Patent Application
20080295589
The invention relates in general to logging techniques.
More precisely, the invention relates to a pressure meter intended to enable evaluation of a geotechnical property of the subsoil, this pressure meter comprising, as sub-assemblies, a bottom tool intended to be inserted into a borehole, surface equipment, and connecting means adapted to at least connect the tool to the equipment, these sub-assemblies themselves comprising at least a first inflatable sleeve held by the bottom tool, a substantially non-deformable reservoir containing complementary volumes of gas and liquid, a pressurized gas source, a first duct connecting the gas source to the gas volume of the reservoir, a second duct connecting the liquid volume of the reservoir to the first inflatable sleeve, flow control means interposed on the first duct, a pressure sensor adapted to provide a signal related to the pressure of the liquid inside the reservoir, and a volume sensor adapted to provide a signal related to the volume of the liquid inside the reservoir.
Devices of this type have been very well-known by those skilled in the art since the invention thereof by L. Ménard in 1955.
The pressure meter enables on-site evaluation of the mechanical parameters of soils.
Once the bottom tool or “probe” has been inserted into a borehole, each inflatable sleeve is subjected to increasing pressure, in increments numbering from six to fourteen, for example, and according to an arithmetic progression.
At each increment, the volume of liquid introduced into the first sleeve from the reservoir is measured, typically 15 seconds, 30 seconds and one minute after the end of pressurization.
The result of these measurements is translated by two loading graphs or pressure measurement curves, one of which provides the variation in volume, measured at one minute, in relation to pressure, and the other one of which corresponds to the variations in volume between 30 seconds and one minute, in relation to pressure.
Existing pressure meters suffer from several defects which very seriously limit the accuracy of the results provided thereby.
As a matter of fact, the purpose of the invention, which falls within this context, is to mitigate these imperfections and to propose a precision pressure meter.
To that end, the pressure meter of the invention, which is otherwise consistent with the generic definition thereof, as provided by the above preamble, is substantially characterised in that the reservoir is supported by the bottom tool, in that the flow control means include a nozzle and a valve, in that the surface equipment further includes pressure control means wherein are stored a plurality of pressure set points of increasing values, a program for successive application of these set points over time, and a law of correspondence associating at least these set points with corresponding respective time periods, and in that these pressure control means are designed to selectively open the valve for the purpose of applying each new pressure set point in accordance with the program, during the time period corresponding to this set point.
Owing to this arrangement, the pressure of the liquid is rigorously controlled, without any human intervention, and without being subject to the influence of the gas flow on the pressure measurement or that of the delay in establishing the pressure inside the reservoir.
The pressure control means are preferably connected to the pressure sensor and are further designed to open the valve during a predefined time period, in response to a deficit in the pressure signal in relation to a set point pressure, when this deficit appears during an increment of this set point pressure and when it exceeds a predetermined threshold.
The pressure control means, for example, include a control unit which actuates the valve and a computer wherein the pressure set points, program, and law of correspondence are stored, said computer being connected to the control unit and controlling same.
In one easy embodiment of the invention, the flow control means can include a solenoid valve which holds both the nozzle and the valve.
In order to further increase the precision of the pressure meter of the invention, it is possible to provide for the volume sensor to include a liquid level detector housed inside the reservoir, and for the connecting means to include a transmission link connecting the level detector to the surface equipment.
Owing to this arrangement, the volume is measured simply and reliably without the measurement obtained being disrupted by various artefacts, such as the weight of the liquid column between the surface and the bottom, the deformation of the liquid duct which, in existing pressure meters, generally connects the first sleeve to the reservoir arranged on the surface, or else the inertia which said duct opposes to the flow of liquid between the surface and the bottom.
In the operating configuration, of the bottom tool inside a borehole, the reservoir is advantageously arranged above the first inflatable sleeve, i.e., closer to the ground surface than said first sleeve.
The volume signal is preferably of the electrical type, the transmission link then comprising an electrical transmission line.
In the preferred embodiment of the invention, provisions are made for the liquid to have a relatively low electrical resistivity, for the level detector to include at least one resistive element which is connected to an electric power generator and partially submerged in the liquid, for the resistive element to have a shape that is elongated in the heightwise direction of the reservoir and a relatively high electrical resistivity, and for the liquid and the resistive element partially shunted by the liquid to form a resistive load for the generator, the resistance of which depends on the level of said liquid inside the reservoir.
In addition, the electric power generator preferably delivers an alternating current in order to prevent parasitic polarization.
In order to simplify measuring and the interpretation thereof, the reservoir can be of a cylindrical shape, the resistive element itself being capable of extending along the central axis of the reservoir.
As in the case of known pressure meters, the bottom tool, of the pressure meter of the invention can further include second and third inflatable sleeves and a third duct connecting the gas volume of the reservoir to said second and third sleeves.
Other features and advantages of the invention will become clear from the description thereof, provided, hereinbelow for non-limiting and illustrative purposes, with reference to the appended drawings, in which:
As stated previously, the invention relates to a pressure meter intended to enable evaluation of a geotechnical property of the subsoil.
A device such as this includes, as sub-assemblies, a bottom tool 1, which is intended to be inserted into a borehole F, surface equipment 2, and connecting means, such as 30 to 33, enabling, in particular, the tool 1 to be connected to the equipment 2.
The bottom tool 1 generally includes three inflatable sleeves, namely a main central sleeve 11, and two auxiliary sleeves 12 and 13, which are adjacent to the central sleeve 11 and situated on both sides thereof.
The main sleeve 11 consists substantially of an annular elastic membrane capable of being inflated via injection of a pressurized liquid L, e.g., water, coming from a reservoir 4 and conveyed via a duct 32.
The reservoir 4, for example, is made of a metal cylinder, which is substantially non-deformable at the pressures involved, and which, above the liquid L, contains a propellant gas such as pressurized nitrogen, the liquid and the gas occupying respective and complementary volumes VI and Vg of said reservoir 4.
The surface equipment 2 typically includes a gas source 21 which is adapted to deliver the pressurized gas G and which is connected to the gas volume Vg of the reservoir 4 via a supply duct 31.
The surface equipment 2 also includes flow control means, such as 220-222, which are interposed on duct 31 and which make it possible to control the flow of gas G from the source 21 to the reservoir 4, and therefore the flow of the liquid L from the reservoir 4 to the sleeve 11 via duct 32.
The pressure meter of the invention conventionally further includes a pressure sensor 5 and a volume sensor 6, the pressure sensor 5 being designed to provide a signal Sp related to the pressure of the liquid L inside the reservoir 4, and the volume sensor 6 being designed to provide a signal. Sv related to the volume V1 of the liquid inside the reservoir 4.
According to the invention, the reservoir 4 is supported by the bottom tool 1 and is arranged, for example, above the central inflatable sleeve 11 when the bottom tool 1 is in place inside a borehole F.
The surface equipment 2 further includes pressure control means, and flow control means comprising a nozzle 221 and a valve 222, e.g., built-in to a solenoid valve 220.
In the embodiment shown, the pressure control means include a control unit 7 adapted to actuate the valve 222, and a computer 8 connected to the control unit 7 and controlling same.
The computer 8 is equipped with a memory wherein are stored a plurality of pressure set points of increasing values Kpi, a program PROG for successive application of said set points Kpi over time, and a law of correspondence CORR which, at least on the basis of the set points Kpi, enables determination of the corresponding respective time periods Tpi.
The function of the program PROG is to determine at what moments the various pressure increments will need to be applied, and, as for the set points Kpi, they define the values of the various pressures that will need to be reached and maintained over the course of these various pressure increments.
The law of correspondence CORR is defined such that, at each new pressure increment, i.e., during the application of each new pressure set point Kpi in accordance with the program PROG, this pressure set point can be reached by opening the valve 222, via the pressure control means 7 and 8, during the time period Tpi corresponding to this set point Kpi.
In actual practice, the gas G flows in a laminar fashion as it is admitted into the reservoir G via the nozzle 221.
Thus, according to Poiseuille's Law, the mass of gas G admitted into the reservoir over a specific time period is represented substantially by a linear function of this time period, it being possible to further account for and correct the minor impact of the variations due to the difference between the pressures existing upstream, and downstream from the nozzle, owing to prior knowledge of the pressure upstream from the nozzle 221 and of the set point Kpi to be reached.
Alternatively, or cumulatively, the law of correspondence CORR can be determined or refined experimentally via pre-calibration.
Finally, the law CORR stored in the form of a mathematical relationship, or more simply in the form of one or more monograms.
Once a new pressure increment has been reached in the manner indicated, above, it is appropriate to maintain the pressure inside the reservoir A at the value of the set point pressure for this increment, until the moment when another pressure increment must be reached.
However, since over the duration, of one pressure increment, the sleeve 11 may push back the wall of the borehole F in a radial direction and thus increase in volume, the pressure inside the reservoir 4 may decrease during this time period and will therefore need to be compensated.
To accomplish this, the computer 8 receives the pressure signal Sp from the pressure sensor 5 to which it is connected and compares this continuously or periodically high-frequency pressure signal Sp to the set point pressure Kpi that must be maintained during the pressure increment in progress.
In the case where, in comparison with the set point pressure Kpi, the pressure signal Sp has a deficit greater than a predetermined tolerance limit, the computer 8 transmits to the control unit 7 the order to open the valve 222 for a predefined time period T0.
In actual, practice, the time period T0 is chosen so that the mass of gas passing through the nozzle 221 during this time period is at least slightly greater than the mass of gas which, in the worst case, would be required in order to compensate for the pressure deficit corresponding to the tolerance limit.
In the case of a high degree of desired precision with regard to maintaining the pressure during the course of an increment, i.e., in the case where the value assigned to the tolerance limit is low, the time period T0 will itself thus assume a low value, suitable compensation for the pressure deficit over the course of a pressure increment being achieved by automatic adaptation of the opening frequency for the valve 222, on the basis of the rate of the drop in pressure inside the reservoir 4.
Furthermore, the computer 8, which receives the volume signal Sv, also ensures the time-correlated recording of this signal and the pressure signal Sv, with a view to processing these signals at a later time.
In order to further increase the precision of the pressure meter of the Invention, it is possible to provide for the volume sensor 6 to include a liquid level, detector 61 housed, inside the reservoir 4.
A transmission link 30 is then provided in order to connect the level detector 61 to the surface equipment 2, this link, for example, consisting of an electrical transmission line, in the advantageous case where the volume signal. Sv is of an electrical type.
In one efficient embodiment of the invention, the level detector 61 is of the resistive type.
To accomplish this, the liquid L is chosen so as to have a relatively low electrical resistivity. For the liquid L, it is possible, in particular, to use water that has been ionized by the presence of impurities, salt or, yet more advantageously, antifreeze.
As concerns the level detector 61, for example, it consists of a resistive, element 610 and a pure conductor such as a copper bar, the resistive element and the conductor being connected to an electric power generator 60 and partially submerged in the liquid L of the reservoir 4.
The generator, for example, delivers an alternating current of constant amplitude and of a frequency equal to 270 Hz.
The resistive element 610 has a shape which is elongated in the heightwise direction of the reservoir 4 and, by definition, a relatively high electrical resistivity, i.e., at least one hundred times greater than that of the liquid L.
The resistive element 610, for example, is wound about the conducting bar 611 without being in direct galvanic contact with said bar.
Under these conditions, the resistive element 610, for example, and the conducting bar 611 are galvanicaliy connected to one another by the liquid L, in the immediate vicinity of the level of said liquid inside the reservoir, the resistive element 610 being shunted by the liquid over the entire submerged length thereof.
In other words, the liquid L, the resistive element 610, and, accessorily, the conducting bar 611, form a resistive load CR for the generator, the electrical resistance of which depends on the level of said liquid inside the reservoir 4, and therefore on the volume of liquid L inside said reservoir.
The volume signal Sv can thus be represented by the output signal of a phase-detection voltmeter 62 installed in parallel on the current generator 60.
Alternatively, the wall of the reservoir 4, also assumed to be conductive, can be used in place of the conducting bar 611 in order to close the current loop.
Level detectors of this type, for example, are described in the patent U.S. Pat. No. 4,188,826.
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As described, the invention thus also includes all of the steps for implementing the pressure meter as just described.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0512771 | Dec 2005 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2006/002754 | 12/15/2006 | WO | 00 | 6/16/2008 |
