Reduction of downhole friction by electrochemical methods (2010)

High torque and drag is one of the major problems encountered by the downhole drilling companies and is exacerbated when directional drilling techniques are used.Thus, reducing friction is key to reducing the requirements of drilling equipment and is one of the most researched areas by the oil and gas companies.This study investigated an electrochemical method to control friction for AISI 4340 steel/AISI 4340 steel, and AISI 4340 steel/sandstone sliding contacts lubricated by baseline solutions and the downhole drilling muds.The tribometer used was a pin-on-disc, modified to incorporate a three-eectrode electrochemical cell allowing potential control of the rotating disc.The overpotential used was in the range of 1 V, whilst the load and speed were kept constant at 50 N and 0.03 m s -1 respectively (to replicate downhole conditions).Lubrication involved the addition of electroactive friction modifier additives to allow electrochemical friction control.Initial experiments using pure electrochemical methods established the the behaviour of steel samples immersed in all the test solutions/lubricants.Evidence of additive adsorption was seen via the limiting current response in the potentiodynamic polarisation test, especially at anodic overpotentials with additive containing lubricants such as octanoate.For the baseline solutions, without the additive, friction for steel/steel was found to be independent of applied potential.Evidence of abrasion and adhesion were also seen using a Scanning Electron Microscope (SEM).However, incorporation of octanoate additive resulted in a decrease in friction at anodic overpotentials, up to 66%, due to the formation of iron octanoate tribofilm.Furthermore, abrasion, adhesion and sample wear rates were also reduced.In the bentonite drilling mud, friction levels were generally higher than the baseline solutions, due to the presense of two-body abrasion induced by entrained particles.Incorporation of octanoate also produced dependence of friction on potential in the mud, in which friction was reduced at anodic overpotentials, reaching levels as low as the octanoate solution (baseline with additive).Abrasion and adhesion was also reduced.The polymer drilling mud showed inherent lubricity, but friction was even lower at anodic overpotentials with the incorporation of octanoate additive (62% reduction cf.no additive).Alternative types of additives were tested and showed different effects on friction, depending on the hydrocarbon chain lengths and polar charge.Butyrate (C 4 ) resulted in less friction reduction cf.octanoate (C 8 ), whilst oleate (C 18 ) showed more reduction.CTAB, however, showed reduction at cathodic overpotentials owing to its positive charge.For the steel/sandstone contact, no dependence of friction on overpotential was seen, using the bentonite mud only.However, the octanoate additive resulted in lower friction at anodic overpotentials.Two contact arrangements were employed, i.e., (i) steel pin/sandstone disc, and (ii) sandstone disc/steel pin, both resulting in similar friction behaviour.X-ray Photoelectron Spectroscopy (XPS) of worn surfaces of steel suggested the formation of a weak octanoate tribofilm at cathodic overpotentials, probably due to the loosely-packed adsorption.However, a robust tribofilm was formed at anodic overpotentials, due to the development of a relatively close-packed adsorption layer.The low friction contacts at anodic overpotentials correlate with low values of current density, which supports the formation of a tribofilm, passivating the surface. Overpotential is the difference between applied potential and the open circuit potential (OCP) Surface Potential Effects on Friction and Abrasion of Sliding Contacts Lubricated by Aqueous Solutions