Fluid inertia effects increase equipment loading and reduce efficiency in high rate shallow wells with very low gas production (producing more than 800 BPD from depths of less than 2,500 feet). In deeper wells this effect is negligible. However, in shallow, high rate rod pump wells that are primarily found in California, fluid inertia is one of primarily found in California, fluid inertia is one of the dominant factors affecting system loading and performance. This effect is one of the main performance. This effect is one of the main reasons most design methods for rod pump systems do not agree with field measurements for shallow wells.
In an attempt to reduce the damaging effects of fluid inertia, a prototype of a downhole pulsation dampener was successfully field tested in a California well with severe fluid inertia effects. As Figure 1 shows, the device consists of a perforated pup joint of tubing covered by a Kevlar reinforced pup joint of tubing covered by a Kevlar reinforced elastomeric bladder. This assembly is enclosed in a sealed steel outer housing. The volume between the bladder and the housing is pre-charged with nitrogen to a pressure equal to the tubing pressure at the point of installation. For the test, the dampener was installed as the first joint of tubing above the pump.
The device operates as follows: On the upstroke, as the traveling valve closes, the plunger picks up the fluid load and accelerates it creating a pressure pulse. The dampener absorbs this pulse as pressure pulse. The dampener absorbs this pulse as bladder expansion compresses the nitrogen. On the downstroke, as the pressure in the tubing drops, the bladder returns to its original position.
The downhole pulsation dampener completely removed the fluid inertia effects. Dynamometer cards showed a dramatic difference in shape between the "before" and "after" cases. Dynamometer analyses for before and after dampener installation show the device was effective in reducing gearbox loading by 36 percent, sucker rod loading by 22 percent and polished rod horsepower by 20 percent. Fluid level polished rod horsepower by 20 percent. Fluid level and production rate remained approximately the same.
High rate rod pumping of shallow wells presents unique problems that are absent from deep well applications. Wells that produce more than 800 BPD from depths of 2500 feet or less tend to have more gearbox failures than deeper, lower rate wells. The primary reasons for these failures include the tendency to overtorque the pumping units because of the large plunger sizes used and the lack of accurate methods of designing rod pump systems for these conditions.
Modern rod pump design methods such as the API RP 11L or wave equation based methods accurately predict system performance for wells of 3000 feet or predict system performance for wells of 3000 feet or deeper. However, there are large errors in predictions for wells of less than 2500 feet, predictions for wells of less than 2500 feet, especially for high rate wells.
One of the main reasons for lack of accuracy is the presence of fluid inertia effects that increase rod presence of fluid inertia effects that increase rod loading on the upstroke. These effects are not present in deeper wells because the elasticity of present in deeper wells because the elasticity of the rod string and the "sponginess" of the fluid help absorb the "shock" of fluid load transfer to the rods on the upstroke. In addition, deeper wells have smaller pump plunger diameters that reduce fluid load on the rods and the rest of the system. In deeper wells rod loading is the limiting factor in system design. Therefore, smaller plungers and lower speeds are used that minimize plungers and lower speeds are used that minimize fluid inertia effects.