First of all, I would like to define what is Gas Migration from an acceptable industry reference. “Gas migration is the invasion of formation fluids (gas in this case) into the annulus due to a pressure imbalance at the formation face, where the fluids (gas) may migrate to a lower pressure zone or possibly to the surface”.
Following primary cementing, it can be “logical” to think that, if we see gas at surface, this gas is “migrating” through the cement slurry. This migration could be probably due to, and this is the accepted theory, an imbalance between the formation pore pressure and the annular pressure; created during the transition time of the cement. The definition of “transition time” is the time when the cement slurry is not longer a liquid, capable of transmitting hydrostatic pressure, but is not yet a solid, able to bond and isolate the formation. During the transition time the solid content in the cement slurry becomes self-supported as a result of hydration (gelled structure) and the remaining porosity filled with water-like fluid becomes underbalance with the formation pore pressure.
Typical methods to prevent this to happen involves a cement additive to either viscosify the resulting fluid inside the porosity or lower the porosity itself. This is considered a mitigation measure, because in both cases the “root cause” (underbalance during the transition period) is not removed; instead the focus is in “stopping” gas to move through the gelled cement.
It is important to mention that, this is the FIRST truth, gas migration through gelled-cement only happen if the cement is in contact with the formation (gas source), in other words liquid-flowing cement was able displace mud from the hole. For a minute let’s think was is “the mud” to be removed:
The thickness of both the gelled mud and filter cake are governed by the permeability of the mud cake (related to mud fluid loss) and the formation permeability. The existence of these two mud zones occurs both in static and dynamic conditions. If the well is properly conditioned most of the gelled mud is removed during pre-job circulation and cementing operation, while the mud filter cake is removed mainly during cement placement (if cement design is properly done). However, in case when the mud is unable to properly form an impermeable filter cake, the gelled zone persists during the cementing operation forcing channelling and incomplete coverage.
Now coming back to gas migration, and this is the SECOND truth, it should be clear that proper cement coverage is fundamental to prevent gas migration. It is actually, what makes sense when a gas migration control slurry is used (high cost). In other words, if proper placement is not happening whatever fancy gas migration control slurry is used, it is actually a waste.
Let’s review for a while what are the path for gas migration after primary cementing or through cement:
Most of the gas migration paths are related to cement slurry properties, and that is the focus for most people, but let me just mention one thing here before I continue; and why not let’s call it the THIRD truth: Just because a gas migration control additive is used does not mean the slurry is able to control gas.
The other two paths for gas migration are related to the mud and cement placement. Some people, even cementing engineers, do not pay enough attention to this and it is perhaps the most important element. This is the FOURTH truth: Pay attention to cement placement and channelling prevention (including rheology, rates, compatibility, friction pressure – for cementers; and rheology, API fluid loss – for mud engineers).
Coming back to gas migration during the transition period of the cement slurry, I mentioned that using gas migration control chemicals is an action to mitigate the risk. Now let’s talk about prevention, for me the best way to understand what are our alternatives to prevent gas migration (lower the probability) is by looking at the below equation:
CSGS is the static gel strength of the cement that results in the decay of hydrostatic pressure to the point at which hydrostatic pressure equals pore pressure
OBP is hydrostatic annular pressure minus the pore pressure,
L is the length of the cement column above the flow zone,
DOH is Diameter of the open hole
DCSG is the outer diameter of the casing
This equation and a detailed explenation can be found in: API STANDARD 65 – PART 2, Isolating Potential Flow Zones During Well Construction, Dec. 2010. This is highly recommended reading material.
What you need to understand from this equation is that the lower the CSGS the higher the chances of gas migration to happen. Particularly when CSGS is lower than 250 lbf/100ft2, the risk of gas migration is high. Looking at the equation, the following are valid assertions:
Cement length, L, has a more significant influence in this equation. This is the FIFTH truth: choose your TOC wisely.
I almost forgot, what happens if the job objective requires a low CSGS value?. In this case you need a gas migration control slurry. Correct?. However, I said that just because a gas migration control product is used does not mean the slurry is able to control gas. So, what is the requirement?. Well if you CSGS is low, you need a cement slurry able to make it higher, in other words a cement slurry able to develop static gel strength.
Static gel strength development, as a cement slurry property is measured as CGSP, Critical Gel Strength Period. CGSP is defined as the time between the development of the critical static gel strength (CSGS) and a static gel strength of 500 lbf/100 ft2. At this last value, 500 lbf/100 ft2, it is accepted that gas cannot move. This transition needs to happen in less than 45 min, as per the API STANDARD 65 – PART 2, in order for the cement slurry to be considered able to control gas migration.
As you can see, the requirement does not mention the use of gas migration control products, however they are probably the easiest way to trigger a sufficiently rapid gel strength development, the concentration and chemical nature plays a very important role. This is SIXTH truth: Next time ask your cementing contractor for the Static Gel Strength Development test / chart and look for this transition to be less than 45 min.
I hope you can find this post useful, and if you have any comments or questions, please let me know or let's start a discussion.