Rod string design involves multiple interdependent variables: rod diameter and taper percentages, rod grade, stroke length, pumping speed, pump size, and sinker bar configuration. The optimal design is the combination of these variables that meets production targets while minimizing peak stress, gearbox loading, and energy consumption. Finding that combination through single-point evaluation - running one configuration at a time and comparing results manually - is time-intensive and frequently leads to designs that are adequate rather than optimized.
Multi-scenario comparison with an iterative base case workflow changes this from a serial process into a systematic optimization method. This article describes the approach and illustrates it with a practical example.
The problem with single-point evaluation
A typical desktop design workflow proceeds as follows: the engineer sets up the well parameters, configures a rod string based on experience or a standard taper table, runs the simulation, reviews the results, adjusts one or two parameters, saves a new file, runs again, and repeats. Each iteration takes 5 to 10 minutes including file management. A thorough evaluation of three taper configurations, two stroke speeds, and two rod grades - 12 combinations - requires 12 separate simulation runs and a manual comparison of 12 result sets.
In practice, most engineers evaluate 3 to 5 combinations rather than 12. The time cost of additional iterations is not justified under normal workload pressure, so the design converges to the first acceptable configuration rather than the optimal one. The difference between acceptable and optimal may be 5 to 10% in peak stress margin, gearbox loading, or energy efficiency - differences that accumulate across a portfolio of wells over years of operation.
Multi-scenario comparison as an optimization tool
A comparison view that presents three to five scenarios side by side with all key metrics visible simultaneously reduces the per-comparison time from 30 to 45 minutes (file-based workflow) to approximately 15 minutes. This alone doubles or triples the number of configurations an engineer can evaluate in the same time.
The more significant capability is the iterative base case workflow. After comparing three scenarios, the engineer promotes the best-performing scenario to the new base case. The comparison resets with the promoted scenario as the starting point, and three new variations can be configured and evaluated. This creates a sequential narrowing process: each comparison round focuses on a different variable while the others are held at their best-known values.
Worked example: optimizing a deviated well design
Consider a well at 9,400 ft MD with 52-degree maximum inclination, a 4.8-degree-per-100-ft build rate, and 180 bbl/d production target. The initial design uses a standard three-taper Grade D rod string: 1-inch from surface to 3,400 ft, 7/8-inch from 3,400 to 6,200 ft, and 3/4-inch from 6,200 to pump depth at 9,200 ft. Pumping speed is 7 SPM with a 168-inch stroke.
Round 1: Taper optimization. Three taper configurations are compared - the initial 34/28/38 split, a 30/30/40 split, and a 38/24/38 split. The comparison view shows that the 30/30/40 split reduces peak stress in the upper section by 4% while maintaining acceptable stress in the lower sections. The 30/30/40 configuration is promoted to the new base case.
Round 2: Stroke speed. With the 30/30/40 taper fixed, three stroke speeds are compared: 6, 7, and 8 SPM. The 6 SPM configuration reduces gearbox loading by 11% while still meeting the production target. The production margin at 7 SPM provides a buffer for future decline, but the gearbox loading is 8% higher. The engineer selects 6.5 SPM (interpolating between the scenarios) as the base case.
Round 3: Rod grade. With taper and speed fixed, three rod grade configurations are compared: all Grade D, Grade KD in the upper section with Grade D below, and Grade K throughout. The KD/D combination reduces the Goodman stress ratio in the upper section from 83% to 71% at a moderate cost premium. For a well with a 4.8-degree build rate through the upper section, this margin improvement is meaningful for long-term fatigue life.
Three comparison rounds, approximately 45 minutes total, evaluated 9 configurations across three design variables and converged on a design that is measurably better than the starting point on peak stress, gearbox loading, and fatigue margin. The same evaluation using single-point iteration with a desktop tool would require 9 separate file-based simulation runs and manual result comparison.
When to use this approach
Iterative multi-scenario optimization is most valuable for wells where the design space is wide: deviated wells with multiple viable taper configurations, wells where production targets can be met at different stroke speeds, wells where rod grade selection significantly affects fatigue life, and wells with a history of failures that suggest the current design is not optimal.
For straightforward vertical wells with established taper tables and proven operating parameters, the standard single-configuration workflow remains efficient. The optimization approach is not needed when the design space is narrow and the correct configuration is well established from field experience.
Multi-scenario comparison with iterative base case promotion is available in RodSim Professional (3 scenarios) and SME/Enterprise plans (5 scenarios). The comparison history is preserved as part of the well record for future reference.
