An amphoteric polymer as a shale borehole stabilizer in water-based drilling fluids

An amphoteric polymer P (acrylamide-styrene-butyl acrylate) (ASBA) was evaluated for use as a shale borehole stabilizer and for its ability to counter the problem of well instability during shale gas exploration and development. The hydrophilic groups in the ASBA increased the adsorption between the ASBA and the shale formation and the hydrophobic groups reduced the hydrophilicity of the sodium bentonite (BT). The addition of 1.6 wt% ASBA resulted in a water contact angle of the BT/ASBA that was more than 40° larger than that of the BT. The amount of linear swelling decreased by 37.81% and the hot-rolling recovery rate of the shale cuttings was more than 60% higher in the 1.6 wt% ASBA solution than in deionized water. The ASBA exhibited micro/nanometer globules with an average diameter of 250.4 nm in aqueous solution. This was in agreement with the results of the morphology tests (scanning electron microscopy (SEM) and transmission electron microscopy (TEM)). The morphology tests, elemental analysis, and thermogravimetric analysis showed that the polymer was evenly dispersed in the aqueous solution and effectively adsorbed onto the BT instead of the free water. The soft globules changed under pressure and effectively plugged the various pores in the formation. Due to the hydrophobicity and the plugging effect, the ASBA prevented the BT aggregation and reduced the filtration loss; the American Petroleum Institute (API) filtration loss of the basic mud containing 1.6 wt% ASBA was reduced by 56.67% compared to the basic mud without ASBA after aging at 150 °C. Due to the excellent inhibition of the shale hydration and the micro/nanometer plugging performance, the ASBA has a potential application as a borehole stabilizer in drilling fluids.