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Version 3.0
A high pressure, direct strain measuring borehole dilatometer inflated by
oil or gas pressure. Strain is determined across three diameters in the same
plane. The instrument is placed in NX size pockets formed by conventional
rotary coring.
A combined hose and cable assembly connects the instrument to a pressure source and readout unit. When oil is used a second hose is fitted to allow gas purging of the hydraulic fluid after completion of a test.
The output of the instrument is an RS232 signal which may be connected directly to the serial port of a computer. Software is supplied to log data on an IBM PC or compatible computer.
The instrument includes an electronic compass so that the orientation of the axes of measurement is known.
| PARAMETER | VALUE |
| Diameter | 73 mm |
| Length | 1 m 80 |
| Expanding length | 455 mm |
| Length to diameter ratio | 6.2:1 |
| Rated Pressure | 20 MPa/200 bars |
| Maximum pressure | 30 MPa/300 bars |
| Displacement Measuring | Six equi-spaced full bridge strain gauged cantilever springs in the same horizontal plane. The output is a function of the radius of the membrane. This allows the diameter of the borehole at three axes to be known. |
| Strain Range | Each displacement follower has a range which translates to a 30% diametric expansion of a nominal 76 mm borehole, a volumetric expansion of approximately 70%. |
| Pressure measuring system | Two downhole strain gauged diaphragm pressure cells |
| Inflation method | Hydraulic oil and hand pump or compressed gas and hand regulator |
| Electronics package | Plug-in. Contains microprocessor controlled multiplexer/ fluxgate magnetometer compass/l6 bit analogue to digital converter and RS232 transmitter. |
| Resolution | 1 kPa in pressure, 1 micron (equivalent to 2.1 x 10-5 in strain), Shear moduli up to 4 GPa measured. Orientation 5° in azimuth |
| Membrane | The standard membrane is a 5 mm thick nitrile rubber sleeve which is the pressure container. This in turn is protected by an 18 strip stainless steel sheath. The upper and lower ends of the rubber membrane are reinforced with a short cone of steel fingers to resist axial extrusion. In weaker materials the standard membrane can be replaced with a membrane whose ends are reinforced with strands of kevlar. This offers sufficient resistance to axial extrusion without the need for steel cones. |
| Membrane protection | l8 strip stainless steel sheath known as the Chinese Lantern |
| Coupling thread | EW pin thread to BS 4019:Part 1:1974 and DCDMA Standards |
| Connections to surface | 13 mm high pressure hose which contains an electrical cable. If gas inflation is used then the outer cover of the hose is pricked as a precaution against leakage. For oil inflation an optional 11 mm return hose is supplied for gas purge. |
| Output of the probe: | RS232 compatible, one scan of all sensors every 10 seconds |
| Electrical Interface Unit | The instrument connects to a small control box which provides power to the instrument and buffers the RS232 signal. Two isolated RS232 outputs are provided. To assist the operator to control the inflating pressure accurately the Unit monitors the output of a downhole pressure transducer and gives a continuous reading on a digital panel meter. |
| Hose length | 100 metres in one length is standard. Longer cable lengths can be supplied up to about 250 metres. For instruments intended for use on wire lines extension cables with couplers can be supplied. In this case the first coupler is usually at 2 metres (just above the instrument) and the extension cables may be any length up to 250 metres. |
| Power Requirements | The instrument and Interface Unit are powered from a single 12 volt lead acid vehicle battery which may also be used to power the computer either directly or via a socket on the Interface Unit. |
| Hand Pump | A hand operated oil pump is provided which can be used to inflate the instrument to a pressure of 20MPa. |
| Handling | The instrument is heavy and normally travels with a calibration cylinder over the membrane with the cable detached and inside a length of plastic pipe. For working on the instrument on site a frame with pipe vice is supplied in four parts. These assemble together to make a very rigid stand. |
| Handling Frame dimensions | 1.2 m long 0·85 m wide 0·90 m high when assembled |
| Weights | Downhole instrument 55kg including couplers and calibration cylinders. Plus 215 m Hose on drum and windlass 72 kg. 215m Oil return line on drum 45 kg. Hand hydraulic pump 19 kg |
| Software | A suite of programs is provided to log the data from the instrument and display the pressure versus displacement curve in real time. The program provides a text file of the instrument output converted to engineering units for analysis by common spreadsheet programs or the separate analysis program. |
| Recovery from dry hole | the hydraulic instrument can be purged of oil using gas pressure |
| Flushing | Holes are provided at the top of the instrument so that a flow of mud or air can be passed to remove any material fallen in on top. |
| Disconnection | If the instrument becomes stuck in the borehole then there is a left hand thread part way down the instrument which allows disconnection of the rod adapter and cable. Recovery by over coring is then possible. |