We can face a common challenge while attempting zonal isolation during primary cementing: the resulting lack of “good bond” – as measured with cement logs – across permeable zones. This is typically seen as a sudden decrease in cement presence (increase in CBL amplitude or decrease in Attenuation in sonic logs or a clearer image, maybe some green or even red, in ultrasonic logs) overlapping with lower gamma-ray readings or micro-fracture formations.
The problem becomes more significant and challenging to address when coinciding with a depleted formation or sub-normal formation pressure. This degree of depletion will be leading to the higher differential pressure between the cement slurry and the pore pressure, as can be seen in the chart below.
From well cementing point of view, the combined effect of higher differential pressure and permeability will cause additional fluid loss from the cement slurry (cement filtrate) into the porous media compared to other formations above and below.
This additional amount of cement filtrate (fluid leaving the cement slurry’s constituents) to the formation creates a reduction in volume (cement available to fill the annular space), eventually leading to less cement once set, due to bulk shrinkage. In other words, the cement becomes denser (density = mass/volume) or the same solids with the less liquid phase, reducing the volume each “mass” of set cement occupies. This process will occur locally and predominantly in front of the permeable zone, and the longer/thicker the permeable zone interval, the more damaging the effect would be.
This volume reduction in the cement can induce radial cracks forming once the cement is set. In simpler terms, as there is not enough cement to fill the annular space, the cement sets in tension being “pull” circumferentially, leading to cracks parallel to the casing extending from the casing to the formation across the cement.
In logs, this can be seen similar to the lack of or poor casing-cement bonding or micro-annulus signature (high amplitude in CBL vs. clear formation arrivals in VDL) but, with the main difference being the clarity of the formation arrivals in the VDL. In the presence of radial cracks, induced by bulk loss of cement filtrate, the imagen of formation arrivals, if present, will be less noticeable or weaker; this is because of the combined presence of inner (casing-cement) and outer (cement-formation) micro-annulus.
Now, the big question?
Is this lack of bond remediable? Or Is squeeze cementing attainable?
More profoundly, is zonal isolation in jeopardy?
Moreover, the answer is YES, zonal isolation is jeopardized, but this is not easily remediable.
Why? Because every time, during a squeeze cementing operation, the casing, or a portion of it, is pressurized, and any micro annulus will tend to close, forcing the fluid inside out; limiting the space for the further flow of fluids away (both occupying fluids and squeezed cement slurry filtrate).
Factors that can increase the success rate are perforating density and phasing, not shot penetration or diameter. Penetration and shot diameter would make the injectivity higher but not favor injection or flow to the affected area in the annular gap.
More importantly, why remediate (perforated casing + squeeze = possible casing leak in the future) if “bulk loss of cement filtrate in from permeable zones” can be prevented? Yes, this situation is preventable, leading to an OK cement log the first and only time. This must be the objective.
Information about the possibility of this problem usually comes from the well design process, with the more professional ones incorporating it in the risk register or well construction (most common drilling) challenges table. The cement engineer should include formal elements to the cement slurry and cement placement process with this information.
The accuracy of the proposed design elements to the job scheme will be determined by “Experience.” Either experience of the engineer her/himself or collective experience in the area; in other words, previous experience in the form of the experience of the engineer in other locations around the world or collective team experience in the same location because of previous similar failures (the problem is happening for some time already).
In a later post, the slurry and job design will be discussed to overcome this problem and get the best cement log right from the first time.