GB2218526A - Method for orientating rock samples - Google Patents

Abstract

A method for orientating a sample of rock, suitably a sample of a bore hole core, comprises successively heating to progressively higher temperatures and cooling the sample after each heating in a number of steps in a zero magnetic field, determining the magnetisation of the sample in each step after cooling, plotting the magnetisation on a vector diagram, determining the discontininuity in the vector which represents the removal of the viscous magnetisation and determining the bearing of the viscous magnetisation with respect to the rock sample and therefore the in situ orientation of the sample.

Description

METHOD FOR ORIENTATING ROCK SAMPLES This invention relates to a method for orientating rock samples, particularly bore hole cores.

While drilling a well it is often necessary to take a core sample, particularly in the reservoir rock of the pay zone. A core is an undisturbed sample of the formation required by the geologist and palaeontologist for correlation and identification purposes, and by the reservoir engineer for permeability and porosity studies and for oil and water saturation measurements. Cores are usually taken on exploration wells and early development wells.

Fragments which reach the surface in drilling mud are incomplete and difficult to interpret. For example, clay in the rock tends to become part of the mud and be lost, while fragments of different size, shape and density travel upwards with the mud at different speeds and so arrive at the surface at different times.

For this reason, special coring equipment has been designed for taking non-orientated, or sometimes orientated, cores.

Orientated coring has recently become of interest because of the need to obtain further information about reservoir properties. An orientated core can provide information about fracture orientation, permeability and porosity in specific directions and bedding plane orientation. This information enables the results of possible treatments to be predicted more accurately.

A barrel used to cut an orientated core is slightly different from a conventional barrel in that the core catcher has three longitudinal scribing blades on the inside. These are placed at non-equal angles around the perimeter such that when the core is recovered, it should not be possible to confuse the scribe lines and hence the orientation.

However, difficulties have been experienced in practise with scribing blades which sometimes fail to engage properly and sometimes become damaged during operations resulting in jamming or slipping of the cores and meaningless markings such as spirals.

We have now devised an improved method for orientating bore hole cores by determining the magnetism of a sample of the core. This method does not rely on scribing and can be used to orientate cores cut with conventional equipment.

Rocks are generally magnetised in the direction of the ambient earth magnetic field at the time of formation of the rock. Such rocks are said to have acquired a natural remanent magnetism (NRM). This magnetism is often very stable and subsequent exposure of the rocks to different directions and strengths of magnetic field may not appreciably change the original magnetisation, although subsequent magnetisation can be added to it, which obscures the original.

A significant type of subsequent magnetism is termed viscous remanent magnestism (VRM) which can be acquired at low temperature in a weak magnetic field over an extended period of time. At higher temperatures such as those encountered by rocks at considerable depths (e.g. 1000C at 3,000 m) the extent of VRM is considerably increased.

The total magnetisation of a rock is due to the sum of all types of magnetisation of which NRM and VRM are the most important.

The overall strength and direction of the earth's magnetic field does not remain constant. Measurements suggest that the strength has varied from about 150Z to 25Z of its present value. The field has also reversed in polarity on several occasions. It is believed that the last field reversal occurred about 0.7 million years ago. Thus for the past 0.7 million years, the earth's magnetic field has been "normal", the north magnetic pole being close to the north geographic pole. This means that the viscous magnetisation direction is approximately due north with an inclination defined by tan I = 2 tan L, where I = inclination and L = latitude. For the North Sea region the average value of I is expected to be about 69.

However, because the process of viscous magnetisation is not linear with time the direction may be heavily biassed by the most recent past. The changes in the direction of magnetisation for Northern Europe for the past 1,000 years are well documented.

A further factor in evaluating the significance of the magnetism of rocks is the question of whether the rock formation has been reorientated since it received its paleomagnetism, for example the extent of movement from the latitude in which it was formed. It is believed that the oil and gas bearing sandstone-and limestone strata in the North Sea Region, now at latitudes in the range 500 - 650N were formed in Equatorial regions. Thus the NRM vector of these rocks is approximately horizontal, the VRM vector is inclined and the total magnetisation vector is offset.

It is known that VRM (and indeed NRM) can be destroyed by thermally treating samples but, unless stringent precautions are taken, such samples quickly acquire a new magnetisation on cooling.

According to the present invention there is provided a method for orientating a sample of a rock, suitably a sample of a bore hole core, which method comprises successively heating to progressively higher temperatures and cooling the sample after each heating in a number of steps in a zero magnetic field, determining the magnetisation of the sample in each step after cooling, plotting the magnetisation on a vector diagram, determining the discontinuity in the vector which represents the removal of the viscous magnetisation and determining the bearing of the viscous magnetisation with respect to the rock sample and therefore the orientation of the sample.

The magnetic determinations after cooling are usually made at room temperature. The sample may be heated in steps up to 300 C or more, usually in steps of 10-50 C, often 200C steps are used.

Suitable equipment for magnetic measurements is described by Shaw et al, "An Automated Superconducting Magnetometer and Demagnetising System", Geophys. J.R. Astr. Soc. (1984), 78, 209-217.

As a check, the inclination of the vector representing the viscous component of the magnetism should correspond approximately to the latitude from which the sample of the rock was taken.

The invention is illustrated with reference to the following Example.

Example A sample from a bore hole core was orientated using the procedure previously described.

The sample was heated to a demagnetisation temperature of 40C and cooled back to room temperature in a zero magnetic field and the magnetism determined at room temperature. This procedure was then repeated heating the sample to 60-C and cooling back, and so on raising the demagnetisation temperature in each successive step by 20-C until the final temperature of 300C was reached.

The following results were obtained and are shown in tabular form below and also in the form of the accompanying vector diagram for values of 180C and above.

Demannetisation Int(l0'6A/m) Bearing Inclination Temp C 40.0 364.4 193.9 64.3 60.0 362.8 193.5 64.3 80.0 361.2 193.5 64.2 100.0 357.6 192.7 63.9 120.0 346.8 190.3 63.5 140.0 323.6 188.3 62.6 160.0 297.6 186.6 61.7 180.0 272.0 184.1 60.9 200.0 250.0 182.3 60.2 220.0 236.8 182.3 59.7 240.0 207.2 180.6 59.0 260.0 185.6 179.9 58.5 280.0 164.0 179.9 58.2 300.0 140.0 179.1 57.5 The diagram shows the projection of the magnetic vector in the horizontal plane (xy) and a vertical plane (xz). The crosses show the projection in the horizontal plane and this allows the angle between the magnetic vector and the x direction to be calculated which corresponds to a mark on the core. In the example this is 190.51".

Because the viscous magnetic vector was isolated then it is known that it points towards true north so we can say that the "x" mark on the core is in a direction 190 E of N. As a check of the validity of the interpretation the inclination of the magnetic vector can be determined from the projection in the vertical plane (circles). In the example shown this angle is 64.02- which should correspond to the present day inclination of the geomagnetic field at the borehole.

Claims (6)

Claims:

1. A method for orientating a sample of rock, which method comprises successively heating to progressively higher temperatures and cooling the sample after each heating in a number of steps in a zero magnetic field, determining the magnetisation of the sample in each step after cooling, plotting the magnetisation on a vector diagram, determining the discontinuity in the vector which represents the removal of the viscous magnetisation with respect to the rock sample, and therefore the orientation of the sample.

2. A method according to claim 1 wherein the sample of rock is a borehole core.

3. A method according to either of the preceding claims wherein the magnetic determinations after cooling are made at room temperature.

4. A method according to any of the preceding claims wherein the sample is heated in steps ranging between 10 and 50"C.

5. A method according to claim 4 wherein the sample is heated in steps of 20"C.

6. A method as hereinbefore described with reference to the Example.