Sucker Rods Information
Sucker rods connect a surface pumping unit to the downhole components of a pumping system in an oil and gas well. They are typically steel rods 25 to 30 feet in length, but fiberglass rods of differing lengths are also produced. The first string attached to the pumping unit is a polished rod that reciprocates through a stuffing box providing a liquid-tight seal. Shorter sections of sucker rods referred to as 'pony rods' are used to assemble the rod string to its required overall length.
Pumpjacks, or "horsehead pumps," are ubiquitous in oil well operations when a well does not naturally flow on its own. Artificial lift can be used to generate flow, either from a well with no flow or to intensify the flow in a low-flow, naturally producing well. Most oil wells will require artificial lift at some point in time. These pump systems consist of a motor or engine, a crank, a walking beam, a horsehead, and a hanger/rod string that connects with the sucker rods and pump.
Sucker rods are the mechanical components that transmit power from the horsehead pumping unit on the surface to the rod pump deep down the well bore. There are several types of sucker rods and each has its own specialized role. These rods can also be constructed of different materials to optimize strength-to-weight ratios for the well depth and oil properties (density, viscosity, etc.) at hand.
A sucker rod string is the assembly of components that links the hanger with the rod pump and other downhole components. A typical sucker rod string is assembled as follows from the top down: The bridle running off the horse head connects to a carrier bar that is connected to a specialized sucker bar called a polished rod. The polished rod passes through a stuffing box at the wellhead to create a seal. From here downward, the polished rod is connected to a series of sucker rods until reaching the rod pump. Each end of the sucker rod is threaded to allow a robust and simple connection between the rods. Rod guides, stabilizer bars, and tubing centralizers help center the rod string and keep the rod couplings off the tubing, reducing tubing wear and maximizing flow up the well.
A detailed view of a typical oil well, showing the location of the sucker rod.
Rod pumping systems are used in hundreds of thousands of oil wells throughout the world. Prominent features of the pumping rig include power systems located on the surface and the pump located at the bottom of the well. However, it is the sucker rods in the pump string that are perhaps the most critical component in keeping the well up and flowing. The large forces on the string demand proper design, installation, and maintenance to keep the well operating with maximum uptime throughout its working life.
Sucker rods take many forms based on their specific functions. Weight and strength are key factors in sucker rod string design. The rod at the top of the string must transmit the weight of all of the sucker rods beneath it as well as the weight of the pump. Add to this the weight of the oil being lifted plus any viscous drag created with the sides of the well bore and loads can reach tens of thousands of pounds. Rod material strength versus weight is an important consideration for well string design.
Steel sucker rods are the traditional sucker rod design. They come in diameters of 0.625 to 1.125 inches. Lengths established by the American Petroleum Institute (API) are 25 and 30 feet. Steel rods are available in varying grades that range in tensile strength from 90,000 to 140,000 psi.
Fiberglass (FRP) rods offer benefits compared to steel sucker rods. Fiberglass sucker rods are stronger, lighter, and more elastic than steel sucker rods. Resulting weight and drag reduction reduces stresses on the rod string. Fiberglass sucker rods also resist corrosion. Fiberglass rods are weak when loaded in compression, however. As a result, steel sucker rods must be used on the bottom of the string to absorb the buckling forces should the pump stick on the downstroke. Several disadvantages of FRP sucker rods are premium cost, greater difficulty in installation, and somewhat greater care requirements in field operations.
Pony rods are shorter than 25 feet and vary in length. API has established that pony rods come in six lengths: 20, 44, 68, 92, 116, and 140 inches. Pony rods are used to bring the entire rod string to the desired working length. They are commonly placed adjacent to the polished rod at the top of the rod string, on top of the rod pump, and on top of the polished rod to prevent the string from falling downhole if the polished-rod clamp slips
Polished rods are the top-most rod in a rod string, and form a tight seal with the stuffing box on the wellhead. These rods come in various lengths and sizes; API polished-rod lengths are 8, 11, 16, and 22 feet. Polished rods are made of various materials, including carbon steel, stainless steel, and Monel. Though API guidelines call out a finish of between 10 and 20 micro inches, a finish of 16 micro inches will provide a balance between wear and clamping efficiency. Polished rods must be properly aligned to minimize wear.
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