Design and application of a borehole flowmeter

Abstract

A borehole flowmeter has been developed to measure low flow rates in NX (76 mm) boreholes. The instrument can be used to determine the standing water level and permeability of a test section of the borehole.The borehole flowmeter is a short, lightweight wireline packer which is inflated pneumatically and contains a turbine flowmeter transmitter unil. The useful flow range of the standard instrument is approximately 0.7 cm3/s to 70 cm3/s. The unit was originally designed to measure small axial flow rates in an array of boreholes which was thought to intersect a series of perched aquifers.Experimentation showed that an increased rate of flow could be induced through the unit by piping water into the top of the borehole, the flow rate being proportional to the excess head of water above the unit. Using this principle, a form of in situ falling head permeability test was devised. The corrected head of water at a zero flow rate was found to correspond to the standing water level in a borehole, and the linear relationship between applied head and flow rate was used to calculate the permeability of the section of borehole below the unit. The results are compared with those of packer tests carried out in the same test sections.The method has several advantages over packer testing in many conditions. The wireline equipment is light and can be operated by one man. The permeability test results are repeatable and are completed on average within one hour, though a satisfactory relationship between flow and head is usually established within ten minutes in a moderately permeable formation.

Results of an experimental programme of in situ permeability testing in rock

“…Fig. 7 also shows the results of some long-section tests carried out with the borehole flowmeter (Wild 1985) and these also agree well with the packer test results.…”

“…One method utilises the borehole flow meter to which reference has already been made (Wild 1985), and which has been shown to produce results comparable with packer tests.…”

“…The existence of low permeabilities and variable water levels made it impossible to complete objective 3 although drawdown tests using a syphon technique were successfully carried out at two sites. The development of a borehole flowmeter to investigate the anomalous water levers, however, proved to be a useful testing tool (Wild 1985). Slug tests were performed but proved difficult to interpret as the recent analysis of Black (1985) It was not possible to drill at the sites previously used due to difficulties with access for the rig but the work was carried out in the same general area of Roxburghshire and in two of the same formations, the Upper Old Red Sandstone and Silurian greywackes with, in addition tests in Lower Carboniferous basaits.…”