The extraction of crude oil by using downhole pumps requires the constant vertical oscillation of a sucker rod inside a tubing in order to pump the crude oil out of the well. This reciprocating motion leads frequently to sliding contact between the couplings used for connecting the single rods and the inner surface of the tubing. The damage caused by the sliding contact is promoted by the corrosive downhole conditions. The consequence is a continuous degradation of the inner tubing surface, ultimately leading to tubing failures, which requires the replacement of the tubing string. One of the parameters that determine the contact conditions and therefore the severity of the damage is the coupling roughness. For this reason, the American Petroleum Institute (API)1 recommends a maximum average coupling roughness of Ra = 1.6 μm. In the present work, we investigate the impact of coupling roughness on friction and tubing damage under controlled laboratory conditions.

Key words: coupling, roughness, sliding, CO2 corrosion