Charles Hart, J.M. Huber Corporation
Even though progressing cavity (PC) pumps have been used by the industrial world for many years on liquids containing abrasive fluids, PC systems are a relatively new means of artificial lift in the oil field. One of the more obvious differences between the newer PC and the traditional beam pump is that the rod string rotates rather than reciprocates. PC pumps are now being used on increasingly deeper wells and on a wider variety of production fluids following their introduction in shallow wells to produce heavy, sand-laden oil. As a result, PC pumps are earning a place in the market and oil field equipment manufacturers are beginning to develop products for PC systems. One example is rod guides which, until recently, have been designed solely for reciprocating rod strings in beam pumped wells. Lower initial investment, less power per unit of production, more tolerance for sand-laden fluids, and greater production capacities are some of the advantages touted by PC systems. However, maintenance can be more expensive. One reason, which is the driving force behind this study, is that tubing wear opposite rod couplings is more concentrated because the rod string rotates in a stationary position. If well bores were truly vertical and crude oil was free of abrasives and water, rod and tubing wear in either beam or PC pumping systems would be of little consequence. However, in the real world, rods and tubing never hang perfectly concentric and few wells, if any, produce crude oil with undiluted lubricity. Consequently, in both reciprocating and rotating systems, rod and tubing wear accelerates as production rates, hole deviations, water/oil ratios, and sand concentrations increase. As these variables increase, the need for rod guides also increases. When PC's were first installed, operators had no choice but to rely on guides which had been developed for reciprocating pumps to centralize rod strings inside the tubing. Two examples are Huber's New Era Turbulence Breaker (NETB) and Patco's Double Plus (DP) shown in Figures 1 and 2, respectively. Both are the end result of years of research and development. Each guide has fins with an O.D. close to the I.D. of the tubing. The fins have been designed to achieve maximum standoff between the rod couplings and tubing with minimum pressure drop. A third example is the cylindrical unfinned poly guide shown in Figure 3. The poly guide has a smaller O.D. than either the NETB or DP, otherwise pressure drop increases beyond acceptable limits. Because the O.D. is smaller, standoff between the rod couplings and tubing is less. Consequently, the unfinned design is at a disadvantage because it has less erodible wear volume (EWV), as defined in Figure 5, to prevent metal-to-metal contact between the rod couplings and the tubing. In all three examples, the guides are bonded to the sucker rod. In fact, the quality of guides for reciprocating rod strings is frequently judged on the basis of bonding power