfacing used.

Plain steel TJ give rise to severe wear in brines or unweighted muds, but only mild wear in oil-base muds or weighted water-base muds.

TJ with exposed rough hardfacing lead to severe wear irrespective of the drilling fluid.

TJ with a few specific, smooth hardfacing materials lead to a minimum rate of wear in all muds.

Recommendation: TJ opposite casing use only qualified smooth hardfacing, which has been shown to cause minimum wear.

Tungsten carbide hardfacing should not be used opposite casing.

Also TJ with overlay covering tungsten carbide should not be used opposite casing.

Severe & dangerous wear rate can occur when overlay starts to wear away over time.

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Minimizing Casing Wear

Casing wear is primarily controlled by the type of TJ hardfacing, mud, & most importantly, contact load between TJ & casing.

Elimination or reduction of casing wear focuses on these factors.

Contact Load

Wear is nonlinearly proportional to the contact load.

Approach – where possible, reduce drillstring contact load. When well builds angle & reduces drillstring tension, wear tends not to be a problem. Shallow unplanned doglegs & correction course increases drillstring tension & contact load.

Wellpath Selection

High contact loads occur when TJ are pulled firmly against the casing over a dogleg zone (build / drop section).

Ensure DLS in build & drop sections of deviated wells ALAP to minimise contact force Use torque & drag software to analyze contact loads for alternative

wellpaths.

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Minimizing Casing Wear

Use of Drill Pipe Protectors

Use rubber DP protectors to distribute contact load over a larger area.

By mounting one protector per single (at mid-joint), contact load at TJ reduced by 50%.

Wear caused by rubber DP approximately 5% of wear due to plain steel TJ under same conditions

Other Ways to Minimize Wear Consider using a mud motor – reduce total rotating hours to drill an interval. Consider using synthetic muds to achieve higher ROP – less time to drill

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Course Discussions

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