Cement needs to develop strength to protect and support the pipe, and to allow subsequent operations in the well. However, just how much strength is needed for proper protection and support of the casing is not well understood. It is likely that, in general, the industry uses cements that develop more strength than is needed.
Cement contamination with well fluids that can dramatically reduce strength so this is good insurance. However, high strength cements are brittle, and recent studies have suggested that the industry needs to consider using lower strength cements (below 2000 psi) across pay zones. Some of these lower strength cements are better able to withstand casing perforating and loads imposed by pressure and temperature changes in the wellbore, without damage to their integ-rity.
The poor understanding of strength requirements is, in part, due to the very sim-plistic crush test used in the industry. This unconfined 2” cube test is really only of any use as an indicator of strength development and as a means of comparing dif-ferent samples. The numbers generated are not useful in design calculations where complex loading needs to be addressed.
The strength development of cements depends mainly on temperature and the water/cement ratio.
Low well temperatures tend to cause slow strength development. This can pre-sents a serious design challenge for shallow casing strings and/or low tempera-ture environments like the North Slope and in deepwater locations. On the other hand, elevated temperatures accelerate the development of strength.
At temperatures above about 230F, powdered silica needs to be added to the cement to avoid the phenomenon known as ‘strength retrogression’.
The compressive strength development of the slurry is normally measured only after the other properties have been determined and optimised. If the strength developement is acceptable, the cement slurry formulation is considered satisfac-tory.
No Permeability
Protection of casing and prevention of fluid migration within the cemented annulus during the life of the well requires set cements to have very low permeability once fully set. Fortunately, cements normally develop very low levels of permeability.
The Table below shows examples of permeability values for several neat cements.
Figure 11: Water Permeability (md) of Some Neat Cements After 7 Days of Curing
(1) course ground, 40% water (2) medium ground, 46% water (3) fine ground, 70% water
Set cements, cured at temperatures less than 230F develop very low permeabili-ties, much lower than those of producing formations and similar to those of shales. However, the final permeability of a cement is a property over which nor-mally, relatively little control can be exercised. The API RP-10B contains a proce-dure to measure the permeability of set cements but, under normal conditions, not much attention is given to the permeability of the set cement, and permeabilities are not often measured.
Cements subjected to high temperatures – higher than 230 deg F – which have not been stabilized against strength retrogression will show increased permeabili-ties with age due to the chemical phase changes. The Figure illustrates this effect.
Stabilisation with around 35% silica bwoc will prevent high permeability.
Figure 12: Effect of Temperature on Permeability
Temperature F API H (1) API A (2) API C (3)
80 0.00218 0.000167 0.0000000537
120 0.000001 0.000241 0.0000000613
140 0.000175 0.0.0213 0.0000000459
160 0.000983 0.0172 0.0000000915
No Shrinkage
Shrinkage is perhaps the most misunderstood and controversial property of oil industry cements. The main reason is that the problem is complex. The API has published a pamphlet (API Technical Report 10 TR2) on expansion/shrinkage of cements that attempts to explain the phenomenon and better define the terms.
Shrinkage and expansion of cements results from the formation of hydration prod-ucts having densities which differ from those of the reacting components. These hydration reactions can cause:
• Changes in pore volume of the cement matrix
• Changes in pore pressure in the matrix
• Changes in the cement physical dimensions
• Changes in internal stresses
The actual behaviour depends partly on whether the cement is able to draw in extra fluid or gas from the surroundings. When cements are sealed in totally impermeable membranes – as they would be if used in an inflatable packer, ECP, – then bulk volume shrinkage of around 2% is measured. If the cement is against permeability – say a water bearing sand – and extra water can be sucked into the sample as the hydration proceeds, then the total of the inner and bulk shrinkage is around 6%. This inner, chemical, shrinkage is related to the development of inter-nal porosity and permeability
Not surprisingly, temperature can also influence the resulting behaviour.
One reason for the confusion around shrinkage is related to the time at which measurement starts. If cement is mixed with water, conditioned, then sealed in a membrane while it is still a pumpable slurry; then the bulk volume change, mea-sured from this early time, follows the curve shown below. It may never ‘expand’
since it will not cross the axis. However, if the sample is allowed to set solid and then the measurements are started then the cement is seen to be expansive (see the new origin ‘Time Zero’ on the curve).
For cement inflated packers, cement shrinks and the sealing has to be compen-sated for by the rubber. In a normally cemented annulus, early shrinkage may be compensated for by movement (compaction) of slurry.
The negative pressure developed during hydration of the cement can draw in gas from the formation. This effect, coupled with other properties of the slurry, can lead to gas migration and an incomplete annular seal.
So-called expanding cements should only be used in extreme cases and where their behaviour is understood in detail. When an unrestrained annular ring of cement expands, the hole in the centre gets bigger not smaller. In a weak forma-tion, cement expansion can produce a micro-annulus.