Shale inhibition by low-molecular-weight cationic polymer in water-based mud

Water-based drilling fluids are cost effective and environment friendly compared to oil-based muds. However, these muds create wellbore instability problems in water-sensitive and highly reactive shale formations (O’Brien and Chenevert 1973 ). Over 75% of the formations drilled for petroleum exploration and production are comprised of shale and it causes about 70% of the wellbore instability problems across the world (Aftab et al. 2017 ), as the shale swell rapidly on absorption of water. Shales are essentially fine-grain sedimentary rocks composed of different clay minerals like kaolinite, illite, montmorillonite, and other minerals like quartz and calcite (Caenn et al. 2011 ; Feng et al. 2018 ). Due to absorption of water by clay minerals in shale, the shale swells that drastically changes the stress regime and the mechanical strength of shale formation decreases. This may result in collapsing of the wellbore during the drilling process. Inhibitive muds are drilling fluids that suppress hydration of clay which ultimately result in stabilized shale formation. Some of the examples are lime muds, gypsum muds, sea-water muds, and saturated salt-water muds (Gholizadeh-Doonechaly et al. 2009 ). One example of inhibitive WBM is silicate mud that consists of sodium or potassium silicate as the inhibitive component. Silicate anions combine with shale formation to stabilize the well bore by sealing microfractures, forming a silicate layer on shales and possibly acting as a less permeable membrane. This inhibit the water infiltration into formation and drilling a well through troublesome shale section is eased off that otherwise might require the use of oil-based mud (van Oort et al. 1996 ; Guo et al. 2006 ). Potassium ions also stabilize cuttings that may disintegrate by minimizing dispersion into finer particles. Inhibition is explained by the possible penetration of small non-hydrated ions into the porosity of the shale, thus forming an effective semi-permeable membrane (O’Brien and Chenevert 1973 ; Fink 2012 ; Gueciouer et al. 2017 ). Silicate systems have been used earlier too, but the control of rheological properties was difficult. Silicate muds along with potassium salt and polymer forms even more effective shale inhibitive combination with better rheological properties (Mahto and Sharma 2004 ; Patel et al. 2007 ; Khodja et al. 2010 ; Mahto et al. 2013 ). High-performance WBM containing amine-based polymers have been reported to have potential to be as effective as OBM. Amine-based compounds and polymers such as polyetherdiamines, hexamythylene diamine (HMDA), polyhydroxylated alkyl ammonium salts, polyethoxylated diamines etc. have been used in shale stabilization, but they have some limitations such as low inhibition, solid tolerance, and toxicity (Patel et al. 2007 ). Polyoxyalkyleneamine has also been used as a shale stabilizer for water-based drilling fluids that had no effect on filtration and rheological properties of water-based mud (Qu et al. 2009 ). Acrylamide-based polymers have also been reported to be effective in reducing API fluid loss and improving shale stability (Deville et al. 2011 ; Jain et al. 2017 ). Amine compounds with cationic sites are known to impart shale inhibition properties in mud and they act as clay stabilizer. Cationic amine compounds stabilize the shale by chemical interaction between cationic sites and anionic sites present on clay minerals in shale or by the cation exchange at single or multiple sites inside the porosity of shale or at the shale surface. The interaction is improved in case of oligomeric and polymeric amines due to multiple site adsorption onto the surface of shale. One advantage with oligomeric shale stabilizers is that they can enter the small pores of shale and stabilize the inner layers and block the water from entering shale pores (Gomez and Patel 2013 ). Oligomers of cationic amine may provide good shale inhibition through cation exchange and encapsulation mechanisms (Patel 2009 ). However, low-molecular-weight polymers may function as the shale stabilizer as well as rheology modifier. Smaller molecules of polymer can act as oligomers to stabilize inner layers of shale and covering large number of clay sites on shale surface that will result in better surface locking to minimize shale dispersion.