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 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 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 .