Pump Positioning vs Liquid Level in Annulus

All wells produce mixture of oil and gas, because of dissolved gas in oil. Gas separate from oil because of pressure decline in tubing and annulus.

As wellbore fluid rises in tubing, pressure decrease continuously. At a certain level gas bubbles will break out and at this point the hydrostatic pressure is equal to the bubble point pressure (Pbub) of the oil. Below this point there is no free gas in the wellbore as all light components are in solution. Above this point the fraction of free gas increases steadily because of:

1. The expansion of gas due to pressure decline;
2. The breaking out of additional components of oil.

The volume of free gas reach its maximum at the dynamic level.

The gas/liquid ratio (GLR) is a measure of the volume of free gas at the pump intake. This ratio is contrary to the gas/oil ratio (GOR), which is measured at the surface. The GLR is calculated as a percentage related to volume of liquid. It depends on several parameters:

• Bubble point pressure of the oil;
• Hydrostatic pressure in the well at the pump intake;
• Percentage of the water in the well production (watercut).

In order to ensure maximum efficiency of the pump it should be installed below the bubble point level.

In figure is an example of illustration of GLR variation between bubble point pressure and dynamic level. The presence of the free gas evidently lowers the volumetric efficiency of pump, because it reduces the volume of liquid oil handled by the pump. This also cause a substantial increase of the GLR.

There are some ways to reduce excessive GLR.

If well completion allows one to do so:

1. Install the pump deeper to increase the pressure at the pump intake;
2. If continuous gas phase developed in the reservoir and produces gas through perforations, the pump should be installed below the perforations.

If completion does not allow deeper installation of the pump:

1. Use a gas anchor or below pump gas separator at the lower end of the pump.

If no of above mentioned methods are applicable in the well, the only recourse would be the reduction of the flow rate. By doing so the dynamic level will rise in annulus and bubble point level will rise in conjunction, thereby increasing the submergence of the pump and reducing GLR at the intake point.

It is important to clearly assess the following fundamental differences:

• The gas involved in the calculation of the GOR is the gas produced by separators, e.g., under surface conditions (pressure and temperature).
• The gas involved in GLR has a different composition and is calculated at the bottom hole conditions (pressure and temperature).