Sand and other fluid entrained particulates can cause substantial operational problems for rod pumped producing wells. These problems take the form of down- hole pump wear, plunger sticking, and/or catastrophic breakage of pump components.
A six year study of 600+ pump investigations shows that problems with particulates account for a substantial number of the total barrel and plunger failures. Many of these failures could have been avoided through the proper application of API and special pump designs as well as certain choices of pump variables. These pump variables and designs will be addressed along with operating parameters.
How hard is sand? How large is it? Should plunger and barrel choices take these variables into account? Also, metal plungers must "slip" fluid for proper lubrication. Too much slippage leads to pump inefficiencies. However, down-hole pump efficiencies based on slippage need to be balanced against pump longevity due to proper selection for particulate production. Some time-proven rules of thumb can be applied to make these choices, and an included chart will make plunger slippage calculations simple and straightforward.
Scale which sticks to pump surfaces dictates the choice of a different style down-hole pump. A straight-forward modification of an RH style API pump has proved successful in these conditions.
Fluid and particulate production with and without attendant gas production requires a different approach to down-hole pump selection. Several successful older pump designs as well as some recently proven new designs will be described.
A test program has been completed and actual applications have shown that an API Tubing Pump derivative can pump large volumes of particulate laden fluid without characteristic sticking of the plunger.
Introduction
A study was conducted of pumps with relatively short run times during the years from 1989 through 1994. These were analyzed with respect to pump barrels or plungers that had problems due to produced particulates. A significant number of the barrels and plungers developed problems due to particulate induced wear or sticking (Figs. 1 and 2).
Costs of new rod pumps and pump repairs are typically closely monitored, perhaps at the expense of overall well production profitability. A different costing focus may be applicable, and appropriate record keeping is necessary for accurate cost analysis. New record keeping methods are available which pinpoint areas for improvement.
Variables For Rod Pumping
Several general pumping choices for the down-hole pump must be logically made for optimum functioning and efficient longevity. These apply to any pump and include; pump metallurgy, plunger minus fit choice, and slippage design for lubrication of the plunger and barrel interface.
Pump Metallurgy. pump wear is the primary consideration when particulates are produced along with the production fluid. Sand grains can be as hard as hardened steel (Table 1), causing contact wear and erosion of pump surfaces. The two surfaces that receive the most wear are the barrel and plunger surfaces. These two parts of the pump form the moving surfaces of the pump that are subject to sliding wear during each stroke of the pump. During a typical 24 hour pumping day at 10 strokes per minute, a plunger and barrel will see over 14,000 cycles of up and down motion contributing to plunger and barrel wear. For a 100" long down-hole stroke length, the total distance of sliding wear is about 45 miles per day.
Barrel Metallurgy. The oldest and most popular choice of inside wearing surfaces for pump barrels is hard chrome plating. The hardness is Rc67 minimum, and the chrome plating seldom wears out. Its weakness is due to its intolerance of hydrochloric acid, which is commonly used for remedial acid treatment of oil wells, and other corrosive fluids which attack the chrome plating bond line. Hydrochloric acid will dissolve chrome plating. Any down-hole formation fluid with a Ph below 7, or other corrosive down-hole fluids will eventually cause chrome plating to flake off, through weakening of the chrome to substrate bond line. This invariably leads to further damage to the barrel and plunger by the hard chrome particles which have flaked off of the barrel. This weakness is one of the reasons that chrome plating seldom wears out. It is more likely that it will eventually flake off in some area before it wears out, even if the barrel has given a satisfactory run time. Therefore, a chrome plated barrel has one likely early failure mode; flaking off of the plating, and two possible normal "used up" modes; wearing away or flaking off of the chrome plating. A significant number of the barrels represented in the failure study had problems with chrome flaking. None had problems with worn chrome.
A nickel carbide coating is available for the inside and outside surfaces of pump barrels. It provides a hard wearing inside surface and an outside corrosion barrier.
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