Engineers, operators, pump techs, production foremen and all other end users are expected to make efficient use of all the production data that technology has to offer the oilfield. This article focuses on two common downhole pump card readings (with gas interference and fluid pound) and the relatively simple ways to correct sucker rod pump inefficiencies.
Downhole pump cards are generally provided by rod pump controllers or other well analyzing equipment. A perfect pump card (see Figure 1) represents a perfect pumping well. This pump card resembles a rectangle in which the smooth top and bottom lines represent the smooth up and down strokes of the pumping unit. This perfection only exists in classrooms, but that should not deter end users from trying to get as close to this perfection as possible.
First, a quick explanation of the pump card is necessary. Pump cards are drawn in a clockwise direction following four points (A, B, C and D). While the pump card technically derives its information from the fluctuating weight within the fluid column impressed upon the rods, the opening and closing of pump valves are represented by these four points.
In Figures 1 through 5, Point A will generally indicate that a fluid load is being picked up until Point B. At Point B, both the standing and travelling valves are closed briefly before Point C occurs. In the downhole pump card, Point C represents the moment when the pump discharge pressure equals the static tubing pressure. Immediately after in the Point C to Point D transition, the fluid in the pump is displaced into the tubing and zero fluid load is on the rods.
Thousands of feet of rods separate the surface and the plunger downhole. As a result, no simultaneous upstroke exists between a surface card and the downhole pump card. This is because when the top rod is pulled or pushed at the surface, it pulls/pushes the next rod which pulls/pushes the next rod and so on and on to the bottom where the plunger is finally pulled or pushed.
Since the transition from Point D to Point A and the transition from Point B to Point C are the points at which both valves are closed, Point A is the point at which the standing valve opens on the upstroke and Point C as the moment when the travelling valve opens on the downstroke.
One of the most common downhole pump cards is one that represents gas interference (see Figures 2 and 3). The upstroke (Points A and B) runs across the top. However, in the transition from Point B to Point C, the line begins to slope down and to the left. This indicates that the initial part of the downstroke had gradual difficulty opening the travelling valve at point Point C. Remember that from Points B to C, both valves are closed. The travelling valve does not open until Point C.
The downward curve indicates a gradual opening of the travelling valve as opposed to a sharp cut from Point B to Point C, as might be seen in a fluid pound situation (see Figures 4 & 5), which is discussed in the next section.
Simply assessing the issues at play can be the most challenging task because so many components are involved in sucker rod pumping. When one piece of equipment begins to fail along the chain, it affects the whole pump system. The pumping unit, surface equipment and downhole equipment can all be negatively affected as a result of poor rod or pump designs, inefficient chemical application, or personnel errors in the field to name a few possible causes.
In this case, the problem is gas interference expressed by the sloping line inward. A few options may be used to remedy the situation, including minor alterations to the pump. By tweaking the cages (or valves) of a pump, operators can minimize the amount of entrained gas that breaks free of the fluid within the pump.
A large pressure drop in the valve allows entrained gas to break free as it is enters the pump chamber, which leads to gas interference. Any increased space in the valve will decrease the pressure drop in this area. An end user can choose a larger cage or implement a smaller ball within the cage, which will decrease the pressure drop. Since both options are reasonably easy changes, an end user may consider implementing both.
Once the pump is ready to be changed, replace the standing valve with an oversized cage to increase the space within the valve. The same ball can be re-run in the new oversized cage if it has no damage. However, a new, larger seat size with a double-lap is required for an oversized cage. Then replace the ball inside the travelling valve with a smaller ball. This alternate pattern of balls and seats is sometimes referred to as a California-style pattern.
Less weight in a smaller travelling valve ball coupled with increased space for fluid flow will help minimize the pressure drop and the gas interference. While this sounds minute, the subtle changes will make a difference in pumping efficiency.
Fluid Pound Reading
A downhole pump card that resembles gas interference is the fluid pound situation shown in Figures 4 and 5. Instead of a sloping downstroke line, fluid pound presents an initial horizontal movement followed by a sharp decline. Whereas gas interference gradually opens the travelling valve (Points B to C), fluid pound does not open the travelling valve until it slams into the fluid (Points B to C).
Fluid pound typically indicates that a shortage of fluid exists in the pump chamber, which creates a plunger freefall followed by a collision with the fluid. Without fluid (or heavy amounts of compressed gas), the travelling valve ball has nothing to push it off the seat to open the valve. This could have several causes, but generally, it means the well is pumped off and does not have any more fluid to pump.
At the surface, operators can slow the pumping unit strokes per minute if a pump off controller (POC) or time clock for pumped off conditions is not available. Aside from this, the pump bore should be down-sized. For example, if the pump bore is 1.5 inches in diameter, simply down-size to a 1.25-inch pump bore. By subtly restricting fluid flow and slowing the pump jack, the fluid pound will occur less frequently.
Targeted Rod Pump Efficiency
Neither of the pump card solutions described in this article will provide the perfect pump card, but they are a great start to targeting rod pump efficiency downhole. Extending pump run times will keep production online. If pump controllers are not an option for a company, many service companies will provide surface and downhole pump cards for a well. The data is available. Target pump efficiency and keep production consistent.