A measuring tool for measuring a physical property of an earth formation in an oil borehole is known per se, for example a measurement-while-drilling (MWD) tool, wherein the drill that is used for forming the borehole is fitted with a sensor. Another manner of collecting formation data is to lower the measuring tool into the borehole after the hole has been drilled and remove the tool before the casing and filling substance is put into place. There is also a need, however, for collecting formation data, such as the fluid pressure in the pores of the rock that makes up the earth formation, after the casing through which the oil is transported to the surface has been placed in the borehole.

Consequently, it is an object of the invention to provide a measuring tool by means of which measurements can be carried out on the earth formation at a large depth while oil is being transported to the surface through the casing, which tool can be placed into the borehole without difficulty and which can also be used in cases where the borehole wall is very irregular in shape and/or in cases where the borehole exhibits local variations in diameter.

In order to accomplish that objective, the measuring tool comprises a holder which can be attached to the outside of the casing, in which holder a sensor is

capable of measuring said physical property is mounted, and the measuring tool furthermore comprises contacting means which are capable of bringing the sensor into permanent direct contact, fluid contact or gas contact with the earth formation, irrespective of the local diameter of the borehole within certain bounds, in such a manner that the sensor will be capable of measuring said physical property in the earth formation. In this manner it is easy to lower the measuring tool to a great depth by mounting it on the casing, without having to take into account the local diameter of the borehole, as long as said diameter remains within predetermined bounds.

The holder that contains the sensor is preferably substantially annular in shape, and it is fitted around the casing.

In a first preferred embodiment, said sensor is rigidly mounted in the holder, and said contacting means are capable of creating a space in said filling substance between the sensor and the earth formation, which space is at least partially filled with a gas or a fluid.

Generally, said space will automatically fill with fluids from the earth formation, at a fluid pressure which equals the pore pressure in the earth formation. In this manner it is possible to make an accurate measurement of said pressure.

In a first preferred variant of the first preferred embodiment, said contacting means comprise an explosive material, which can be fired from the surface after the filling substance has cured. The explosion will create the desired space between the measuring tool and the earth formation.

In order to prevent damage to the sensor, said sensor is preferably disposed substantially on a side of the casing remote from the explosive material, wherein the holder furthermore comprises a channel which is positioned between said sensor and said explosive material. Also preferably, the sensor is screened by

protective means during the explosion of the explosive material, which protective means can be removed by remote control from the surface after the explosion of the explosive material. In a first preferred variant, said protective means comprise a porous material which is permeable to gases and/or fluids, but which is capable of stopping the shockwave of the explosion at least partially. In a second preferred variant, said protective means comprise a bimetal or a shape-memorizing alloy. In this way the firing of the explosive material and/or the removal of the protective means can take place by introducing heat into the casing.

In a second preferred variant of the first preferred embodiment, said contacting means comprise a projecting part of a hard porous material, which can be positioned between the sensor and the earth formation, for example by rotating the casing until said projecting part abuts against the borehole wall, thus effecting a fluid-permeable connection between the measuring tool and the earth formation.

In a third preferred variant of the first preferred embodiment, said contacting means comprise a flexible holder, which is filled with a fluid, in which or on which holder the sensor is mounted. Said fluid-filled flexible holder can be positioned between the casing and the borehole wall, thus effecting the desired fluid connection between the sensor and the earth formation.

Preferably, said fluid is a low-viscosity substance, and the flexible holder is provided with small holes, through which said substance can leak out slowly. In this way the fluid in the holder is prevented from being subjected to pressure, which would result in the pressure sensor arriving at an incorrect measured value.

In a second preferred embodiment, said sensor is flexibly mounted in the holder, and said contacting means bring the sensor into direct contact with the earth formation through the filling substance. The direct

contact between the sensor and the earth formation obviates the need to provide a gas connection or a fluid connection. Preferably, the contacting means comprise a bimetal or a shape-memorizing alloy in that case, which makes it possible to move said contacting means by introducing heat into the casing.

The invention furthermore relates to a method for measuring a physical property, such as the pressure, in an earth formation, wherein a borehole whose diameter may or may not be variable is formed in an earth formation, into which borehole a casing is placed, through which oil and/or gas may be transported to the surface, wherein the space between the casing and the borehole wall is at least partially filled with a curable filling substance, such as cement, wherein a measuring tool is attached to the outside of the casing, which measuring tool comprises a holder, in which holder a sensor capable of measuring said physical property is mounted, wherein contacting means bring said sensor into permanent direct contact, fluid-contact or gas contact with the earth formation, irrespective of the local diameter of the borehole within certain bounds, in such a manner that the sensor will be capable of measuring said physical property in the earth formation.

The invention will now be explained in more detail by means of a number of exemplary embodiments as shown in the figures, wherein like parts are indicated by like numerals, and wherein: Figure 1 is a vertical sectional view of a casing and a first embodiment of a measuring tool; Figure 2 is a horizontal sectional view of the casing and the measuring tool of Figure 1 ; Figure 3 is a horizontal sectional view of a casing and a second embodiment of a measuring tool; Figure 4 is a horizontal sectional view of a casing and a third embodiment of a measuring tool ;

Figure 5 is a horizontal sectional view of a casing and a fourth embodiment of a measuring tool ; and Figure 6 is a vertical sectional view of a casing and a fifth embodiment of a measuring tool.

Figure 1 shows a borehole 1 that has been formed in an earth formation 2 for the purpose of extracting oil that is present at a great depth in the ground. As is shown in Figure 1, wall 3 of borehole 1 may be irregular in shape as a result of, among other things, erosion caused by fluid. In order to transport oil from said earth formation to the surface, a metal casing 4 is placed into borehole 1. The external diameter of said casing 4 is smaller than the smallest diameter of the borehole, so that a space surrounds casing 4. In order to fix casing 4 in position, said space is filled with a curing substance 5, such as cement. In practice, a layer of liquid mud 6 will form between wall 3 of borehole 1 and the outside of the cured cement 5.

In order to be able to carry out fluid pressure measurements in the pores of the earth formation 2, an annular holder 7, in which a pressure sensor 8 is present, is introduced into the borehole 1 and fixed in position therein at the level where the measurements are to be carried out, which is done before the casing 4 is placed into the borehole. The pressure sensor 8 is mounted in the holder 7 in such a manner that sensor 8 is adequately protected against harmful influences from outside, in particular against the force of an explosion, as will be described hereafter. Electrical conductors 26 are used for transmitting the data measured by sensor 8 to the surface, where the data are registered.

In order to carry out said measurements on the earth formation 2, a free path, in which liquids and/or gasses can move freely, must be constructed between the sensor 8 and the earth formation 2. To penetrate the cement 5 and the mud invaded zone 6 a heat sensitive shaped explosive charge 9 is used, which is put in a hole 10 of the

holder 7, which hole 10 is located opposite the sensor 8.

At the back of the shaped charge 9 a circular channel 11 is made to provide an open space between the hole 10 and the sensor 8. To protect the sensor 8 from the shockwave during firing of the shaped charge 6, sealing pins 12 of a shape memorizing alloy are placed in the holder 7 such that the pins 12 will stop the shockwave travelling through the channel 11. The sealing pins are attached to the holder 7 in such a way that the part of the pin 12 that blocks the channel 11 can be removed by shortening the pins 12. For this purpose the pins 12 are tightly fit in the holder 7 at one side and at the other end the pins are less tightly fit in the channel 11, while around the central part of the pins a hole 13 with a larger diameter than the pins 7 is provided.

Once the casing 4 and the attached holder 7 are put into place, and the cement 5 has cured, the shaped charge 9 is fired by heating the inside of the casing 4, for instance by filling the casing 4 with a heated liquid, or by applying heat locally with a heating element.

Alternatively, the shaped charge 9 may be triggered by any other trigger mechanism, such as a time clock or a remotely controlled unit, which is activated by electromagnetic, acoustic or electric signals. A suitable trigger mechanism may comprise a piezo-electric element which is activated by a hammer which bounces against the inner surface of the casing 4. The explosion caused by the charge 9 forms a hole 14 in the cured cement and in the wall 3 of the earth formation 2. After the explosion unseating, by shortening, of the sealing pins 12 is also achieved by applying heat to the inside of the casing 4, whereafter a free path between the sensor 8 and the earth formation 2 is achieved. It will be clear to the skilled person that the distance between the holder 7 and the wall 3 of the borehole 1 that can be leared this way may vary between a minimum and a maximum value, depending amongst other things on the power of the charge 9 and the

characteristics of the cement 5, as well as the local pressure within the cement 5. The hole 14 as well as the channel 11 will be filled with mud and/or other fluids, thus allowing the sensor 8 to measure the local pressure in the earth formation 2 through said fluid path.

Figure 3 shows the same arrangement as figures 1 and 2, however the shaped memorizing alloy sealing pins 12 have been replaced by sintered steel bars 15. These bars 15 are porous and will provide a connection between the sensor 8 and the earth formation 2. They will also damp the shockwave travelling through the channel 11 which provides the connection between the earth formation 2 and the sensor 8.

An alternative embodiment as shown in figure 4 is the use of a sharp edged holder 7 around the casing 4 with the sensor 8 and the channel 11 built in. The sharp edged tip 16 of the holder 7 consists of porous sintered steel, and is in open connection with the channel 11 through a hole 17. The required free path between the sensor 8 and the earth formation 2 can be achieved by rotating the casing 4 until the tip 16 is in firm contact with the wall 3 of the borehole 1. The minimum and the maximum distance of the porous connection between the casing 4 and the wall 3 is determined by the shape and dimensions of the tip 16.

A further alternative embodiment is shown in figure 5. A circular flexible plastic container 18 is mounted on the casing 4 and contains a fluid 19 having a low viscosity such as grease, and furhtermore contains a sensor 8. The container 18 will be pressurized due to the fact that the hydrostatic head is present in the borehole 1, having a maximum value of the depth times the cement fluid gradient. This gradient is typically higher than the geophysical gradient in the earth formation 2.

To enable pressure release in the flexible container 18 fine perforations 20 are made allowing grease 19 to exit the container 18 therewith adjusting to the geophysical

gradient available in the earth formation 2. In the situation as shown in figure 5, the container 18 does not reach the wall 3 of te borehole 1, therefore a down hole measurement should be carried out at the area where the sensor 8 will be placed in order to find a spot where the diameter of the hole 1 is small enough for the container 18 to rest against the wall 3. Thereby the required fluid connection between the sensor 8 and the earth formation 2 will be established, having a minimum and a maximum distance depending on the dimensions of the container 19.

Figure 6 shows a further embodiment. The sensor 8 is now mounted on a spring bow 21, which is being activated using a shape memorizing alloy rod 22. The rods 22 have a contraction of approximately 3% of their length. Pending the required shortening, the length of the rods 22 can be calculated. The rods 22 are being activated to shorten by applying heat to the casing 4. When shortening the spring bow 21 travels outwards till the sensor 8 reaches the wall 3. This action can take place at any moment prior to curing of the cement 8. The spring bow 21, the rod 22 and the electric wire are at one end mounted on a mounting ring 23 which is affixed to the casing 4. At the other end the spring bow 21, the rod and the wire 25 are being held together by a movable clamp 24.