1 COMPUTER IMPLEMENTED METHOD FOR USING 2D ORTHOGONAL CT CORE SCANS TO GENERATE CORE GONIOMETRIC DATA. BACKGROUND OF THE INVENTION [0001] The recording and measurement of the orientation of geological features / surfaces visible on the external surface of a core cut in an oil well or mineral boring is termed core goniometry. The data generated are here termed "geological feature dips", and might represent "bedding" or "tectonic features", the true orientation of which can be determined if the core can itself be oriented. Understanding the orientation of geological features of this type is particularly important for geologists in the resource industry sector. Examples of how these data are used include, aiding understanding of the internal geometry of petroleum reservoirs, and how geological bedding fabrics might impact fluid flow within them; using geological bedding fabrics to help understand the depositional processes that impact upon petroleum reservoir distribution; or understanding the orientation of fracture systems which might aid or impede fluid flow in petroleum reservoirs, or host mineralisation in mineral deposits. Core orientation can be achieved by either: [0002] Orienting the core and hence goniometer data gathered from it by using scribe lines of known orientation which were scored along the core during its acquisition; or, [0003] By referencing the derived goniometer data i.e. bedding and feature dips gathered from the core; to data from the same features which has been gathered an independent source, e.g. a borehole image log, the precise orientation of which is known. [0004] Existing goniometry methods thus use data from the circumference of the cylindrical core (e.g. circumferential core scans, or features traced from the circumference of the core) in order generate goniometry data sets. When cores are sampled in oilwells, it is desirable that they are cut or "slabbed" along their long axis in a direction that parallels the dip direction of the geological features or "bedding" within the core. This provides the optimal oriented cross section in which geologists working with the cores can view the relationships between the different geological features present. Since the cores recovered from an 2 oilwell are typically recovered encased within a protective liner of aluminium or some other material, examination of the core in order to identify the optimal line along which to cut it is not always possible. In this case it is standard practice to subject the core to a CT scan whilst it is in the protective aluminium liner in order to understand the orientation of features within the hidden core. When this is done, it is routine practice for the core analysis companies undertaking the core preparation and CT scanning to provide the CT scan data for each section of core to their clients as two orthogonal CT scan sections through the core axis (Figure 1). Geological features (e.g. bedding) will be evident as inclined to flat, planar contrasts representing surfaces of different orientation in the orthogonal CT scans, e.g. as illustrated by the lines representing a geological feature defined by points 1 and 2, and points 3 and 4, shown at "c" and "d" on Figure 1. [0005] Geological features which are planar at the scale of their sampling by a wellbore or core (e.g. such as bedding or tectonic fractures) will have a circular to ellipsoid intersection with the borehole margin depending upon their inclination relative to its axis, and these inclined features are represented by a sinusoidal trace if the cylindrical borehole is unwrapped and displayed as a 2D map. In standard borehole images or core images, this trace is used to define the dip of the geological feature, typically by manual "picking' of the feature by a geologist, or by use of automatic computation algorithms. Techniques for determining the orientation of dipping geological features within oriented borehole images of wellbore walls have long been established, and utilise trigonometric analysis of the position of geological features on the borehole wall, together with borehole deviation azimuth and direction in order to determine the true orientation of geological features within the borehole images. This feature orientation can be done by an interpreter manually picking dips in a computer environment, as has been long established e.g. R.A. Plumb, S.M. Luthi "Analysis of Borehole Images and Their Application to geological Modeling of an Aeolian reservoir" SPE Formation Evaluation Symposium, December 1989. Pp 505-514. Alternately, feature measurement and orientation can be accomplished using advanced automated computing methodologies. E.g. as described in US patent No. 5162994 A and 5960371. A description of the trigonometric relationships encountered when a dipping geological feature or event penetrating a cylindrical borehole is projected onto a 2D surface is provided in US Patent 5162994 A.
3 [0006] The following invention describes a new and innovative computer assisted technique to generate core goniometry data using 2D orthogonal CT scans of cores, which are routinely provided to oil companies as large format digital images. The invention describes how, by placing 2D CT core scans within an orthogonal "pseudo-reference" system, they can be used in a similar manner as borehole images to generate a goniometric data set for geological features, which can then either be used to investigate and interpret the relative orientations of the geological features ; or which can subsequently be oriented using additional data to provide a data set with correct true geographical referencing. [0007] The invention is a process whereby : [0008] Two orthogonal CT scans of core which are routinely gathered in the petroleum industry, are loaded into an interpretation environment (represented by "a" on Figure 1), where they are displayed depth matched next to one another, and they are designated arbitrary North-South and East West orientations (as illustrated at "b" on Figure 1). Top and bottom depths of the core CT scans are recorded into the system. NOTE: Adjacent CT scans (scans are typically 0.3m, 0.5m or 1m in length) which obviously represent continuous sections of core which "match" in that they show orthogonal CT scans with features in exactly the same orientations matching across the break between the core sections, could be spliced into continuous scan sections at this stage, but this is not essential. [0009] An interpreter then picks 4 depth points upon the same geological feature seen across the two CT scan images. These points are the left and right depth of the feature in the "North-South" CT scan image (points 1 and 2 shown at "c" in Figure 1) , and the left and right depth of the same feature in in the "East -West" CT scan image (Points 3 and 4 shown at "d" in Figure 1). [0010] The North, South, East and West depth points picked are then displayed in a sinusoid fitting track (shown at "e" in Figure 1), and used to generate a goniometry dip, which is an apparent dip within the arbitrary North-South, East-West reference system. The goniometry dip is thus a pseudo dip which is not correctly oriented in space, but the 4 computation of the goniometry dip does take into account the wellbore deviation and azimuth. The goniometry dip is calculated automatically by computer software using the four input depth points (North, South, East and West), which define a sinusoid as shown by the points numbered 1 to 4 in "e" on Figure 1. Resultant goniometry dips are recorded as text file information, and are also displayed as standard "tadpole type" displays (shown in "f" on Figure 1) in which the head of the tadpole records the goniometry dip magnitude and the tail of the tadpole the goniometry dip azimuth, or as depth based displays of dip and azimuth. [0011] The goniometry dips are re-oriented into their true orientations by using a borehole image log to identify a "calibration dip" i.e. a feature of known orientation evident in both the orthogonal CT scans and an independent oriented borehole image log. The goniometry dip of the calibration feature (represented by G1 shown in "f" and "g" on Figure 1) is simply rotated using stereographic projection (e.g. using Task Geomodelling attitude software) to match the known orientation of the calibration feature as determined from a borehole image log (generating the corrected dip C1 shown in "g" on Figure 1). During rotation of the calibration goniometry dip (G1) to match its known true orientation (Cl), exactly the same rotation is applied to all the other goniometry dips for this continuous CT Scan core section, thus also correctly orienting them. [0012] Since core CT scans will record many more geological surfaces "features" than a borehole image, the technique has the benefit of providing an increased volume of oriented information for bedding fabrics within cored reservoir sections. This is particularly true where borehole images are acquired in electrically non-conductive oil based mud systems, where bedding resolution is typically reduced in the borehole images compared to images acquired in conductive mud systems. [0013] Glossary of key terms: [0014] Core. Cylindrical sample of rock cut from an oil well or mineral borehole.
5 [0015] CT scan. Computer Tomography Scan. In this context, a 2-dimensional x-ray image of a slice or section through a 3-dimensional core recovered from an oilfield. [0016] Goniometry. The measurement of an angular relationship. In this case, it specifically refers to the orientation of geological features in a core.