In the drilling fluids service industry the term “inhibition” has come to mean the suppression of hydration of clay, which means inhibit the water to transform the shale into swelled, soft, sticky shale, while her ewe mean the inhibition of swelling pressure. After this knowledge it is acceptable to define inhibition as Reduction of swelling pressure.
Before tending to inhibitive muds we would like to introduce some basic issues concerning swelling pressure and hydraulic pressure.
Swelling pressure vs. hydraulic pressure: Inhibitors cannot prevent pore pressure increase induced shale problems because the inhibitors (in the diffusion front) will always lag behind the pressure front. Swelling pressure can therefore not be reduced down to zero. An effective tensile force is remaining supporting the shale. When the net tensile forces overcome the shale’s tensile strength (low in shale), yielding will occur at the weakest sites, which will trigger subsequent full-scale failure. Pressure fluctuations (from S&S) will change hydraulic support, and may deliver the final blow to already weakened shale.
This time-lag in inhibitor transport is regarded as the main reason behind inhibitors shortcomings as shale-stabilizers. When inhibitors are arriving together with water they may lead to very small to low pswell (as opposed to large pswell if no inhibitor was present).
Under these circumstances something more than inhibition is needed. The answer is: Prevent water flow to suppress pressure penetration. This is how to achieve it:
- Apply radial support through proper MW (prerequisite).
- Maintain support by reducing filtrate invasion.
- Use inhibitive mud.
Pressure inside the cuttings: Cuttings are exposed to the same mechanisms as the wellbore, except that
- Geometry and stress condition are different.
- Timing is different. Exposure is typically only 1 hour.
In Figure 1 the forces action on a shale cuttings is presented.
The resulting radial forces can be expressed as:
δr = phydr – ppore – pswell
and will be in tension if phydr < ppore + pswell and disintegrate if pcohesion is overcome.
The following will take place:
- phydr will lead to slow invasion and equalize ppore, but normally not within 1 h.
- A bigger problem is the phydr reduction as cuttings are transported up the wellbore, combined with slow increase of pswell, probably the most detrimental effect.
- Shut off water by enhancing viscosity of filtrate.
- Use inhibitive mud.
Inhibitive muds can be grouped into three types. We will now compare their pressure transmission-ability through shale, and present the most common representative mud for each mud type.