USGS_GeophysicalLoggingInSLV_Report

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ANALYSIS OF GEOPHYSICAL LOGS Well flow recorded on the logs is the flow that passes through the measurement section of the flow- Geophysical logging and analysis of the driller?s meter. A flow-diverting skirt can be used to concen- and geophysical logs were performed to provide better trate all flow in a well through the measurement information about wells in the CAWN. The logs section of the flowmeter when flows are small. Flow- provided information on (1) construction of the wells, diverting skirts were not used during this study (2) lithologies or lithologic zones open to the wells because (1) the diameter of most wells in the CAWN is through saw- or torch-cut slots or stainless-steel too large to successfully use a skirt, (2) discharges screens or in uncased portions of boreholes, and under free-flowing conditions were too large to allow (3) inflow or outflow zones providing water to the the use of a skirt, (3) the annular space of many wells wells. The logs recorded in each of the wells were appeared to lack gravel or sand packing that poten- plotted side-by-side on graphs. This method of plot- tially could allow flow to bypass the casing and occur ting the logs allows data at particular depths to be in the annular space, (4) the tops of some well liners correlated between the logs. The logs plotted in the were bent, or (5) the inside of most wells is rough, figures for each well do not start at zero on the charts reducing the effectiveness of the skirt. because it was necessary to lower the logging tools Flowmeter data plotted in the figures showing 10?20 ft into the well before the logs could begin to be all well logs for a well are shown in gallons per minute recorded. The upper, unlogged 10?20 ft of the wells or feet per minute, as measured through the measure- typically does not yield additional information that ment section of the flowmeter. Stationary and trolling would change the analyses that are presented in this flowmeter measurements were selected for presenta- report. tion in the figures, based on which logs showed clearly In the description of the logs for each well, definable information about flow zones within the reference is made to features of the well casing or the wells. These flow measurements are used to identify generalized lithologic layers that occur at depth. All zones of inflow rather than to quantify the amount of depths given in the well descriptions are depths below flow from each of the zones contributing flow to wells. land surface. Thus, the flow logs show differences or changes in In most cases, the flowmeter logs are the flow within each well as the flowmeter was advanced primary logs used to indicate zones of inflow to the upward or downward in each well to record logs or to wells. Specific-conductance and temperature logs are obtain stationary measurements. useful in indicating inflow to the wells, but these logs Most of the figures showing trolling flowmeter are supplementary to the flowmeter logs. Specific- logs contain a reference line called the ?tool-velocity conductance and temperature logs are the primary logs line.? This line shows the response the flowmeter used to define zones of inflow to wells when no flow- would record if there was no flow in a well and the meter logs are available for a particular well. only velocity was that induced by the downhole move- ment of the flowmeter. When the trolling logs are Thirty-two wells in the CAWN were logged; recorded in shut-in wells and if there is no flow in the 10 other wells were not logged because (1) known well among flow zones (interzonal flow), the flow- well construction indicated that the zone of inflow to meter log should match the tool-velocity line. Quanti- the well was very narrow, (2) well owners denied tatively, the velocity of water through the flowmeter is USGS access to the wells, (3) too much oil was equal to the velocity at a point minus the downhole floating on the surface of the water column, (4) wells velocity of the flowmeter during logging (usually were obstructed, or (5) well construction prevented 20 ft/min). The tool-velocity line is not shown on access for logging. A complete set of geophysical logs graphs with a large range in flow rates because the line consisted of video, caliper, water specific-conduc- at 20 ft/min approximately coincides with the left-axis tance, water-temperature, and free-flowing or pumped of the graph in those cases and is of little use in the and shut-in or static water-column well-flow logs. analysis. The tool-velocity line is also not shown for Logging was attempted in several wells but the logs wells in which the trolling rate was less than 20 ft/min yielded insufficient data for analysis, and several wells due to the shallowness of the well or due to the need to were constructed in a way that prevented complete slow the flowmeter down to get past irregularities in logging. Table 1 lists the availability of logs for each the casing or borehole. well in the CAWN. ANALYSIS OF GEOPHYSICAL LOGS 7