Casing Damage in Horizontal Wells

Casing Deformation Causes – George E. King, P.E., April 19, 2021

Engineers and Geomechanics professionals have been engaged over the last year in increasing industry knowledge of casing deformation and damage in multifractured horizontal wells. SPE supported and held a three-day virtual SPE Summit meeting in March and the work continues on several fronts. The following comments are my summary of the work of many people on identifying casing damage causes. Most of the following are supported by field examples. The damage causes vary across basins and across fields and many are not associated with fracturing. Some are singular but most are in combination with other causes. Rock fabric reactions to drilling, stimulation, production and to events not associated with hydrocarbon production have been identified. Following is a partial list of causes:

Casing damage causes that routinely rise in rank or risk can include:

·      fault & bedding plane slip or natural fracture tension-driven slips from frac pressure,

·      highly variable initial stress states, and very local stress increases,

·      well path & orientation with respect to stresses and hazards,

·      well design - particularly coupling design & casing weight (D/t<12),

·      perf cluster location & small spacing can cause fracture interference stresses.

·      fracture treatment design – volume, rates, ramp-up, stage numbers are factors in some cases,

·      fatigue from cyclic application of stresses (multiple fracturing) - affects both casing and cement,

·      frac driven interactions (or frac hits) can cause casing damage & production losses in offsets.  

·      dog leg severity (over 20o/100 ft.) hidden by use of drilling DLS information averaged over a stand of drill pipe rather than continuously. Correct tools use can spot it before casing damage.

·      poor cementing selection and placement may be a fatigue or damage factor.

·      thermal changes to formation and/or casing using cold frac water on a hot formation,

·      rock fabric variances including changes in rock fabric or stresses.

·      Subsidence in zones above or below the pay zone can cause casing damage – often during later stage production.

When during a frac and where along the lateral is damage likely to occur? Why?

“When” is controlled by areas where damage-inducing conditions are highest – which can be impacted by fracturing application, production and pressure depletion. The highest areas of risk may be triggered by pressure, temperature or stress changes in a specific area.  Casing deformation can be witnessed during pump-down or setting of a frac plug, or during plug mill-out.

“Where” is impacted by fatigue or fracture-to-fracture linkage from previous stages. A slight majority of casing deformation (ovaling of the pipe) is reported in the heel, but some areas report seeing deformations scattered along the lateral. However, few instances of damage area reported in the vertical section.

The “Why” question is probably the most elusive unless well-by-well analysis can identify the most likely damage mechanisms.

Frac driven interactions (frac hits) may be a factor when an active well fracture connects with a well that is unprepared for a direct influx of pressure, fluid volume or proppant carried by invading frac fluid. A growing frac takes a path of least resistance & perf clusters in the same fracture plane between active and offset wells will easily connect the wells, targeting depleted sections.

Recognizing casing damage before and during fracturing requires:

• A workflow of casing deformation risk, constructed by addressing when, where, why and causes.
• Areas with higher risk can be estimated – and labeled. 
• Monitoring offset well pressures can supply good information on timing and type of FDI.
• Treating each set of wells as independent with a step-by-step workflow. Fracturing guidelines can be written to assist field personnel on spotting fracturing problems real-time. 

I hope this small summary will help communicate the issue.