(Meth)acrylic copolymers as rheological additives in drilling fluids and drilling fluids comprising such copolymers

A drilling fluid in the form of a water-in-oil (W/O) emulsion comprising at least components (A) to (D), each being different from one another, namely

• (A) at least one hydrocarbon as continuous phase,
• (B) water or an aqueous solution of at least one organic and/or inorganic salt as discontinuous phase,
• (C) at least one weighting agent, and
• (D) at least one (meth)acrylic copolymer,
• wherein the (meth)acrylic copolymer (D) contains at least one of structural units (D1), (D2) and (D3), which are different from one another,
• structural unit (D1) comprising at least one hydroxyl group and not comprising any amino groups,
• structural unit (D2) comprising at least one amino group and not comprising any hydroxyl groups, and
• structural unit (D3) comprising both at least one hydroxyl group and at least one amino group,
• and wherein the (meth)acrylic copolymer (D) further contains at least one structural unit (D4), which is different from any of structural units (D1), (D2) and (D3), structural unit (D4) containing the part structure (P)
• wherein
• the symbol
• denotes a covalent bond of part structure (P) to the backbone of the (meth)acrylic copolymer, and
• R denotes a C4-C25 aliphatic residue, an aromatic C6-C25 residue or an araliphatic C7-C25 residue.