USGS_GeophysicalLoggingInSLV_Report

depths in the aquifer. These natural, measurable view of the borehole wall behind the well casing. If the temperature differences are useful in helping to define annular spaces are open or not completely packed, zones of inflow to the well and to verify or assist in vertical flow may occur between the well casing and interpretations of results from other logs. Temperature the borehole wall. An open annular space would cause logs were recorded in flowing wells under free- the effective cross-sectional area of a well to increase, flowing conditions and in nonflowing wells under depending on whether annular flow was occurring. pumped and static conditions. The trace of the flowmeter log would indicate outflow Well-Flow Log. The well-flow log is a measure from a well as flow entered the annular space and of the velocity or rate of flow through a flowmeter at inflow to the well as flow went from the annular space each depth for which a measurement is recorded. Flow into the well casing. Flow in the annular space of a in the well can be recorded as a continuous profile well would make the smaller inflections in a flow- (trolling log) or as point measurements at specific meter trace difficult to interpret. Therefore, the anal- depths (stationary measurements). Inflections in this ysis of the flowmeter data is qualitative rather than log indicate either increasing flow (an inflection to the quantitative because a quantitative analysis is not right) or decreasing flow (an inflection to the left) in possible in poorly constructed wells. the well. Flowmeter measurements were recorded at two Two types of flowmeters were used to record flow rates: free-flowing and shut-in in flowing wells, data: (1) an electromagnetic (EM) flowmeter and (2) a or pumped and static water column in nonflowing spinner flowmeter. The EM flowmeter measures the wells. The rate of natural flow in fully open flowing velocity of flow by applying a magnetic field to the wells or the highest pumping rate that was obtainable water and measuring the distortion of the magnetic for nonflowing wells ranged from less than 10 gallons field caused by water moving through the detector per minute (gal/min) to more than 1,000 gal/min. section of the tool. The manufacturer of the EM flow- Except for one well, the shut-in and static water- meter reports its measurement range as 0.3 to 260 feet column flowmeter logs were always recorded immedi- per minute (ft/min), making it useful for measuring a ately after the free-flowing or pumped flowmeter logs wide range of flow velocities, including flow at very were recorded. The shut-in measurements were slow velocities. The spinner flowmeter has a propeller, recorded a day after the free-flowing measurements mounted in a protective housing, that spins in propor- were recorded in well SAG.2. Logging the wells under tion to flow velocity. The manufacturer of this flow- free-flowing or pumped conditions allowed the identi- meter reports its measurement range as 9.75 to 200 fication of the predominant flow zones in each well. ft/min. The spinner flowmeter was used in three wells Shut-in measurements were recorded with flow from because the EM flowmeter was not available. each flowing well almost entirely closed off. Shut-in The free-flowing trolling EM flowmeter measurements were obtained to detect head differ- measurements in four wells exceeded the manufac- ences between flow zones that may exist naturally in turer?s recommended maximum measurable flow of the aquifer or because of preferential flow during free- 260 ft/min. The free-flowing trolling flowmeter logs flow conditions resulting in lowered pressures in more that were recorded in two of those wells appear to permeable zones. Flowmeter measurements recorded reasonably represent the changes in flow in the wells under conditions of no pumpage in the static water because the shut-in stationary flow logs (within the column of nonflowing wells, did not indicate inter- measurement range of the flowmeter) showed the zonal flow. same profile as the measurements that exceeded the Flowmeter measurements were obtained at flowmeter?s limits. different flow rates with the flowmeter being lowered One of the difficulties in obtaining accurate down a well (trolling) or with the flowmeter stationary flowmeter measurements in the CAWN wells is the within the well. Not all the flowmeter logs were possibility that many of the wells may have annular recorded successfully because of the variability of spaces (the space between the borehole and the casing conditions in the wells. The objective in attempting to pipe) that are not completely sand or gravel packed. record trolling and stationary flowmeter logs in each Many videos of the CAWN wells show a lack of well was to obtain as much useful information as packing in the annular space through many of the possible to be able to decide which logs were most torch-cut slots that were sufficiently open to allow a instructive in delineating flow conditions in each well. 6Geophysical Logging to Determine Construction, Contributing Zones, and Appropriate Use of Water Levels Measured in Confined-Aquifer Network Wells, San Luis Valley, Colorado, 1998?2000