Trouble-Shooting Rod-Pumped Wells

Based on a workshop presented by Oklahoma's Marginal Well Commission and PTTC's South Midcontinent Region, Tulsa, Oklahoma, August 19, 2003

BOTTOM LINE

Failure and operating cost reduction begins with an understanding of the basics—equipment, terminology, and sound design principles. Several software packages exist to aid in lift system design. Common failure mechanisms for different components (pumping units, rods, pumps) are known, as are accepted equipment and operating practice solutions. This workshop relayed both the theory and practice of rod-pumping operations, stressing the importance of the team environment in failure reduction efforts.

PROBLEM ADDRESSED

Artificial lift is a fact of life in mature domestic producing operations, and rod pumps are the most prevalent equipment used. Reducing component failures and pumping costs requires one to combine science with field savvy. This workshop relayed information at both levels, providing something for both the novice and expert practitioner.  

KEY WORDS:

Failures (and Corrective Actions), Predictive Programs, Rod Failures, Rod Pumping Basics, Wellbore Management/Failure Reduction

SPEAKERS

W.L. (Bill) Maxey, Harbison Fischer 

TECHNOLOGY OVERVIEW

Each pumping unit has an API Model Number, which reveals basic information about the pumping unit. For example, the model number C-640D-256-144 refers to a conventional pumping unit with a max torque rating of 640,000 in-lbs at the crankshaft, a maximum polished rod load of 25,600 lbs, and a maximum stroke length of 144 inches. The initial letter denotes the type of pumping unit, as summarized below:

The basic components of a rod-drawn pump are: (1) barrel, (2) plunger, (3) standing valve and (4) traveling valve. A fifth basic component, the hold-down seal assembly, may be either a cup-type or mechanical-type. Both work well, although when bottomhole temperatures exceed 250 °F, mechanical-type seals should be used. Mechanical hold-downs with spring steel should not be used in hydrogen sulfide environments. In H₂S environments, stainless steel with Monel or copper seats has had some success.

Common sucker rod types include:

• Grade C (Carbon Steel 90,000/115,000)—common in shallow wells
• Grade K (Nickel-Moly 90,000/115,000)—for more corrosive environments
• Grade D (High Strength 115,000/140,000)—for deeper, heavier loads
• Grade KD (115,000/140,000)
• Grade 97 (Super High Strength 140,000/150,000)—not for H₂S since susceptible to embrittlement

API convention for sucker rods is: 5=5/8", 6=3/4", 7=7/8" and 8=1". An 86 tapered rod string would represent a 1" x 7/8" x3/4" string. To keep the rod string in constant tension, it is common to add sinker bars. Some operators run larger rod strings on bottom in lieu of sinker bars. 

Rod-Pumping Predictive Programs
There are two different types of programs used today—the generalized API 11L method developed in the 1960s and more modern stress wave programs. API RP11 has limitations (steel rods only, full pump fillage only, high slip motors only, not accurate for shallow wells, conventional pumping units only, vertical wells only, unit presumed to have no structural imbalance). Choices for more modern programs include:

• Rod Star (www.theta-ent.com/rodstar.asp)
• Lufkin LOADCALC (www.hfpumps.com/calc.html)
• Echometer's Q Rod (www.echometer.com/software/index.html)
• NABLA S ROD (www.lufkin.com/oilfield/index.html)

Of these, S ROD is widely recognized as the Cadillac program. It can handle vertical or deviated wells and is the only program that will handle fiberglass rods. 

Upon prior arrangement, Harbison Fischer offers a free two-day rod-pumping school at its facilities in Fort Worth. If interested, contact the speaker for further information. 

Pumping Unit Failures
There are seven major causes for pumping unit failures.

1. Maintenance—greasing bearings, checking and changing gear box oil, belts
2. Overloaded
3. Not counterbalanced
4. Gas or fluid pound
5. Sticking plungers
6. Alignment—note that improper alignment of the bridal and carrier bar causes 99% of polish rod breaks. Harbison Fischer offers a PRO-ALIGN alignment tool. There are also stuffing box/wellhead solutions to reduce failures and minimize environmental damage should failures occur.
7. Stroking too fast—the speed that rods can fall limits prudent operations to 1,440 inches (for example, 10 SPM x 144" stroke) per minute

Sucker Rod Failures
There are ten common causes for rod failures.

1. Mishandling
2. Improper coupling makeup—leads to pin breakage, clean and lubricate (w 90 wt oil and corrosion inhibitor) when making up
3. Overloading
4. Fatigue—too many cycles (i.e., an old string) or pulling too hard (limit to 90% of maximum tensile strength when pulling on stuck pumps)
5. Improper design
6. Corrosion—chemical treating programs often required
7. Sticking plungers
8. Gas or fluid pound (pumping off)
9. Compression (not running weight bars)
10. Wear or rubbing on tubing

Attendees received a copy of Norris's special report, "Sucker Rod Failure Analysis." Much of that information is available online (www.norrisrods.com/index3.html).

Pump Problems

• Fluid Pound (pumped off)—slow the unit down, time clock, pump-off controllers (costs are coming down), variable speed motor (still expensive), STR (shock absorber effect) tool, sliding top valve, variable slippage pump.
• Gas Interference (gas locked)—slow the unit down, proper gas anchor/mud anchor design (key is "quiet zone," contact speaker for HF calculator), specialized plungers and valves, Pampa Gas pump (good at handling sand and FeS), Gas Chaser pump (expensive, last resort). For wells that tend to gas lock because they flow, increasing flowline backpressure will help prevent flowing off.
• Solids (sand, iron sulfide and iron oxide)—loosening or tightening barrel/plunger fits, three-tube pump, P.A. and groove plungers, Texas Stripper pump (similar to three-tube pump but lower volume), Pampa Rod pump, bottom discharge valve (allows 25% fluid discharge to keep solids stirred up/washed upward), top holddown pump, tubing purge valve.
• Abrasion/Wear—harder barrel tubes, harder plungers, wear-resistant ball and seats (titanium carbide popular for corrosion/abrasion, ceramic susceptible to chipping, silicon nitride very expensive), stellite-lined cages, rubber-lined cages
• Corrosion (H₂S, CO₂, bacteria, electrolysis)—barrels (brass, 304 stainless, 501 stainless, Monel), plungers (Monel pin, nickel-plated ID & pins), fittings (alloy steel, 304 stainless, 316 stainless, brass, Monel). If a corrosive environment, metallurgy/materials selection and an effective chemical treating program are required.

Failure Meetings, Team Approach to Reducing Failures/Costs
Led by efforts initiated by majors (and their vendors) in the early 1990s, industry has learned that team efforts involving company field and office staff and vendors (chemical, pumps, rods, tubing, etc.) that are focused on identifying failure causes and initiating solutions can yield significant results—as much as ten-fold improvement in failure rate. Smaller independents can incorporate the same concepts to reap benefits, albeit on a smaller scale. NOTE — see the PTTC-developed "Produced Water & Associated Issues" manual (www.pttc.org/pwm/produced_water.htm) for further information about proven practices for failure and cost reduction. 

CONNECTIONS:

W.L. (Bill) Maxey
Harbison Fischer
P.O. Box 95127
Oklahoma City, OK 73143
Phone: 405-354-9389
Email: bmaxey@hfpumps.com

For information on PTTC’s South Midcontinent Region and its activities contact:

Charles Mankin, Director, Oklahoma Geological Survey
100 E. Boyd St., Room N131, Norman, OK 73019-0628
Phone 405-325-3031, Fax 405-325-7069, Email cjmankin@ou.edu

Disclaimer: No specific application of products or services is endorsed by PTTC. Reasonable steps are taken to ensure the reliability of sources for information that PTTC disseminates; individuals and institutions are solely responsible for the consequences of its use.

The not-for-profit Petroleum Technology Transfer Council is funded primarily by the US Department of Energy’s Office of Fossil Energy, with additional funding from universities, state geological surveys, several state governments, and industry donations.

Petroleum Technology Transfer Council, 16010 Barkers Point Lane, Ste 220, Houston, TX 77079
toll-free 1-888-THE-PTTC; fax 281-921-1723; Email hq@pttc.org; web www.pttc.org