Downhole fluid separator in re-entry multilateral well

A system comprising:

• an orienting liner hanger disposed in a main bore of a multilateral well in a position downhole from a junction of the multilateral well;
• a lower completion secured to a downhole end of the orienting liner hanger;
• a fluid separator assembly disposed at least partially within the junction of the multilateral well and uphole from the orienting liner hanger, wherein the fluid separator assembly is configured to receive formation fluid flowing uphole from the lower completion, wherein the formation fluid includes oil and water, wherein the fluid separator assembly is configured to at least partially separate the formation fluid into formation oil and formation water, wherein the fluid separator assembly is configured to output formation oil to an upper production tubing, and wherein the fluid separator assembly is configured to output the formation water into a lateral bore of the multilateral well;
• and
• an upper completion packer secured within the main bore in a position uphole from the fluid separator assembly, wherein the upper production tubing extends from the fluid separator assembly to the upper completion packer, wherein a junction annulus is formed in the main bore between a wellbore wall of the main bore and radially outer surfaces of the fluid separator assembly and the upper production tubing, wherein the junction annulus is in fluid communication with the lateral bore, and wherein the upper completion packer is configured to seal an upper end of the junction annulus and the orienting liner hanger is configured to seal a lower end of the junction annulus such that formation water output into the junction annulus, via the fluid separator assembly, is directed to flow into the lateral bore from the junction annulus.

A method, comprising:

• drawing a formation fluid into a fluid separator assembly, wherein the formation fluid includes oil and water, wherein the fluid separator assembly is located at a junction of a multilateral well, wherein the fluid separator assembly is configured to receive formation fluid flowing uphole from a lower completion, wherein the lower completion is in fluid communication with a downhole end of an orienting liner hanger, wherein the orienting liner hanger is disposed in a main bore of the multilateral bore in a position downhole from the junction of the multilateral well, and wherein the fluid separator assembly is disposed uphole from the orienting liner hanger;
• separating, at least partially, the formation fluid into formation oil and formation water via the fluid separator assembly;
• drawing formation oil up hole to a surface via an upper production tubing, wherein the fluid separator assembly is configured to output the formation oil to the upper production tubing;
• and
• injecting formation water from the fluid separator assembly into a lateral bore of the multilateral well.