Running a Mark 2 Permeameter Test

Techref Number: 
Authored by: 
Robert Whittle

1) Calibrating the system

  1. The pressure cell and flow pump must be calibrated against a standard.

  2. The probe itself must be demonstrated to be pressure tight. This is done before field work starts by capping off the lower end of the instrument then operating the full permeameter system as if it were a real test.

  3. A successful demonstration would show that even at the lowest flow rates of which the system is capable the internal pressure continues to rise - a leakage path shows itself as an apparent steady state.

  4. The other major source of error is temperature change. A temperature transducer is fitted to the system and the best results are obtained when the temperature changes by no more than a degree over the course of a particular test.

  5. Special drill string where the joints are sealed by 'O' rings must be used for permeability testing. Other forms of sealing are not sufficiently reliable, especially when the joints are subject to flexing as happens when self-boring is taking place.

2) Creating the pocket

  1. This description assumes the pressuremeter has already been self bored to some depth and the system has been flushed to remove cuttings.

  2. Although for research purposes pocket heights at or near zero are important, for site investigation purposes the longer pockets are likely to be of greater interest. It is likely that the first action will be to retract the pressuremeter.

  3. Before moving anything, record the initial stickup.

  4. All movements of the pressuremeter must be done slowly, with the casing pipe open to atmosphere and full of water to avoid suction.

  5. Record the new stick-up and calculate the length of the test pocket.

  6. The suspended probe must be secured so that it cannot move.

3) Bleeding the system prior to starting the test

  1. The casing pipe must be full of water. Initially water is poured down the casing until it appears full. Watch the level for a minute to check for air bubbles and to check that the level is not visibly dropping - this would imply a formation better tested by the falling head technique rather than constant flow. It may indicate significant leakage around the probe - see 4(d) below for a possible fix.

  2. A special LH thread Water Line Adapter is then fitted to the top of the casing. It has an automatic de-airing device (ADAD) on the top but at this stage it is left open. A water line plugs into the side of this and at the other end is connected to a header tank of water. If the reservoir head is small the tank is pressurised to drive water through the water line, filling the pipe and flowing out of the top of the adapter.

  3. Once water is flowing freely from the ADAD the top pressure on the header tank is removed. The observed flow should now be gentle and smooth, free of bubbles.

  4. Now bleed the rest of the system. The header tank is transferred to the flow pump input, and the water line to the flow pump output. The Flow Control Unit (FCU) is set to a fast rate (typically 100mL/Hr) and used to drive the syringes, forcing any air out of the system as water is pumped into the open water line. The syringes and transparent lines are checked for bubbles of air.

  5. When as much air as possible has been driven out, the flow rate is set to zero and data logging starts. The current state is used as a reference for the flow pump pressure cell. The ADAD is now capped, giving a sealed delivery system.

Side view of pump used in permeamter test

Side view of pump

4) Running the test

Front panel of flow control unit in permeameter test

Front panel of Flow Control Unit

  1. How the test is run depends on the number of geometries to be tested. At all times the pressure in the test area must be below the level required to induce hydro-fracture, remembering to add the standing head of water in the pipe to the pressure cell readings taken at ground level.

  2. Water forced into the formation for one geometry must not be allowed to influence the next, an issue in low permeability formations. The easiest way to arrange this is to start with the longest pocket and ensure that the maximum head applied to a particular geometry is less than that required for the first step in the next shortest geometry. Alternatively after testing a short pocket wait for at least as long as the test took before trying a longer pocket length.

  3. Although it is a constant flow test, in practice the head is known and the flow is discovered. Initially a fast flow rate is used to drive the pressure in the cavity to the intended point. The flow rate is then reduced in stages until a value is discovered at which the applied head remains constant for a period of time, typically 5 to 10 minutes. It is important to record and observe the temperature.

  4. Once the first plateau has been established, continue logging and check that inflating the SBP membrane has no effect. There is no need for a full inflation, 1% cavity strain is sufficient. If water in the test cavity has been leaking past the instrument shoe edge then using the membrane to seal the cavity will make the pressure in the test cavity rise. A lower flow rate will have to be discovered at which a steady state condition is once more achieved. If this does change the observed characteristics then the test pocket length is the distance from the bottom of the borehole to the start of the membrane, not the cutting shoe.

  5. Once more set a fast flow rate and drive the pressure in the test pocket to the next intended level. As before, hunt for the equilibrium flow rate and observe for 5 to 10 minutes.

  6. This process is repeated until four or more steady state readings are achieved. A plot of head and flow for all the steady states can be done while the test is underway and a check made that a linear relationship has been established.

  7. While the last steady state is still being logged, vent the pressure in the membrane and watch for an effect. If the ground has sealed to the instrument then there should be none.

  8. When the test is ended, remove the cap from the ADAD so that the delivery pipe is once more open to atmosphere and check that the pressure cell reading returns to its start value.

5) Resetting the system

  1. The probe may be pushed down to achieve a shorter test pocket.

  2. The delivery pipe must be open to atmosphere.

  3. Measure the stick-up to obtain the pocket length. Note that when pushing down to the zero length case it is critical not to overpush. For this reason it may be better to carry out a zero height test immediately after self-boring, noting that there must be a delay before testing longer pockets. See 4(b).

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