CA2383379A1 - Determination of the orientation of geological beds from stratigraphic data in deviated wells - Google Patents

Abstract

A method is presented to determine the probable orientation of a geological bed near a drilling during drilling of a wellbore. The method requires the obtaining of all possible orientations of the geological bed penetrated by the wellbore, and all possible bedding orientations that are parallel to a fold axis or a local structural trend of the geological bed. These orientations are then compared to arrive at no more than two choices for the true orientation of the geological bed. Stereonet presentations of these orientations may be employed where superimposing the stereonet presentations provides at most two points of intersection representing the poles for no more than two choices of the true orientation of the geological bed.

Description

s.
TEM Docket no. 503.1 TITLE: DETERMINATION OF THE ORIENTATION OF GEOLOGICAL BEDS
FROM STRATIGRAPHIC DATA IN DEVIATED WELLS
s FIELD OF THE INVENTION
The present invention relates to directional drilling of wellbores in the oil and gas industry, and in particular to a method of determining the probable orientation of geological strata near a drillbit during drilling in order to ascertain the structural position so of the wellbore.
BACKGROUND OF THE INVENTION
Current drilling practices in the oil and gas industry often involve the controlled steering of a drillbit in a direction that is strongly deviated from the vertical, in some 15 cases toward a sub-horizontal direction. This deviated or horizontal drilling is generally employed with the intent of positioning a wellbore within certain desirable geological strata (i.e. "beds") or features. To optimise this process, a knowledge of the position of the drillbit relative to these geological features during the drilling operation is fundamental. Additionally, a knowledge of the orientation of the geological bedding at 2o the position of the drillbit is required to determine the preferred direction of continued drilling.
Prior art methods, as described in US Patent 5,311,951 (Kyte et al.) for instance, have proposed navigation of a borehole by comparing characterising information from nearby boreholes to information from the borehole being drilled. However, these prior art methods suffer from an important shortcoming in that none are able to determine the orientation of bedding in the target formation.
What is therefore desired is a novel method which overcomes the limitations and s disadvantages of the existing approaches. Preferably, it should provide a determination of the probable orientation of the geological bedding near a drillbit during the operation of drilling of deviated to horizontal oil and gas wells. This determination should require as input: (1) true stratigraphic thickness of the formation of interest from nearby reference wells and (2) an estimation of the local structural trend and plunge.
SUMMARY OF THE INVENTION
In one aspect the invention provides a method of determining the probable orientation of geological strata near a drillbit during drilling of a wellbore comprising:
obtaining all possible orientations of the geological strata penetrated by the wellbore;
is obtaining all possible bedding orientations that are parallel to a fold axis or a local structural trend of the geological strata; and, comparing the orientations of steps a) and b) to arrive at no more than two choices for the true orientation of the geological strata.
In another aspect the invention provides a method of determining the true orientation of a geological bed at a wellbore comprising:
obtaining a first stereonet presentation showing at least one two small circle segment representing all possible bedding orientations relative to the wellbore;

-2-obtaining a second stereonet presentation showing a great circle representing all possible bedding orientations that are parallel to a fold axis or a local structural trend; and, superimposing the stereonet presentations of steps a) and b) to obtain no more than two points of intersection between thye at least one small circle segment and the great circle representing the poles for no more than two choices for the true orientation of the geological bed.
In yet another aspect the invention provides a method of determining the orientation of a geological bed at a wellbore comprising:
obtaining a graphical presentation showing at least one first graphic representing possible to bedding orientations relative to the wellbore;
superimposing on the graphical presentation a second graphic representing possible bedding orientations that are parallel to a fold axis or a local structural trend; and, obtaining at least one point of intersection between the at least one first graphic and the second graphic indicating a true orientation of the geological bed.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:
Figure 1 (a) illustrates the inclination (O) of a geological bed relative to a 2o wellbore, which is calculated trigonometrically from a determination of the apparent (Ta) and true (Tt) stratigraphic distances between two points in the wellbore;
Figure 1 (b) illustrates the pole to the bedding surface. which makes an angle ~
with the wellbore as shown in Fig. 1 (a);

-3-Figure 1(c) illustrates how the azimuth (a) and plunge (r~) of the wellbore are measured;
In Figure 2 the poles to the family of planes identified in equation [1] form a right conical surface that is symmetric about the wellbore, with a half apical angle ~, defined s by eqn. [2];
Figure 3(a) illustrates a cylindrically folded surface wherein all parts of the folded surface are parallel to the fold axis;
In Figure 3(b) the pole to any part of the bedding surface lies in or is parallel to the plane that is perpendicular to the fold axis;
1o Figure 3(c) illustrates how the bearing (8) and plunge (~) of the fold axis are measured;
Figure 4(a) shows the poles to the two planes that satisfy eqn. [ 1 ] and which are also aligned with the local structural trend or fold axis wherein the two poles are the only lines that are common to the cone of Fig. 2 and the plane of Fig. 3(b);
is Figure 4(b) shows the bedding surface orientation that is represented by the pole SOa shown in Fig. 4(a);
Figure 4(c) shows the bedding surface orientation that is represented by the pole SOb shown in Fig. 4(a);
Figure 4(d) illustrates how the dip azimuth (a) and dip magnitude (8) of a plane 2o are measured;
In Figure 5(a) a plane and a line passes through the centre of a sphere wherein the plane intersects the surface of the sphere in a "great circle," and the line intersects the surface of the sphere at two points;

-4-In Figure 5(b) the same plane and line as shown in Fig. 5(a) intersect a hemispherical surface in a half circle and a single point, respectively;
Figure 5(c) illustrates the stereonet projection representation of Fig. 5(b) wherein the stereonet projection is the view of the lines and points of intersection as seen looking down into the lower hemisphere "bowl" of Fig. 5(b);
In Figure 6(a) two right circular cones, symmetric about the centre of the sphere, intersect the surface of the sphere in "small circles" wherein the conical surfaces contain all the possible bedding poles that make the angle ~ with the wellbore of Fig.
2;
Figure 6(b) illustrates haw the "small circles" appear on the stereonet projection;
1o In Figure 7 the plane that is perpendicular to the local fold axis or structural trend of Fig. 3(a) forms a "great circle" on the stereonet projection;
In Figure 8 the two bedding poles which represent surfaces that intersect the wellbore at the angle O of Fig. 1(a), and which are also parallel to the local fold axis or structural trend of Fig. 3(a), are the two points of intersection between the great circle of is Fig. 7 and the small circles of Fig. 6(b);
Figure 9(a) illustrates three rock units (beds) within a fold (hangingwall anticline) that is expected to contain hydrocarbons wherein the target zone for a well being drilled into this feature is in unit 3 near the crest of the fold;
Figures 10(a), 10(b) and 10(c) illustrate the wellbore path and the calculated 2o bedding orientation (viewed parallel to the fold axis) as the wellbore penetrates the top surface of units 1, 2 and 3, respectively; and, In Figure 10(d) the actual path of the wellbore within the fold is shown.

-5-v, DESCRIPTION OF PREFERRED EMBODIMENT
The steps in the method of the present invention are set out in greater detail below.
Correct True Stratigrraphic Position and Depth Determination s Currently the stratigraphic position of a wellbore is conventionally determined using petrophysical logs. The most common log used for stratigraphic correlation is likely the gamma ray log. This type of log may be obtained after a drilling operation has stopped, or during the drilling operation using MWD ("measurement while drilling") technology. The specific stratigraphic position of any point in the wellbore is determined 1o by correlating the signal of the gamma ray log in the wellbore of interest with a reference log. Such correlations are conventional approaches in the oil and gas industry.
In the procedures of the present invention, it is important ( 1 ) to determine the true stratigraphic position of several points along the wellbore very accurately, and (2) to know the exact stratigraphic thickness between these identified points. It is therefore 1s necessary to have a reference log that has been fully adjusted to reflect accurate stratigraphic thickness. Currently there is commercially available software to allow these adjustments to be made using dip information from one or more reference wells.
Calculation of bedding dip relative to the wellbore Sedimentary rocks are generally deposited as approximately horizontal layers.
2o These planar units are referred to as "beds" or "strata". Subsequent to their deposition and consolidation, these geological beds may be tilted and contorted by tectonic forces.

-6-.. ,w Even where these beds are complexly folded on the large scale, however, they generally retain a nearly planar character at the scale of the wellbore diameter. The orientation in space of these planar beds or bed segments and the variation in their orientation from point to point are important for the understanding and mapping of the geological features s of interest and for positioning the wellbore in the desired geological beds.
Refernng now to Figures 1 (a) - (c), the inclination of a bed or bedding 30 relative to a wellbore 32 is made by accurately determining the stratigraphic and physical position of two points 34, 36 in the wellbore of interest. Wellbore deviation surveys, obtained incrementally during the drilling operation, allow the physical position of any point in the 1o wellbore to be calculated. The physical distance between the chosen two points 34, 36 represents the "apparent" stratigraphic thickness (To) of the sequence of interest. The true stratigraphic thickness (i.e. the distance measured perpendicular to bedding 30) between these points, namely Tt, is derived by correlation to the reference log. The angle between the wellbore and the "planar" bedding is O
15 O = asin (TdTa). [ 1 ]
This equation (referred to herein as "eqn. [ 1 ]") relates solely to the relative angle between the wellbore 32 and the plane or surface 38 of the geologic bed. This does not define a single bedding orientation, but rather a family of surfaces.
A plane is a two-dimensional surface. However, the orientation of any plane (not 2o the position) may be uniquely represented by the orientation of a line or vector 40 that is normal to this plane 38 (this line is referred to as the "pole" to the plane).
These one-dimensional representations (i.e. the poles to the planes) are used henceforth in this application for all presentations and computations related to bedding planes.

The angle between the wellbore 32 and the pole 40 to the plane (~) is derived from eqn. [ 1 ], namely ~ = 90 - O = acos (Tt/Ta). [2]
The poles that satisfy this equation are a family of lines that describe a circular cone 42 that is a body of revolution about the wellbore (see Fig. 2) at the point of calculation for eqns. [ I ] and [2].
Determination of true bedding_orientation In order to choose which pole 40 within this family of poles 42 (see Fig. 2) represents the actual bedding orientation, some additional constraining information is 1o required. Specifically, a knowledge or estimation of the local structural geology is needed. The particular information required is the "strike" of the bedding or the local structural "trend," e.g. the "bearing" and "plunge" of a local fold axis. Itr is noted that geologists conventionally describe the orientation of a linear element in terms of its bearing and plunge, as illustrated in Fig. 1 (c). The bearing is the azimuth of the linear is feature, measured from north (N), and the plunge is the angle between the horizontal plane and the linear element. A positive plunge angle indicates the linear element is directed below the horizontal surface. This information may be derived from a knowledge of the regional geology, seismic data, offset wells or other sources.
In many locations where geological strata are tectonically deformed, the 2o geological beds are folded and faulted. Referring to Fig. 3(a), the folds can usually be described as a cylindrical surface 44, at least locally. A "cylindrical" fold is defined by a system of lines that are parallel to the axis 46 of the fold, as illustrated in Fig. 3(a).
_g_ Equivalently, the pole 44a to any point on the surface 44 of a cylindrical fold is perpendicular to the fold axis 46, as illustrated in Fig. 3(b). The fold axis 46 also generally defines the local structural trend. Where distinct folds are not developed, the local structural trend is taken as the line that is statistically perpendicular to the poles to bedding in the area of interest. Finally, where the local fold axis or structural trend line is horizontal, this linear reference is parallel to the strike of bedding.
A knowledge or estimation of the local fold axis or the local structural strike or trend provides the information needed to limit the possible orientation of the beds intersected by the wellbore. Specifically, the pole to the bedding plane that is intersected to by the wellbore at the location of interest must lie on the conical surface 42 defined by eqn. [2] (Fig. 2) and in a plane 48 that is perpendicular to the local fold axis 46 (Fig.
3(b)). The pole that satisfies both of these conditions is the line or lines of intersection SOa and 50b (Fig. 4(a)) of the cone and the plane. There will be, at most, two lines of intersection for these two surfaces. The plane could intersect the cone at just one line in s5 the special case of tangential contact between this plane and the conical surface. It is also possible that the two surfaces do not intersect at all, which indicates that an error exists either in the assumption of the local structural trend or the calculation of the bedding orientation relative to the wellbore.
The lines) of intersection SOa, SOb between the conical surface 42 and the plane 20 48 may be determined graphically (e.g., using a stereonet presentation) or analytically.
Presented below are, first, the analytical approach and then the equivalent stereonet (graphical) procedure.

Analytical approach Both the cone 42 and the plane 48 are loci of lines (viz. the poles to certain families of planes). Our interest is in the orientation of these lines, not their position in space. Consequently, all of the poles should be considered as lines radiating from or s passing through the origin of a common co-ordinate system. As illustrated in Fig. 4(a), the plane 48 representing the poles 44a perpendicular to the fold axis passes through the origin 52 of this co-ordinate system, and the apex of the cone 42 coincides with the origin 52.
The structural trend or fold axis 46 may be described by its direction cosines A, B
1o and C. The values of A, B and C are related to the bearing (A) and plunge (~) of the fold axis (Fig. 3(c)) by the following equations:
A = cos ~ * sin 8 [3a) B = cos ~ * cos 0 [3b) C = -sin ~ [3c) is The plane of interest (indicated by 48 in Figs. 3(b) & 4(a)) is perpendicular to this local fold axis or structural trend, and may thus be described, in the x-y-z (E-N-up) co-ordinate system that is shown in Fig. 4 (a), by the equation:
Ax+By+Cz=0. [3) The equation for the circular cone of interest 42 shown in Figs. 2 & 4(a) is expressed 2o most simply in a x'-y'-z' co-ordinate system, also shown in Fig. 4(a), where the y'-axis is parallel to the wellbore 32 and, thus, to the axis of the cone. The conical surface that contains all poles making an angle ~ with the wellbore is described by:

x~z + z~z = Kz * y~z [4]
where K = tan (d~) [4a.I
The following computations will be made in the x'-~y'-z' co-ordinate system.
Consequently, the description of the plane 48 that is perpendicular to the fold axis 46, given in eqn. [3], must be transformed from the x-y-z co-ordinate system to the x'-y'-z' co-ordinate system. This is performed using the standard vector transformation:
axx axy axz x x' ayx ayy ayz y = y' [4b]
1 o azx azy azz z z' where the components of the transformation matrix are defined in terms of wellbore bearing (a) and plunge (r)), as shown in Fig. 1(c):
axx = cos a ayx = cos r1 * sin a 1s azx = sin r) * sin a axy = cos (a +90) _ -sin a ayy = cos a * cos r1 azy = cos a * sin r~
axz = cos (90) = 0 2o ayz = cos (r~ +90) _ -sin rl azz = cos r~

In the x'-y'-z' co-ordinate system the plane 48 that is perpendicular to the fold axis is described by the equation:
Ex' + Fy' + Gz' = 0 . [5]
where E, F and G are the values of A, B, and C from eqn. [4] transformed through eqn.
s [4b]. As described above, there are, at most, two lines SOa, 50b (Fig. 4) that lie in both the conical surface 42 and the plane 48 of interest and which, thus, satisfy both eqns. [4]
and [5]. If we also define the unit sphere x'2 + y'2 + z'2 = 1. [6]
Then, there are, at most, two points that are common to the three surfaces defined to by eqns. [4], [5] and [6]. The x', y' and z' values of these points, which are designated xs', ys' and zs', respectively, are the direction cosines of the two poles SOa, SOb shown in Figs. 4(a) - (c) that are common to the plane 48 and the cone 42 of interest.
In order to determine the common solutions (xs', ys', zs') to eqns. [4], [5]
and [6], first substitute eqn. [6] into eqn. [4], which produces:
i5 1 - ys'2 = K2 * ys'2, or (K2 + 1) ys'2 = 1 , so [7]
[8]
ys' = t[1/(K2 + 1)]1/2 = ~R~
where R is introduced as a simplifying variable. Because the conical surface 42 shown in Fig. 4(a) is defined for positive values of y' only, 2o ys' _ +R [9]

. ~ , CA 02383379 2002-04-25 Now inserting eqn. [9] into eqn. [4] yields xs'2+zs'2=Kz/(KZ+ 1)=S, [10]
where S is introduced as another simplifying variable, so that ZS' _ [S - XS'2]~~2. [11]
Using eqns. [$] and [11] in eqn. [5] produces:
E*xs' + F*R = -G*[S - xs'Z]lie . [12]
Squaring both sides of eqn. [ 12] and expressing the result as a quadratic equation in x' xs'2(EZ + G2) + xs'(2*F*R*E) + (F2*Rz - G2*S) = 0. [13]
Expressing eqn. [13] as:
p* xs'2 + q* xs' + r = 0, [14]
where p=EZ+G2 [14a]
q = 2*F*R*E [14b]
r = F2*R2 - GZ*S, [14c]
then the standard solution for a quadratic equation is expressed here as:
xs' _ [-q t (q2 - 4*p*r) 1~]/(2*p) = T. [15]
Note that there are two solution values for xs'. However, a negative value inside the radical of eqn. [15] indicates there is no real solution, i.e. that the plane and the cone do not intersect. Finally, using eqns. [5], [9] and [15], zs' _ -(ET + FR)/G [ 16 ]
Eqns [15], [9] and [16] provide the direction cosines xs", ys', zs', within the x'-y' z' co-ordinate system, of the poles to the planes that satisfy eqn. [2] and are parallel to the local structural trend. These are converted back to direction cosines u, v, w in the x-y-z s co-ordinate system using the transformation axx ayx azx xs' a axy ayy azy ys' [ 16a]
axz ayz azz zs' w Because xs', ys', zs' satisfy eqn [6], it will also be the case that so u2 + v2 + w2 = 1 [ 16b]
The bearing ([3) and plunge (p) for each of the solution poles SOa, SOb shown in Fig. 4(b) are:
(3 = atan (u/v) [17]
p = asin (-w). [18]
1s Each pole defines a single bedding plane 54a, 54b shown in Figs. 4(b) and 4(c), respectively. Geologists conventionally describe the orientation of bedding planes (or other planar surfaces) in terms of the dip magnitude (8) and dip azimuth (a) of the surface, as shown in Fig. 4(d). These values are derived from the bearing and plunge of the pole (eqns. [17] and [18]) via the equations:
20 8 = 90 - p [19]
a = 180 + (3. [20]

There are two bedding orientations obtained from this analysis, although there is only one actual bedding orientation. The determination of which of these two solutions is appropriate cannot be made solely from these geometric calculations. Rather, the choice must be based on consideration of reasonable structural style and continuity with available orientation information.
Graphical (stereonet) approach The determination of the two bedding orientations that both intersect the wellbore 32 at the angle O and are parallel to the local structural trend may be done graphically using a stereonet projection.
so The stereonet projection is used to determine angular relationships between lines and planes. Referring first to Fig. 5(a), the stereonet projection begins with a sphere 60.
Any line and plane of interest is defined to pass through the centre of this sphere. This may be done because the stereonet projection is concerned only with orientation of the line or plane and not its position in space. A plane 62 passing through the centre of the sphere intersects the sphere in a circle 64. A line 66 passing through the centre of the sphere intersects the sphere at two points (one point 68 is shown, and the other is hidden from view).
The stereonet display uses only half of this sphere, as shown in Fig. 5(b).
Either the upper hemisphere or the lower hemisphere 70 shown in Fig. 5(b) may be used. The 2o chosen hemisphere and all of the points and lines of intersection between this hemisphere and lines or planes are projected onto a. horizontal plane 72 shown in Fig.
5(c). This is the stereonet projection. In this display, a line is represented by a single point of intersection 68 with the hemisphere. Any specific point on the stereonet projection represents a unique orientation. The intersection of a plane with the hemisphere is a smooth arc that intersects the perimeter of the stereonet display on diametrically opposing points. This arc is referred to as a "great circle."
s In the present case, the poles to all planes that intersect the wellbore 32 at angle O
describe the conical surface 42. This conical surface intersects the sphere 60 in a "small circle" 73 shown in Fig. 6(a). The centre of a "small circle," in contrast to a "great circle," does not coincide with the centre of the sphere. On the stereonet projection, this small circle intersection will form either a closed curve or two arcs 74a and 74b shown in to Fig. 6b. Because the stereonet display uses only one hemisphere 70, any part of the conical surface 42 that does not directly intersect this hemisphere is projected through the centre point of the sphere 60 to the opposite side of the hemisphere of interest.
The plane 48 containing the poles 44a to all planes that are parallel to the local structural trend or fold axis 46 in Fig. 3(b) forms a single great circle 76 on the stereonet 15 display, shown in Fig. 7. The fold axis 46 itself is represented by a single point 78.
An important aspect of the present invention is the representation in Fig.8 which shows the two points of intersection 80a and 80b between the great circle 76 and the small circles 74a, 74b. The points of intersection 80a, 80b represent the poles SOa, SOb to the two planes shown in Fig. 4(b) and 4(c) that satisfy both of the earlier noted geometric 2o conditions, i.e. they intersect the wellbore at the angle (J and are parallel to the local fold axis or structural trend. The orientations derived here graphically are the same as those given in eqns. [ 17] & [ 18]. As discussed for the case of the analytical approach, the user " CA 02383379 2002-04-25 must then determine which of these two choices is the appropriate orientation of the bed 30 based on some other criterion, such as knowledge of local geology.
The method and many advantages of the present invention may now be better understood. Prior to commencing drilling of a wellbore, a user first derives a model of the structural feature to be drilled based on seismic data, existing wells, etc. This provides an estimate of the type of geometric information contemplated in Figs. 3(a) and 3(b). This information is embedded in eqn. [3] and is graphically represented on a stereonet in Fig. 7 as a great circle of all possible bedding orientations for this structural geometry. Additionally, petrophysical logs should be obtained from nearby reference 1o wells and adjusted/corrected to obtain a "true" stratigraphic thickness.
Then, when drilling a subject well and the wellbore enters a geological bed of interest, the angle of penetration of the wellbore into the bed is determined trigonometrically by comparing reference markers in this well with the reference well(s), as contemplated in Fig. 1 (a).
This identifies the type of geometric possibilities contemplated in Fig. 2.
These possibilities are embedded in eqn. [4] and are graphically represented on the stereonet presentation as in Fig. 6(b) showing two (and possibly one) small circles of the determined bedding orientation relative to the wellbore. The small circles 74a, 74b of the determined bedding orientations are then overlaid on the great circle 76 of possible bedding orientations from the structural interpretation to obtain graphically at most two points 80a and 80b of intersection, as contemplated in 1 ig. 8. These points of intersections are the poles to the only two possible bedding surface orientations that agree both with the wellbore information and the assumed local structural trend.
Alternately, an analytical approach may be used to arrive at the same two possible bedding orientations, given in terms of dip magnitude and dip azimuth in eqns. [19] and [20]. The user may then combine the two possible bedding orientation values with other sources of information (such as knowledge of regional dip, seismic data and other available data) to determine which is the most likely true bedding orientation for the purpose of making s decisions on the direction of further wellbore drilling.
An example of the application of the invention In order to illustrate how the invention described in this application would be used to benefit during a drilling operation, we consider the case of a well being drilled into a 1o fold (anticline) 82 developed above a thrust fault 84 shown in Fig. 9. The objective of this drilling operation is to position a horizontal wellbore at the target location 86 within geological unit 3 such that it runs along the crest of this fold. 'Che fold crest in unit 3 lies 3 km below the earth's surface. The general geometry and orientation of this fold is well defined by seismic data. The axis of this fold plunges 5° on a hearing of 150°. However, 1s the exact position of the fold crest in the subsurface could not be precisely determined.
As the wellbore 88 nears the geological unit l, as shown in Fig. 10(a), the inclination of the wellbore is reduced in preparation to becoming horizontal.
At the point that the wellbore 88 enters geological unit 1 it is plunging at an angle of 20° along an azimuth of 150°. Using a comparison of the apparent (drilled) thickness of beds near the 2o top of this unit with the true stratigraphic thickness of these same beds, it is determined, using eqn. [ 1 ], that the wellbore is inclined at an angle of 12.7° to the bedding. This is the angle O of Fig. 1. Combining this information with the knowledge of the local structural trend allows two possible bedding orientations to be determined, either graphically (Fig.

8) or analytically (eqns. [19 and [20]), using the invention described in this application.
These two possible bedding orientations are:
bed dip azimuthdip ma 'tulle solutaon 068 32 A

solution 232 32 B

s Both solutions indicate that the beds are moderately steeply dipping and that they must be on the flanks of this fold rather than near the fold crest, as was desired. Solution A would indicate that the well is actually on the right side of the fold as shown in Fig. 9, but solution B would indicate that the well is on the left side of the fold.
The geologist believes that solution B is the more likely case. A view of the last segment of the 1o wellbore path and the calculated bedding orientation, viewed parallel to the fold axis, is shown in Fig. 10(a). In order to reach the desired position on the structure with the wellbore, the wellbore path is altered to a bearing of 130° and a plunge of 10°. Along this new wellbore path an intersection angle O between the wellbore and bedding of 14.5° is determined. Combining this determination with the fold axis value results in the 15 calculation, using the invention described in this application, of the following two bedding orientation solutions:

bed dip azimuthdip ma nude solution 232 31 C

solution 063 61 D

Solution C is similar to solution B above. The fact that this solution was obtained from the two different wellbore paths supports this as the correct solution.
With continued drilling, the well encounters unit 2, as depicted in Fig.
10(b). At s this position it is determined that the intersection angle O between the wellbore and the bedding is 9.8°. Using this value and the same fold axis orientation, two possible bedding orientations are calculated using the invention described in this application, viz.:
bed dip azimuthdip ma nitude solution 220 15 E

solution 065 47 F

so The SW dip azimuth solution (solution E) is chosen as the more reasonable of the two, and a view (parallel to the fold axis) of the wellbore path and the bedding orientation at this point in the drilling operation is shown in Fig. 10(b). This determination indicates that the beds at this point have a much lower dip than the was encountered at the top unit l, which suggests that the wellbore is close to the crestal region of the fold. The wellbore is path is readjusted back to a bearing of 1 SO° and the plunge is increased to 20° in order to enter unit 3 more quickly. As the well penetrates the top of unit 3, an intersection angle O of slightly less than 15° is determined, which, with the fold axis value, allows the determination of the following two bedding orientations:
bed dip azimuthdip m nitude solution 120 O6 G

solution 180 06 H

Both of these solutions indicate that the beds now have a very gentle dip, as shown in the view along the fold axis in Fig. 10(c). The fact that both of these solutions indicate quite low bed dips indicates the well is now very close to the crest of the structure, as desired, and it is also now in unit 3. The wellbore path is now adjusted to a so bearing of 150° and a plunge of 5°, such that it is parallel to the fold axis in the target zone.
The overall path of this wellbore relative to the structure of interest is shown in Fig. 10(d). The corrections made to the wellbore path during the drilling operation, which are based on the invention described in this application, have allowed the operator 1s to successfully position the wellbore in the desired location within the geological structure.
The above description is intended in an illustrative rather than a restrictive sense, and variations to the specific configurations described may be apparent to skilled persons in adapting the present invention to other specific applications. For example, the information obtained from the wellbore being drilled may suggest an adjustment of the local structural interpretation, thus altering the orientation of features contemplated in Figs. 3 and 7. This change would alter the results obtained from Fig. 8 or eqns. [ 19] and [20]. Such variations are intended to form part of the present invention insofar as they are s within the spirit and scope of the claims below.

Claims (12)

We claim:

1. A method of determining the probable orientation of geological strata near a drillbit during drilling of a wellbore comprising:
a) obtaining all possible orientations of the geological strata penetrated by said wellbore;
b) obtaining all possible bedding orientations that are parallel to a fold axis or a local structural trend of said geological strata; and, c) comparing the orientations of steps a) and b) to arrive at no more than two choices for the true orientation of said geological strata.

2. The method of claim 1 further including the step of obtaining the angle between the wellbore and the geological strata based on the true and relative stratigraphic thickness of said geological strata prior to said step a).

3. The method of claim 1 wherein said step a) further includes obtaining a first set of poles to all possible planes intersecting said wellbore at said angle of penetration.

4. The method of claim 2 wherein said step a) further includes obtaining a first set of poles to all possible planes intersecting said wellbore at said angle of penetration.

5. The method of claim 1 wherein said step b) further includes obtaining a second set of poles to all planes which are parallel to said fold axis or said local structural trend, and then determining a single plane which encompasses said second set of poles.

6. The method of claim 2 wherein said step b) further includes obtaining a second set of poles to all planes which are parallel to said fold axis or said local structural trend, and then determining a single plane which encompasses said second set of poles.

7. The method of claim 1 wherein:
said step a) further includes obtaining a first set of poles to all possible planes intersecting said wellbore at said angle of penetration;
said step b) further includes obtaining a second set of poles to all planes which are parallel to said fold axis or said local structural trend, and then determining a single plane which encompasses said second set of poles; and said step c) further includes superimposing said first set of poles with said single plane to obtain no more than two points of intersection indicating no more than two possible bedding orientations at said borehole, one of which being the true orientation of said geological strata.

8. The method of claim 2 wherein:
said step a) further includes obtaining a first set of poles to all possible planes intersecting said wellbore at said angle of penetration;
said step b) further includes obtaining a second set of poles to all planes which are parallel to said fold axis or said local structural trend, and then determining a single plane which encompasses said second set of poles; and said step c) further includes superimposing said first set of poles with said single plane to obtain no more than two points of intersection indicating no more than two possible bedding orientations at said borehole, one of which being the true orientation of said geological strata.

9. A method of determining the true orientation of a geological bed at a wellbore comprising:
a) obtaining a first stereonet presentation showing at least one small circle segment representing all possible bedding orientations relative to said wellbore;
b) obtaining a second stereonet presentation showing a great circle representing all possible bedding orientations that are parallel to a fold axis or a local structural trend;
and, c) superimposing the stereonet presentations of steps a) and b) to obtain no more than two points of intersection between said at least one small circle segment and said great circle representing the poles for no more than two choices for the true orientation of said geological bed.

10. The method of claim 9 further including the step of obtaining the angle between the wellbore and the geological bed based on the true and relative stratigraphic thickness of said geological bed prior to step a).

11. A method of determining the orientation of a geological bed at a wellbore comprising:
a) obtaining a graphical presentation showing at least one first graphic representing possible bedding orientations relative to said wellbore;

b) superimposing on said graphical presentation a second graphic representing possible bedding orientations that are parallel to a fold axis or a local structural trend;
and, c) obtaining at least one point of intersection between said at least one first graphic and said second graphic indicating a true orientation of said geological bed.

12. The method of claim 11 further including the step of obtaining prior to step a) the angle between the wellbore and the geological bed based on the true and relative stratigraphic thickness of said geological bed.