SENSITIVITY OF FLOWMETER CALIBRATION TO INCOMING FLOW THROUGH A WELL SCREEN

Field, laboratory, and numerical-modeling evidence indicates that a flowmeter is sensitive to flow entering a borehole above the entrance of the barrel of the flowmeter, if used without a skirt or packer to force flow through the barrel. The electromagnetic flowmeter we are using has a 0.42 meter long barrel through which the flow rate is measured. The ratio of mass flux going through the barrel to flow in the borehole can change by a factor of two.

The effect is noticeable in field data where two sections of screened casing are connected, forming a short unscreened segment. In our field data, a decrease in the measured flow occasionally occurs about a meter below the unscreened segment. A reduction in the rate of increase between measurements in this region is common. In the laboratory data, a 2% drop in flow occurs below the unscreened section compared to a 7% rising trend in other regions. In both field and laboratory data, measured flow increases within the unscreened intervals.

Acceleration of flow along the outside of the flowmeter barrel when flow is entering along the sides of the barrel causes the seemingly phenomena. The acceleration is driven by the pressure drop along the outside of the barrel. Since the pressure drop through the barrel must match the pressure drop outside the barrel, more flow enters the barrel when incoming flow occurs compared to when flow is blocked by the unscreened section.

A momentum balance simulation of the fluid flow within the flowmeter and borehole is embedded within a two-dimensional axis-symmetric version of MODFLOW-2000. With this model, we simulate the influence of the unscreened section. The model is then used to estimate a one-dimensional hydraulic conductivity distribution along the borehole. The interpreted hydraulic conductivity varies significantly from an interpretation that does not include the influence of incoming flow along the sides of the flowmeter barrel.