Drag Bag

Key Takeaways

• The fundamental measurement limitation of conventional stationary production logging (stopping the tool with the packer fully inflated at each depth and recording a steady-state flow measurement) is that each stationary measurement requires the tool to be stopped and the wellbore flow to stabilize before a valid reading is obtained, making a conventional production log a time-consuming operation (typically 30 minutes per measurement station multiplied by 10 to 20 measurement stations equals 5 to 10 hours of rig or wireline time just for the measurements), and restricting the data to discrete depth points rather than a continuous profile; the drag bag technique addresses this limitation by replacing the stationary measurements with a continuous upward pull at a controlled speed (typically 15 to 30 feet per minute), with the partial packer providing enough flow diversion to maintain an approximately representative flow rate measurement without completely stopping the tool; the resulting continuous profile shows the flow contribution of every perforation cluster and every foot of the completed interval, revealing contributions from individual perforations that are too small to be captured by the widely spaced stationary measurement points of a conventional production log and exposing channeling or crossflow between zones that occurs between the discrete measurement points.
• The partial inflation (drag) of the packer in the drag bag technique is a trade-off between measurement quality and tool mobility: a fully inflated packer completely diverts all fluid through the flowmeter and provides the most accurate measurement of cumulative flow at each depth, but requires the tool to be stopped at each measurement station (because a fully inflated packer cannot be pulled continuously without excessive drag force that would damage the inflatable element or the casing) and cannot provide a continuous profile; a deflated or partially inflated packer (the drag bag configuration) allows continuous upward movement but does not completely divert all fluid through the flowmeter -- some fluid bypasses the partially inflated element through the gap between the element and the casing wall, introducing a bypass flow that reduces the apparent flow rate measured by the flowmeter below the actual production rate; the degree of partial inflation is calibrated so that the bypass flow fraction is consistent and measurable (rather than uncontrolled), allowing the measured flow rate to be corrected to the actual total flow rate using a bypass correction factor determined from calibration runs at the same inflation pressure; the correction factor depends on the casing inside diameter, the packer element OD at the partial inflation pressure, and the fluid viscosity.
• Continuous spinner (turbine) flowmeters are the most common sensor used in drag bag production logs: the spinner is a helical turbine mounted on the flowmeter body whose rotation rate is proportional to the upward fluid velocity past the sensor; as the drag bag tool moves upward through the wellbore at a known speed while producing fluid flows upward, the spinner records the combined velocity of the upward-flowing fluid and the upward tool movement; a downward pass at the same speed (against the flowing fluid) records the combined velocity of downward tool movement and upward fluid flow with opposite signs; the difference between the upward and downward pass readings at each depth cancels the tool velocity and gives the true upward fluid velocity at that depth; the fluid velocity multiplied by the cross-sectional area of the wellbore at that depth (from the casing inner diameter) gives the volumetric flow rate; modern production logging tools replace the spinner with array spinner sensors (multiple small spinners distributed across the wellbore cross-section) or with capacitance, conductivity, or optical sensors that simultaneously measure the holdup of each fluid phase (oil, water, gas) and the velocity of each phase, providing multi-phase flow profiles rather than single-phase total flow profiles.
• Phase identification in drag bag production logging is required to determine whether the fluid entering the wellbore from each producing zone is oil, water, gas, or a mixture, because the production allocation decision (shut off a water-producing zone, stimulate an oil-contributing zone) depends on which fluid each zone is contributing rather than just the total volumetric flow; the sensors used for phase identification include capacitance sensors (which detect the difference in dielectric constant between oil and water), conductivity sensors (which detect the difference in electrical conductivity between the conductive formation water and the non-conductive oil and gas), and density sensors (which detect the density difference between oil, water, and gas using either gamma-ray absorption in a density section or a combination of pressure gradient sensors); in gas wells with liquid loading (water accumulating in the wellbore), the drag bag technique can detect the depth at which liquid loading begins (the depth above which the gas velocity is insufficient to lift liquids, allowing them to accumulate as a downward-flowing liquid film while gas flows upward in the center of the wellbore) by identifying the depth at which the flow meter shows a sharp transition from upward total flow to a combination of upward gas and downward liquid.
• Comparison of drag bag production logs before and after workovers, stimulation treatments, or zone shutoffs provides a quantitative measure of the effectiveness of the intervention: a drag bag log before stimulation identifies the contribution of each zone to total well production; a drag bag log after stimulation (typically 30 to 90 days after stimulation to allow the production to stabilize and the stimulation effects to be fully realized) shows how the contribution has changed -- an increase in the contribution of the stimulated zone, a decrease in the contribution of a bypassed zone, or evidence of a new zone contribution that was not producing before stimulation; the before-and-after comparison provides the production engineer with data to evaluate whether the stimulation achieved its objective and to identify zones that may benefit from additional stimulation in future workovers; in waterflooded fields, periodic drag bag production logs provide the water cut profile by zone that is essential for monitoring waterflood conformance (whether injected water is breaking through evenly in all producers or is channeling preferentially through high-permeability layers and leaving low-permeability layers unswept).