PHPA (partially hydrolized polyacriamyde) is one of the common shale stabilizer in the industry. Typically, it’s the high molecular version and the molecular weight range up to 20 million 😯
How is it made?
It’s better to know how it was made before jump to operational practices, so we have better understanding. PHPA is a copolymer, which is polymer that contains two or more different types of monomer, consist of acrylate and acrylamide. The two monomers linked together in random fashion to form linear, carbon backbone.
Actually, the acrylamide is insoluble in water. So, it’s necessary to co-polimerize with sodium acrylate to achieve solubility. The carboxyl group in polyacrylate contributed to make the PHPA to be an anionic polymer. That carbon – carbon linkage make PHPA has an exceptional thermal stability and resistance to bacteria.
Inhibition Mechanism :
- Encapsulation. the PHPA wraps around clay platelets and wellbore as the polyacrylate has too much affinity with positive edges of clay. Since PHPA has a high molecular weight and is relatively long, it combines with several site along the wellbore, this has effect of coating the wellbore and resisting water from entering the clay. The encapsulation process also prevent water to enter interlayer structure of clay. See above figure!
- Viscosifying the water phase. It slows movement of the fluid into interlayer structure of clay.
- Adsord free water by polymer. It reduce the amount of water available to enter the structure of clay.
Limitation (Very Important!):
- Calcium. It will precipitate the polymer. Ca++ is strong cations and will create a strong cationic bonding with PHPA (Remember, PHPA is anionic polymer!) and drop it to the system (precipitated). i’s recommended to treat soluble calcium to below 300 mg/l
- pH has to be less than 10. Otherwise, it will hydrolyse the PHPA and releasing ammonia. Actually, hydrolysis of PHPA happens at any pH, however at pH 10, a more rapid hydrolysis begins.
- Solid. to maintain stability of the system, it’s important to polymer concentration at proper range and maintaining clay-solids content at below 6%. In terms of mud weight, maximum is 12-13 ppg due its solid intolerant.
- Mix in separate tank and in acid environment. Bleed slowly to active system. Unless, PHPA will blind the screen and causing overflow in the shaker.
- Use coarse shaker, in the first or two cycles and changed to finer one once the mud has already incorporated some solids.
Reference: MI Handbook and Baker Mud School