I’ve worked in six different oil-wells until now. All of them used KCL-Polymer system mud. I never work with another mud system so far (High performance mud, Oil base mud, synthetic base mud, etc)… How pity I am! . Well yeah, this KCl-Polymer system is still considered as a powerful mud system to combat reactive shale or clays. It’s firstly used in mid-1950 and started widely used from late 1960’s to present. So actually, this mud system is not really sophisticated . But It’s inhibitive enough, tolerant of contaminants (salt, gypsum/anhydrite, and cement) and help reach high mud weight (since the KCl salt contribute to raise mud weight pretty well). That’s why it’s been widely used all over the world.
Now there’s might be a question comes up , why Shale or Clay must be combated? well, Because it possibly swells so badly once it is contacted to water (like a rubber when it expose to diesel oil). It’s like become 8 – 10 times bigger than its original size. It is obviously dangerous to drilling operation as It will cause some problem such as (stuck pipe, rheology problem, etc). Another question that might be come up later is why shale/clay works that bad? because the shale has molecular structures that possibly invites water and other particle to come onto. The particle consists of 2 layers that have different charges in it. The basal plane (flat planes) is predominant of negative charges (-), and edge side is mostly positive charge (+).
Why it is? Hmm,,,I need separated chapter to tell it why. it’s totally chemistry🙂 , physical chemistry actually. Let’s pick up a smectite, one of the clay minerals that is considered as the least stable and the most susceptible to hydration and diagenetic alteration. Two other terms often associated with the smectites are montmorillonite and Bentonite. Since bentonite has been exposed to seawater and other sources of cations, some of the silicon and aluminium cations in the structure have been replaced. Iron and magnesium typically replace aluminum and aluminum typically replaces silicon. This replacement causes an imbalance in charges in the structure. This causes the bentonite sheet to be negatively charged. Cations (positive charged particles) are attracted to this negatively charged surface.
As time goes on, water molecules begin to seep in between the platelets. Some of the molecules of water adhere to the clay platelets and some of them go to the individual cations on the platelet surface. When a clay platelet is fully hydrated, it is surrounded by a cloud of water and hydrated ions. The Greatest concentration of water molecules and hydrated cations are near the surface of the clay. And the swelling just happened.
Well, back to the KCl system. How does it work? Which of the parameter that important to monitor periodically?
Here we go! The KCl-Polymer system provides potassium ion (K+). It works strongly because particular size of the ions in relation to the clay crystal structure. The K+ ion size really fit to the shale/clay pore (10 Angstrom) once it arrived at the surface of clay or shale. Then it inhibits the water to come into it and reduce the dispersion of clays. And the swelling is just avoided. Cool right?🙂 So, things that really important to measure is the K+ concentration in a mud. There’s a couple method to measure it, but the method I ever had is the hand crank centrifuge (I’ll explain it later!).
Although, it is recognized as a powerful mud system, there’s some limitation though which are: The Toxicity of KCl—typically 3-10 % by weight concentration, disposal cost, mud system cost at hight KCl levels. But I don’t know why the HSE officer told me that the treated mud (water from mud that’s been treated) is really good for fertilizer. He dumped in a yard and the grass then grew nicely. I think it comes from the K, which accidentally has good concentration for the grass growth. kinda miracle😯
Anyway, the combination of KCl with the polymer will enhance its effectiveness regarding to combat shale. The polymer that used is PHPA, that’s why sometimes it called ‘ The KCl-PHPA mud”. PHPA stands for Partially Hydrolyzed Poly Acrylamide. It was introduced by Shell Oil Co in late 1960’s. The PHPA helps to encapsulate (coats) the clays/shale surface with a protective film. This blocks or slows Base Exchange and hydration. But the poor solid control prevented wide spread use. Yep, it definitely needs a good solid control equipment (Shale shaker, etc).