Preparation and Mechanism Analysis of a Hyperbranched-Copolymer Shale Inhibitor for Water-Based Drilling Fluids

The problem of wellbore instability caused by clay hydration and expansion during shale oil and gas drilling has not been properly solved; therefore, high-performance shale inhibitors are urgently needed. Herein, a highly active hyperbranched polydimethylsiloxane (HPNH 2 ) with hydroxyl and amino end groups was synthesized via ester exchange condensation, using 3-aminopropyltriethoxysilane (APTES) and 2-amino-1,3-propanediol as raw materials. Then, an innovative hyperbranched copolymer shale inhibitor (i.e., HPNH 2 -DDB) was prepared by the initiated polymerization of N, N -dimethylacrylamide (DMAA), diallyldimethylammonium chloride (DMDAAC), and butyl acrylate (BA), with cerium ammonium nitrate (CAN) and HPNH 2 as initiators. HPNH 2 -DDB possessed a highly branched three-dimensional network structure and exhibited an initial degradation temperature of up to 275 °C. The inhibitory performance of HPNH 2 -DDB was evaluated through a series of inhibition tests, including a bentonite immersion test, capillary suction timer (CST), linear swelling test, and shale rock fragment hot rolling recovery test. HPNH 2 -DDB had a better inhibitory effect on shale compared to traditional inorganic salt (KCl) and branched polyethylenimine (BPEI). When the addition amount was 1 wt %, the linear swelling rate of bentonite pellets decreased from 73.12% to 27.51% at 160 °C, and the rolling recovery rate increased from 26.43% to 84.62%. Furthermore, the inhibition mechanism of HPNH 2 -DDB was analyzed using contact angle testing, zeta potential, particle size distribution, and X-ray diffraction (XRD). The results showed that HPNH 2 -DDB could be adsorbed on the shale surface through hydrogen bonding and electrostatic interactions, compressing the diffuse electric double layer and reversing the wettability of the shale surface, thereby inhibiting the hydration and swelling of the shale. Therefore, the unique hyperbranched structure of HPNH 2 -DDB endowed it with excellent inhibition performance, making it a promising candidate for use as a high-performance shale inhibitor in drilling operations.