High-density water-based drilling fluid suitable for horizontal shale gas wells

A water-based drilling fluid comprising

• a nano-plugging agent,
• a bionic wall bracing agent,
• a bionic shale inhibitor,
• a filler,
• an emulsifier, and
• an amphiphobic wettability reversal agent,
• wherein the nano-plugging agent is modified silicon dioxide nano-particle including a modifying group, the modifying group on the modified silicon dioxide nano-particle includes a modifying copolymer chain, and the structural units in the modifying copolymer chains are provided by one or more of monomers represented by formula (1) and one or more of monomers represented by formula (2):
• wherein one of R6-R10 is -L″-SO3H and the rest of R6-R10 are independently selected from H, —OH, halogen, and C1-C10 alkyl;
• L, L′ and L″ are independently selected from C0-C10 alkylene;
• wherein the bionic wall bracing agent is carboxymethyl chitosan with a dopamine-derived group represented by the following formula (I-1) grafted on its main chain:
• the bionic shale inhibitor is composed of structural units represented by the following formula (3) and structural units represented by the following formula (4):
• wherein the molar ratio of the structural units represented by formula (3) to the structural units represented by formula (4) is 0.2-6:1, and the weight-average molecular weight of the bionic shale inhibitor is 800-4,000 g/mol;
• wherein the filler consists of calcium carbonate of 1,600-2,500 mesh, calcium carbonate of 1,050-1,500 mesh, and calcium carbonate of 500-1,000 mesh at a weight ratio of 1:0.55-6:0.55-6;
• the emulsifier is one or more of compounds represented by the following formula (i):
• in formula (i), each of the two R11 groups is independently selected from C14-C30 alkyl optionally substituted by group Y and C14-C30 unsaturated alkyl with carbon-carbon double bonds optionally substituted by group Y, and the group Y is independently selected from the groups represented by the following formulae:
• n′ is an integer within a range of 1-8;
• wherein n′ X′-es are independently selected from H and —C(O)—R21, and at least one X′ is —C(O)—R21, R21 is selected from carboxyl, C1-C6 alkyl, C3-C8 cycloalkyl, C1-C6 alkyl substituted by carboxyl, C3-C8 cycloalkyl substituted by carboxyl, C3-C8 cycloalkyl substituted by carboxyl and C1-C4 alkyl, C2-C6 unsaturated alkyl with a carbon-carbon double bond, C3-C8 unsaturated cycloalkyl with a carbon-carbon double bond, C2-C6 unsaturated alkyl with a carbon-carbon double bond substituted by carboxyl, and C3-C8 unsaturated cycloalkyl with a carbon-carbon double bond substituted by carboxyl and C1-C4 alkyl;
• and
• wherein the amphiphobic wettability reversal agent is a dual-cation fluorocarbon surfactant of which the cation part is represented by the following formula (a):
• wherein, each R1 is independently selected from C1-C6 alkyl, each R2 is independently selected from H and C1-C6 alkyl, each R3 is independently selected from C1-C10 alkylene, each n″ is independently selected from integers within a range of 3-15, and m is selected from integers within a range of 1-10.