A mini-reservoir device may be used to screen or otherwise determine a composition of one or more treatment fluids, additives, and other fluids. Such fluids may be for use in a subterranean formation. Methods of determining a composition may include visual analysis of each of two or more fluids, each from a plurality of candidate fluids, flowed through a mini-reservoir device, and selection of one of the plurality of candidate fluids based at least in part upon that visual analysis. Certain methods may include determining an oil recovery factor for each of one or more fluids flowed through a mini-reservoir device. In particular methods, multiple treatment fluids and/or additives, such as surfactants, may be selected based at least in part upon visual analysis of the fluids' flow through a mini-reservoir device.