Parameter Measurement While Drilling (MWD)
The real-time measurement of drilling parameters during rotary drilling (MWD), is a technique new to the UK site investigation industry. However, this technique adds considerable value to rotary drilling investigations.
The information gathered can offer significant improvements and benefits to the standard rotary drilling techniques, both when drilling open hole using downhole hammers and when coring:
- Accurate depth definition of zones of weak and fractured strata, voids, backfilled mine workings etc. when open hole drilling – ideal for correlation between boreholes and probe drilling.
- Use of standardised rotary / percussive sounding methods (Swedish and Norwegian) if required.
- Rapid and cost effective correlation and confirmation of continuity of ground conditions over the site, with repeatable qualitative data recorded from all boreholes.
- Ability to provide semi-quantitative assessment of ground quality by processing the parameters recorded to provide a measurement of the specific energy required to progress the borehole.
- Allows the analysis of drilling technique when used in conjunction with core drilling.
- Minimal additional costs with similar production rates to standard techniques
Emerson Moore Geosciences offers the measurement and recording of the following parameters while rotary drilling:
- Rotation speed
- Torque
- Flush Pressure
- Feed force (bit pressure)
- Penetration rate
- Downhole Hammer Pressure
The specialist instrumentation allows the recording of these parameters against depth for later reporting, as well as real-time display on the rig computer allowing informed decisions to be made on site as to hole depth and program.
Case Study – Open hole drilling for voids
The following example illustrates the results from drilling parameter measurement during an investigation to establish the presence of voids and weak ground in landslide and volcanic strata on the flanks of Mount Vesuvius, Italy, prior to railway construction. The borehole was drilled with rotary open hole techniques, using water flush with a generally constant rotation speed. The following illustrates the presentation of the recorded parameters:
From the raw data and the interpreted logs the following interpretation was made:
0 – 16m Soft / loose highly variable soils – probably loose recent landslide materials. Note the large rapid variations in parameters recorded.
16 – 26m Denser material with boulders – probably older, partially cemented landslide materials.
26 – 29m Dense soils with an infilled 1m void between 26m and 27m.
29 - 33m Void – note zero specific energy and rapid drop in flush pressure with slow increase as void fills with drilling mud.
Note that the parameters are sometimes difficult to interpret individually. To provide a semi- quantitative assessment of the energy required to progress the borehole (specific energy) the measured parameters have been combined using the following formulae, graphically presented on the far right hand column of the log:
| Es = F/A + (2πNT)/AV) | where: |
| Es = Specific Energy Mj / m3 | |
| F = Bit Load (kN) | A = Area of bit (m2) |
| N = RPM (RPM) | T = Torque (kNm) |
| V = Rate of Penetration (m/s) |
These results illustrate the advantages that can be gained by using MWD with
open hole drilling techniques. Obviously the interpretation can be improved
dramatically by correlating open hole boreholes with cored holes, geophysics
and other existing data, making this technique ideal for infill drilling and
probe drilling where the requirement is to maximise the data gathered while
minimising the cost of the investigation.
MWD - Summary of Advantages
- Significant increase in data available from open hole drilling.
- Improved ease and accuracy of interpretation.
- Low additional cost compared with conventional open hole drilling.
- Can reduce the number of cored boreholes required therefore reducing investigation costs.
- No increase in time required for drilling program.
- Results available in real-time.
- Ideal for probe drilling and correlation between boreholes.
- Can be used to assess affect of drilling technique on core recovery.