Method of improving performance or efficiency of a wellbore pump associated with a wellbore and/or for increasing the rate of production of reservoir fluid from a reservoir, in which a wellbore pump is arranged to pump wellbore fluid within the wellbore to a surface. The method includes (a) selecting a wellbore which includes an associated wellbore pump associated with a production tube and arranged within a casing. An annulus is defined between the casing and the pump/production tube, the annulus including reservoir fluid which has a hydrostatic head which is at least 15 m above the level of an inlet of the wellbore pump, and (b) contacting a reservoir fluid in the annulus upstream of an inlet of the wellbore pump with a treatment formulation. The treatment formulation has a first polymeric material which includes —O— moieties pendent from a polymeric backbone thereof, and the first polymeric material is optionally cross-linked.