In rod-pumped systems, the locations to install rod guides along the rod are determined through the analysis of calculated side forces exerted on/by the rod string. The side forces are computed from axial forces in combination with information about the trajectory of the rod string in the well. Traditionally, the trajectory of the rod string is estimated from directional survey data, obtained by magnetic or gyroscopic surveys at typically 100ft reporting intervals. However, because the surveys are run in open hole, in casing, or in tubing, it is the trajectory of these structures that are described by the directional data. None of them are accurate representations of the actual trajectory of the rod string, implying that the resulting side forces may suffer from reduced accuracy, and therefore are not optimal for decisions on rod guide placement. Additionally, due to the low resolution survey data, local variations in the wellbore trajectory within the survey intervals, which may affect the forces on the rods, may be undetected.
In a previous paper we developed a method for analyzing the small-scale tortuosity of a wellbore from high-resolution (1ft) survey data. One important outcome of this analysis is the finding that high small-scale tortuosity substantially narrows the free passage through a wellbore or tubing. The narrowing is quantified in terms of the reduction in the effective diameter of a device that can be placed in the wellbore. In this paper, the technique has been extended for the calculation of points along the wellbore where the rod string is expected to make contact with the tubing. This is an important result by itself, indicating where rod guides may be needed along the rod string. With these points of contact, the trajectory, or shape of the rod within the production tubing is estimated. The use of this estimate instead of the traditional directional survey data in the calculations of side forces is expected to improve the accuracy of the results.
The technique has been applied to a number of field cases, and two are presented in this paper. The results show that the forces on the rod calculated from the proposed technique are similar to, and exhibit the same general trend as the forces calculated with conventional methods. However, there are some differences, due primarily to the use of the estimated shape of the rod string in the proposed method. The forces on the rod at the estimated points of contact of the rod and the tubing can be extracted and used for rod guide placement decisions.
By providing improved estimates of contact point locations and the forces acting between the rod and the tubing, the method may help to optimize the rod pumping system for producing wells. It may help to explain occurrences of pump failures, erosion of the production tubing by the rods, and other issues that are not fully understood. This may result in reduced failure rates, energy savings, and cost savings resulting from reduced workovers and production losses.