Modular systems for extracting hydrocarbons from subterranean volumes and associated methods are generally described. Methods for extracting hydrocarbons from a subterranean volume are also described. Some of the described methods comprise flowing a mixture comprising hydrocarbons and water from the subterranean volume to a plurality of fluidically connected separation units and at least partially separating the hydrocarbons from the water using the separation units to produce a first stream comprising higher purity water than the mixture. In certain cases, the method includes fluidically disconnecting at least one of the separation units from the remaining separation units while the separating step is being performed. Some methods include fluidically connecting an additional separation unit to the separation unit while the separating step is being performed.