Frequently Asked Questions

Does casing expansion compress, crack or shatter cement?

Often the perceptions of the properties of cement are formed by observations of how it behaves in construction applications such as roadways, sidewalks, patios and basements. By extension, in the oil and gas industry the behaviour of cement behind well casing is not well understood. Unless there is a failure in the cemented casing, the attention is directed to what happens inside the casing, not outside.

But construction cement is not the same as that injected into a wellbore. Surface construction cement is actually concrete: cement plus an aggregate (most often gravel). The aggregate introduces millions of shear points within in the cement allowing it to crack or shatter. Cement on surface has no pressure on one side, the that’s visible. By contrast, wellbore cement is pure cement with no aggregate, and its properties are materially different than surface cement.  

Primary cementing in wellbores has advanced significantly over the decades to achieve the best possible cement seal. Cement enters the annulus at the bottom of a casing string and is displaced up the outside of the pipe. To improve uniform distribution, casing centralizers are employed, as is reciprocating the casing while the cement is being displaced and prior to setting. Under most conditions, cement is uniform and its density increases with depth.

Cutaway showing cement compression caused by casing expansion.

 

When casing expansion is applied to normal cement, it is compressed, not cracked or shattered. This is because under force, annular wellbore cement has nowhere to go. Cracking or shattering can only occur when the cement is in immediate proximity to an area of lower pressure or density.

Laboratory tests conducted on surface samples of cured API cement reveal that after controlled casing expansion, cement is compressed resulting in significant reductions in porosity and permeability.

Annular leakage in cement occurs primarily for the following reasons:

  • Poor primary cementing across porous, gas or water-bearing intervals;
  • Cement shrinkage resulting in the development of micro-annular passages between the cement and the wellbore, or the cement and the casing;
  • The existence of channels due to poor primary cementing across intervals with water or gas inflow greater than cement pressure;
  • Existence of channels on the high side of the hole in slant or horizontal wells caused primarily by gravity after cementing displacement pressure is released and before the cement sets up.

Copies of the laboratory testing is available upon request.

Does casing expansion weaken or damage the casing?

Extensive surface testing of CET on all common grades of API casing does not result in the casing splitting except under the most extreme circumstances. Steel is more flexible one might imagine. Under controlled expansion, casing may undergo several changes, depending on whether the casing is expanded elastically or plastically as follows:

  • Elastically, where the expansion occurs at lower internal pressures and the casing returns to its original shape when the pressure is released.
  • Elastically and/or plasticly where expansion and occurs at a higher pressure, where the casing partially returns to the original shape when the pressure is released.
  • Fully plasticly where the casing expands, and due to permanent molecular realignment, does not return to its original shape.

Winterhawk has taken samples of its permanently deformed casing to metal laboratories for analysis. Testing has revealed the following:

  • Increase in Vickers hardness;
  • Implied increase in tensile strength (tensile strength cannot be tested on a bent steel sample);
  • No evidence of cracking on weakening on the external face of the bulge.

Copies of the laboratory tests are available upon request.

Does casing expansion damage old or corroded casing?

The type of casing in which controlled the CET is likely to be employed is rarely available for testing. Remedial work is done in older wells where the casing has been cemented in place for many years, possibly decades.

Winterhawk approaches this issue in these ways:

  • Winterhawk has researched the strength, burst and plasticity ratings of all popular API casing sizes and weights. There exists a variation in these ratings within the API designation. Ideally, the well owner will have the mill specification of the casing within the well allowing Winterhawk to design its program accordingly.
  • Casing expansion to re-seal existing cement is targeted at intervals with gauge borehole and the best cement possible. Caliper logs of previous CET tests has shown that expanding casing to the point it would split or crack in these desired intervals is likely impossible.
  • An analysis of all common grades of API casing has been done factoring in the Barlow equation which adjusts casing strength for measured internal pitting. If pitting is known, Winterhawk will adjust casing properties accordingly.
  • If CET is being employed to augment remedial cementing in wellbore intervals with no annular cement, external casing damage will not be measurable using internal pitting analysis. A more thorough casing inspection including casing wall thickness may be required to assure expansion in uncemented borehole without damage.