Image courtesy of Alberta Oil Tool.
Sucker rods, the metal or fiberglass rods that connect the beam to the downhole suction device, commonly fail by breaking or 'parting.' This means that the broken section is left in the tubing string and the broken rods must be replaced before the well can produce oil again. Rod parting negatively affects both the revenue and cost sides of the profitability equation. Costs are incurred to pull the attached section of rods, to 'fish' the broken section, and to purchase and install new rods. Revenues are lost while the well is not producing. Obviously, profitability can usually be increased by reducing rod partings.
Sucker rod partings are caused predominantly by three factors: abrasion, corrosion, and/or 'pounding.'
40% of rod partings occur at connection joints. These failures are primarily caused by stresses generated as a result of the rods 'pounding' on the top of the fluid. This occurs when the pump is lifting fluid out of the wellbore faster than the formation is feeding it in. Eventually the fluid level drops below the pump and the bottom of the rod string begins pounding on the top of the fluid.
1. Assure that connections between each joint of the rod string are correctly torqued when they are installed in order to assure that there are no 'weak links' in the rod string if pounding should occur. It is common to have several connections torqued incorrectly. Incorrect tightening of the connections allows abnormally high stresses to occur during pounding resulting in pin and box failure at the rod connections.
2. Time the pump to go off before the fluid level drops too far. Cycling the pump on and off, so that it is operating only when there is enough fluid in the hole saves costs on electricity and reduces or eliminates pounding. Installation of a pump-off controller (POC) can improve overall beam-pump efficiency. (Case study: Oil & Gas Journal, 9/13/99, pp.86-97.)
3. Slow the pump down so that its speed is matched to the fluid inflow into the wellbore. A new device marketed as "Pump Trac" is touted as being an 'intelligent' Variable Frequency Drive (VFD) that allows speed variations within each pump stroke. This Permian Basin supplier claims the device can/will increase production, reduce electrical costs, and reduce mechanical stress within the pumping system.
Cause: Crooked holes in the Permian Basin are common and are the most common cause of abrasion of the rod string. Continual rubbing of the rods against the sides of the well bore as it moves up and down the hole can eventually wear through the rod and cause failure.
Fiberglass versus Steel
Rod Partings: Operator Experience
A large independent in East Texas instituted a program of dynamometer testing of pumping oil wells using a PC program to evaluate the results. This program reduced rod partings on 177 wells from 11% per year to 2% per year.