Borehole flow rate

Figure 55. Geothermal response test with combined heat extraction/heat injection and different energy levels, shown are the borehole heat exchanger flow and return temperatures and the calculated heat flux. Borehole flow rate is not shown in the graph...

Borehole flow rate recorded as 11/h Methane concentration in borehole = 23%... 

It is important to record (in addition to the monitoring parameters from the instrument such as gas concentrations, borehole flow rates, atmospheric pressure and vapour concentrations where appropriate) ... 

Any activihes in the area that may affect readings (e.g. dewatering or excavation). In one case at a football stadium, operation of the underpitch air drying system was affecting the borehole flow rate... 

A comparison of the results of the gas monitoring with ground conditions can help to identify the main source of gas on a site. For example, it is often useful to group together the results of gas concentration or borehole flow rate for wells with response zones in a particular soil type. 

The method described in CIRIA Report 152 assmned that the actual entry rate of methane into the cupboard was the diffusion rate of methane from the adjacent landfill. However, this possibly does not adequately represent the rate at which methane will enter a cupboard. A more realistic indicator of methane surface emissions into the cupboard can be gained using borehole flow rates or calculated surface emission rafes. One mefhod of doing fhis, if there is a sufficient data set, is by xmdertaking statistical analysis of the gas monitoring results (see Box 6.5). 

The underfloor venting has been determined by calculation (see Chapter 7) to be effective up to a borehole flow rate of 1.61/h. We can estimate the probability of this value being exceeded (assuming the gas concentration remains constant). The example also assumes that all the data is representative of the gas source on tire site (any data from boreholes that are not representative should not be included in the analysis). 

Figure 6.11 Borehole flow rate and pdf for normal distribution...

The pdf for borehole flow rates measured in boreholes in the site is given in this case by a normal distribution shown in Figure 6.11 (note that the pdf chosen must be appropriate to the data being analysed). 

So the probability that the borehole flow rate is greater than 1.6 1/h from the above graph (mean of data is 0.5 1/h, standard deviation = 0.22 1/h) is given by ... 

Borehole flow rate 13.6 l/h N/a Borehole flow rate is maximum recorded in the site and therefore no sensitivity analysis has been used for this parameter... 

Maximum methane concentration = 0.6% v/v Maximum carbon dioxide concentration = 6.8% v/v Maximum borehole flow rate = 21/h... 

A site is proposed for a shopping centre development and has a maximmn gas concentration of 26% and borehole flow rate of 0.11/h. The source of the gas is a tiiick layer of alluvium with peat lenses and groundwater levels in flie area are known to rise and fall quickly in response to rainfall events. The development will have a suspended cast in situ reinforced concrete floor slab fliat is 200 mm thick. 

So, we can consider that the borehole flow rates from the gas monitoring (0.11/h) are not representative. We can estimate that if groimdwater levels rise we may obtain higher rates in the short term (say, for up to 8 h). So, we will estimate that 2 1/h borehole flow rate is more representative of the site. 

Gas concentration measured in a monitoring well multiplied by the measured borehole flow rate... 

The GSV (in litres per hour) is as defined in Wilson and Card (1999) as the borehole flow rate multiplied by the concentration of the particular gas being considered. 

Gas screening value (litres of gas/hour) is calculated by multiplying the maximum gas concentration (%) by the maximum measured borehole flow rate Site characterisation should be based on gas monitoring of concentrations and borehole flow rates for the minimum periods defined in Section 5... 

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