Automatic Downhole Card Generation And Classification
Authors: Filho M.A. Barreto , Tygel M. , Rocha A.F. , Morooka C.K. 
Published: December 2007

Sucker rod pumping system is a very important artificial lift method. The surface dynamometer card (SDC) is a plot of the load versus the pumping cycle measured at the polished rod. The SDC shape is assumed to reflect the actual pumping conditions. However, SDC is a composition of the actual downhole pump dynamics and the noise added during the information transmission along the sucker rod. The difficulty in recognizing specific SDC shapes augments as the amount of noise increases mainly as a function of the well depth. Filtering algorithms may be developed with the purpose of obtaining the downhole dynamometer card (DDC) from the recorded SDC. Artificial Intelligence may provide the adequate tools for DDC classification. The present paper describes an intelligent system which automatically calculates DDC by SDC algorithm filtering, and classifies DDC taking into consideration a set of patterns associated to the most frequent abnormal pumping operating conditions. This system is actually running at different Brazilian Oil Fields. The paper describes the main characteristics of the filtering algorithm; it shows how linear mathematical programming and neural nets are used for DDC classification, and presents results collected at different pumping conditions. These results show that the system is highly reliable in correctly identifying the actual pumping conditions through DDC classification.

Introduction

The sucker rod pumping is the most common artificial lift method and it is largely applied on onshore brazilian oil fields. The system consists basically (Fig. 1) in a surface pumping equipment and a prime mover, usually an electric motor, which provides necessary energy into the system. The pumping equipment induces alternate motion of a sucker rod string connected to a piston type subsurface pump located at the bottom of the well. In sucker rod pumping unit operation, Surface Dynamometer Card (SDC) is commonly used to verify faults in the pumping unit and make fault diagnosis. Moreover, SDC can be useful to optimize the pumping efficiency by means of adjustments of the pumping capacity to the optimal reservoir production rate. SDC is a load vs. position plot at the polished road (Figs. 1 and Fig. 2). SDC is transformed into the Downhole Dynamometer Card (DDC) by means of a mathematical filtering which considers the dynamic behavior of the sucker rod string. The DDC is the fundamental information source to support a downhole pumping malfunction diagnosis.

Many methods has been proposed for calculating forces and displacements along the rod string during sucker pumping operation for obtaining DDC from SDC. Most of them are based on wave equation for modeling the rod string dynamics. Firstly, Snyder uses characteristic curves method to solve the wave equation where the damping effects were taken into account as a friction force estimated for each point through the rod string. Gibbs and Neely propose an analytical method for solving the wave equation considering the damping terms. Pavlik based on Gibbs and Neely results, add gravitational effects to the filter model and the possibility to use fiberglass rods is included. Furthermore, in this case the damping coefficient is determined from an iterative procedure where the annular flow with interactions between rod, production tubing and are taken into account. Finally, Everitt proposes a finite difference numerical method to solve the damped wave equation in order to obtain the downhole dynamometer card for a combined rod.

Several companies has been started to automate the monitoring and control of sucker rod pumped wells. Dickinson and Jennings describes a deterministic grid method approach for DDC pattern-recognition and Rogers et al proposes a pattern recognition scheme utilizing Fourier descriptors and neural network model.

P. 311