A comparative study of four rod load reduction techniques for deep-rod pumping

J Petrol Explor Prod Technol (2018) 8:475–483 https://doi.org/10.1007/s13202-017-0367-6 ORIGINAL PAPER - PRODUCTION ENGINEERING A comparative study of four rod load reduction techniques for deep-rod pumping Yi Zuo1 • Xiaodong Wu1 Received: 7 March 2017 / Accepted: 17 June 2017 / Published online: 27 June 2017 Ó The Author(s) 2017. This article is an open access publication Abstract In deep-rod pumping wells, polished rod load tends to be very high and even exceeds the operating range of the pumping unit. To tackle the problem, mainly four rod load reduction techniques have been designed till now. They are side-flow pump, rod load reducer, fiberglass sucker rod and deep pumping with small-diameter pump. However, some of them still cause certain problems when put into use. That indicates the fundamental mechanisms of these techniques have not yet been fully studied. In this paper, on the basis of conventional polished rod load calculation models, the new calculation models for these techniques are respectively established. The rod load reduction effects are comparatively analyzed by calculation. The results indicate that the differences in mechanisms lead to different reduction effects. Side-flow pump and deep pumping with small-diameter pump can largely reduce the liquid pressure load, but its reduction effect for the polished rod load is very limited. On the contrary, fiberglass sucker rod can greatly reduce the polished rod load by decreasing the weight of the sucker rod. Rod load reducer does not change the previous load; instead, it can create an additional reduction force. The potential disadvantages are also discussed. When using a side-flow pump, the resistance on the plunger caused by the liquid pressure would increase. This resistance may cause the sucker rod to bend when it is moving downwards. Deep pumping with small-diameter pump needs to use very small pump in order to get a relatively good reduction effect, so its application is limited. These disadvantages should be considered in practical application. & Yi Zuo 1 China University of Petroleum, Beijing, China Keywords Deep-rod pumping  Polished rod load  Load reduction  Side-flow pump Introduction Sucker rod pumping is a traditional and widely used method of artificial lifting in the world’s oil industry. In recent years, on the one hand, as the natural energy has been depleted and working fluid level is going down, most of old oilfields in the late development stage need deep pumping techniques to increase oil production; on the other hand, deep reservoirs are gradually being developed. For example, the average reservoir depth in Tahe oilfield of Northwest China is more than 6000 m. With the complex geological conditions and bad fluid properties, the working fluid level is very low and rapidly dropping to the limit depth of conventional sucker rod pumping system. The limit depth mainly depends on the polished rod load which increases with the pump depth. However, high polished rod load is disadvantageous to the whole system and even tends to exceed the operating range of the pumping unit. At present, aiming at the problem about how to reduce the polished rod load in the deep sucker rod pumping wells, mainly four rod load reduction techniques have been designed. They are side-flow pump, rod load reducer, fiberglass–steel sucker rod and deep pumping with smalldiameter pump. The side-flow pump has a special structure. It uses solid plunger instead of hollow plunger, and its traveling valve is substituted by side-flow valve (Zhao and Liu 2009). Compared with the conventional pump, this pump can cause a smaller polished rod load; thus, a deeper pump depth can be achieved. 123 476 J Petrol Explor Prod Technol (2018) 8:475–483 The rod load reducer is a device installed at certain depth between the well head and the pump. With dual plungers, it can produce a hydraulic feedback force to reduce the polished rod load (Meng et al. 2002). In contrast to steel sucker rod with relatively heavy weight, fiberglass sucker rod is much lighter. So it can reduce the polished rod load (Hicks 1986). Considering that fiberglass sucker rod cannot bear compressive stress, a length of steel sucker rod is needed to be connected to the bottom of the fiberglass sucker rod. Deep pumping with small-diameter pump is aimed to solve the problem in old oilfields at their late development stage (Feng 1990). With the existing pumping units, using small-diameter pump can reduce liquid pressure load which is a part of the polished rod load. Therefore, the pump depth can be deepened. That helps to improve the pump efficiency. The rod load reduction effects of the four techniques have been proved by applications on the fields. However, some of them still cause certain problems when put into use. That indicates the fundamental mechanisms of these techniques have not yet been fully studied. Moreover, the four techniques have not been compared with each other before. In the following section, on the basis of conventional polished rod load calculation, the polished rod load calculation models for these techniques are respectively established. The rod load reduction effects are comparatively analyzed by calculation. From the result, the mechanisms of rod load reduction are figured out and the potential disadvantages are discussed. Maximum polished rod load Conventional polished rod load calculation model Side-flow pump Polished rod load includes static load and dynamic load. Static load includes the sucker rod gravity load and liquid pressure load. And dynamic load is mainly comprised of vibration load, inertial load and friction load. The API RP 11L method (API 1977) is adopted to calculate the polished rod load in this paper. Compared with other simplified models, the calculation model in API RP 11L method takes into account both static load and dynamic load, so it is more precise and reliable. API RP 11L method has been widely used in the global petroleum industry. Considering that the specific calculation formulas for the static load are not given in API RP 11L method, we only listed them in detail and the dynamic load can be calculated according to API RP 11L method. For a conventional pump system, the maximum and minimum polished rod load can be estimated by the following model based on API RP 11L method. As shown in Fig. 1, the side-flow pump is made up of two solid plungers with different diameters and two pump barrels with different internal diameters (Luan et al. 2011). The lower pump barrel and the tubing casing annulus are connected by the breathing hole. Compared with a conventional pump, the side-flow pump makes use of solid plungers instead of hollow plunger and replaces the traveling valve with a side-flow valve. In the conventional pump system, the standing valve closes and fluid in the pump barrel pushes the traveling valve open and flows into the tubing in downstroke. In upstroke, with the traveling valve clo (...truncated)