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FIELD OF THE INVENTION 
     The present invention relates to the field of interactive displays for use in oilfield services applications. In particular, the invention relates to a three-dimensional interactive display for rock stability applications relating to wellbore construction. 
     BACKGROUND OF THE INVENTION 
     Wellbore instability and its associated drilling problems are a major source of lost time and excess cost when drilling. Planning for the management of instability is now becoming routine, but communicating information on instability prediction, for example to the diverse members of an asset or drilling team, can be difficult. This is because many parameters enter into the prediction, and displaying the influence of varying them all over their potential ranges is not possible with conventional charts or plots. There are also many outputs. 
     Three-dimensional displays for wellbore instability have been used in a demonstration drilling simulator. See, IADC/SPE 59121 , When Rock Mechanics Met Drilling: Effective Implementation of Real - Time Wellbore Stability Control , I. D. R. Bradford, J. M. Cook, E. F. M. Elewaut, J. A. Fuller, T. G. Kristiansen, and T. R. Walsgrove (presented at the 2000 IADC/SPE Drilling Conference held in New Orleans, La., Feb. 23-25, 2000); and SPE/IADC 67816 , Meeting Future Drilling Planning and Decision Support Requirements: A New Drilling Simulator , H.-L. Balasch, J. Booth, I. D. R. Bradford, J. M. Cook, J. D. Dowell, G. Ritchie, and I. Tuddenham (presented at the SPE/IADC Drilling Conference held in Amsterdam, The Netherlands, Feb. 27-Mar. 1, 2001). These displays were implemented using a scientific programming language known as Matlab. 
     Colored polar plots have been used to display the results of instability planning. For example polar colormap plots of the severity of potential instability for wells at different orientations have been implemented by Baker Hughes and Geomechanics International. These techniques show the influences of changing the well azimuth and deviation, with all other parameters fixed. The color used at a particular point in the polar plot depends on how much instability is predicted at the appropriate orientation. However, techniques such as these are of limited use due in part to the following: 
     1. the person viewing must have an appreciation of how a polar plot presents information; this is not a display method familiar to many people outside geology and crystallography; 
     2. the instability function must be integrated around the circumference of the well, in order to generate a single value for the colormap; this masks useful details of the circumferential variation (e.g., its potential use in image log interpretation); and 
     3. the plots are relatively slow to generate, since they have to cover a wide range of parameter space, but are then fixed; any change in the earth parameters means a time-consuming recalculation of the whole plot. 
     Finally, Three-dimensional displays have recently been used successfully to convey instability information for a fixed trajectory in a fixed earth model. However, these techniques suffered in that they were not interactive with the user. This is primarily because if the parameters of the trajectory or earth model are changed, considerable recomputation is required to display the new results, and there is no user-friendly method of changing the trajectory of the wellbore. 
     SUMMARY OF THE INVENTION 
     Thus, it is an object of the present invention to provide a system and method for interactively displaying rock stability information to a user in three-dimensions. 
     According to the invention a system is provided for interactively displaying estimated stability of rock surrounding a wellbore comprising: 
     a three-dimensional display adapted to display to a user an orientation of a portion of the wellbore and an estimation of stability of the rock surrounding the portion of the wellbore; 
     a user input system adapted to accept user input representing changes in orientation of the portion of the wellbore; and 
     a processing system adapted to accept the user input from the user input system and calculate and communicate to the display a revised estimation of stability of the rock based on the user input. 
     Also according to the invention, a method is provided for interactively displaying estimated stability of rock surrounding a wellbore comprising the steps of: 
     displaying to a user a three-dimensional representation of a first orientation of a portion of the wellbore and a first estimation of stability of the rock surrounding the portion of the wellbore associated with the first orientation; 
     receiving user input representing a second orientation of the portion of the wellbore; 
     calculating a second estimation of stability of the rock associated with the second orientation; and 
     displaying to the user in real time a three-dimensional representation of the second orientation of the portion of the wellbore and the second estimation of stability of the rock. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an interactive stability display according to a preferred embodiment of the invention; 
     FIG. 2 shows features of the display screen, according to a preferred embodiment of the invention; 
     FIG. 3 is a flow chart showing processing steps according to the invention as implemented on a computer; 
     FIG. 4 is a flow chart showing steps of planning and drilling a well according to a preferred embodiment of the invention; 
     FIG. 5 is a diagram showing the implementation of an interactive stability display used to create a well plan and drill a well, according to a preferred embodiment of the invention; 
     FIG. 6 shows a portion of an interactive stability display according to another embodiment of the invention; and 
     FIG. 7 is a flow chart showing steps of making a completion plan and perforating a well according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to a preferred embodiment of the invention, an interactive display is provided that displays the predicted failure state of the rock around the wellbore directly and graphically, using a 3-dimensional display with a “click and drag” interface to change the well orientation, and simple methods to choose earth and drilling parameters. The display can be used to convey, quickly and convincingly, the differences between drilling in different directions or at different deviations, and the effects of changes in mud weight, in-situ stress and rock properties. It can also be used as an interpretation tool for comparing predicted deformation patterns against well data, for example to establish bounds on the stress state. According to one embodiment the display is used as a fully-functional instability predictor by a oilfield service engineer who is planning wells for a client. According to this embodiment, any of the parameters can be changed by the user. 
     According to another embodiment, the display is used by an oilfield asset owner or operator. According to this embodiment either some or all of earth and rock parameters are hardwired in, and the user is only allowed to change well orientation, mud weight and a limited number of other parameters. For example, the users could view a 3-D display on their own computer, examining the effects of changes in well orientation, but not being able to alter the stress state. 
     The wellbore instability predictions that are displayed according to the invention are preferably based on calculations of the stress state around the wellbore, and of the response of the rock to these stresses. Even more preferably, the predictions are based on an elastic model of rock behavior, which are known to be conservative but having a clear advantages in terms of speed, intelligibility, and amount of rock data required. Examples of these types of calculations that can be used form the basis of codes used by Schlumberger such those known as Roxan™, RockSolid®, and IMPACT®. See also, Peska and Zoback, J. Geophys. Research, 100, 1995 12791-12811; and Fjaer et al, Petroleum Related Rock Mechanics, Elsevier, 1992, Chapter 4. Alternatively various other mechanical models can be used, with some being more complex models of rock behavior, for example by incorporating plasticity. In accordance with the invention, an elastic model is preferred because calculations of plasticity around the wellbore can be time-consuming. However, under some circumstances where response time is less important or a high degree of processing power is available, a more complex model such as one incorporating plasticity could be used. According to the invention a relatively fast response time is an important feature of the interactive display so that the results of the instability calculations can be viewed by the user as the wellbore is moved around. The fast response time advantageously increases the usability and appeal of the display among a wide range of users. 
     FIG. 1 shows an interactive stability display according to a preferred embodiment of the invention. Interactive stability display  100  comprises a display screen  102 , a processor  107 , a storage system  108 , and user input devices including a keyboard  104  and pointing device  106 . According to a preferred embodiment Interactive display  100  is implemented on a personal computer, and even more preferably on a laptop personal computer. The interactive stability display  100  can be programmed in a language such as Matlab®, although it is preferably implemented directly in a Language such as C++. The display screen  102  is a two-dimensional personal computer display, and even more preferably a LCD laptop screen. Display screen  102  may comprise a number of windows or other information associated with other applications or processes running on the personal computer. Providing interactive display  100  on a laptop computer greatly enhances the wide range of working environments for the user. Keyboard  104  is preferably a laptop keyboard. Pointing device  106  is preferably a mouse, track pad, trackball, joystick, but could alternatively be any other pointing devices useable with a personal computer. 
     FIG. 2 shows features of the display screen, according to a preferred embodiment of the invention. One of the main windows displayed on display screen  102  is graphics window  110 . Graphics window  110  comprises primarily a three dimensional (3-D) display  112  and parameter information  114 . As used herein the phrases “three-dimensional display”, “three-dimensional representation” and “3-D display” include true three-dimensional display techniques (e.g. volumetric and holographic displays), stereoscopic three-dimensional display, and two-dimensional representations of three-dimensional (e.g. perspective projection and parallel projection). According to a preferred embodiment, 3-D display  112  is a parallel projection display. This has the advantage of not requiring high levels of processing power or special hardware beyond an ordinary monitor for a personal computer. The 3-D display  112  displays to the user a three-dimensional representation of stability information of rock surrounding a wellbore. The 3-D display  112  preferably shows: a bounding box  116  to aid in 3-D orientation; a North/East/down coordinate system shown by broken lines  120 ; the orientations and relative magnitudes of the principal stresses indicated with axes  128 ; a hemispherical grid  118  to guide the user in orienting the portion of the wellbore; and a portion of the wellbore itself  124 , whose orientation can be changed with the pointing device  106  using the small ball  122  (preferably brightly colored) as a handle. The portion of the wellbore displayed is preferably relatively short such that the parameters including rock properties, orientation and mud weight, do not vary over the length of the portion. This allows for rapid recalculation of the predicted stability information. For example, it has been found that portions of 1 meter are suitable. In general the suitable length depends in part on the variability of the particular rock surrounding the wellbore. However, if sufficient computational speed is available, the portion of the wellbore used could be longer, up to the entire length of the well. It is also preferable that the aspect ratio of the width and length of the displayed portion of the wellbore is maintained to improve usability. In practice, portions of less than 5 meters are preferred if the displayed aspect ratio is to be maintained. 
     Buttons  134  are used for rotating the axes to manipulate the viewing angle. It will be appreciated that buttons  134  can also be used to display a plan view to the user. 3-D display  112  importantly displays a prediction of the stability (or instability) state of the rock surrounding the portion of the wellbore  124 . This information is preferably displayed using an outline surface around the portion of the wellbore, where the color of different parts of the surface indicates the predicted stability of the corresponding surrounding rock. For example, in FIG. 2 the shaded part of the outline surface  126  is preferably displayed in a red color and the unshaded portion is displayed in blue color. In the example of FIG. 2, the red shaded part  126  indicates clearly to the user that rock failure is predicted in those portions of the rock surrounding the portion of the wellbore. 
     The parameter information portion  114  of the display comprises a number of boxes for entering and displaying various parameters relating to the stability of the rock surrounding the portion of the wellbore preferably including: stress magnitudes and orientations, rock strength parameters, and the true vertical depth. The true vertical depth is preferably used only to convert fluid density (e.g. mud weight) to fluid pressure (e.g. mud pressure). Parameter information portion  114  also includes boxes that can be used to display and change wellbore azimuth and deviation, and mud weight. However, according to a preferred embodiment, these parameters are more easily changed using the three-dimensional display  112  and a mud weight slider  136  respectively, and the boxes  130  display the values of the parameters. Although the parameter information portion  114  is shown to display certain preferred parameters, according to other embodiments other parameters could also be displayed and/or manipulated by the user, such as rock plasticity parameters, fluid flow rates, temperatures, chemical and electrochemical properties, and time since drilling. 
     According to a preferred embodiment, the instability predictions displayed in 3-D display  112  are based on inputs including: the magnitudes of the three principal stresses in the earth at the depth of interest; their orientation relative to North; the pore pressure; the rock strength, friction angle and Poisson&#39;s ratio; the azimuth and deviation of the well; and the fluid pressure (e.g. mud pressure) in the wellbore. These are used to rotate the in-situ stress field into the wellbore coordinate system; then to calculate the stress concentration around the portion of the wellbore preferably using an elastic model; then to compare the maximum and minimum local principal stresses to an appropriate failure criterion (for example, the Mohr-Coulomb criterion). The result is a function representing the extent by which the local stress state exceeds the strength of the rock; in simple terms whether the rock has failed and by how much. This function is evaluated at a number of points around the portion of the wellbore circumference and displayed in real-time to the user via colored shading such as shaded part  126  around portion of the wellbore  124  in 3-D display  112 . 
     Whenever a parameter is changed, or the portion of the wellbore orientation is changed, the equations for the stress state and failure conditions around the wellbore are re-calculated, and the color shading  126  of the portion of the wellbore  124  is re-mapped to the value of the failure function. Although any coloring scheme could be used, the invention preferably makes use of colors that are quickly and clearly distinguishable by the user. According to a preferred coloring scheme, the color of the surface around the wellbore changes from blue through mauve to red as the failure function moves from negative or zero (no failure under the local stress state) through small positive values (mild rock failure) to large positive values (severe rock failure). Because the calculations are preferably carried out using an elastic model, they are very quick, and so the wellbore color map, indicating the failure state, is updated as the portion of the wellbore is moved with the mouse, giving a high degree of interactivity with the user. 
     According to another embodiment, the shape of the surface surrounding the wellbore is distorted such that the cross section of the surface is no longer round, in order to display to the user an indication of the severity of the rock failure. As with the simple color shading approach, this is very rapid and changes as the wellbore is moved. The shape distortion can be used alone or preferably in combination with the color shading approach. 
     Since the display shows both the extent of the potential damage to the wellbore, and its location, it can be used both as a tool to examine and demonstrate the effects of drilling at different orientations and with different mud weights, and also to interpret image logs that show wellbore damage, such as resistivity at the bit (RAB) logs. Interpretation of the position of the damage can help clarify the orientations and magnitudes of the principal stresses in the earth. 
     FIG. 3 is a flow chart showing some processing steps according to the invention as implemented on a computer. In step  210  the program is initialized and a set of default parameters are read from memory. These default parameters could be originally obtained from an earth model, or may be setup for the particular region which the invention is intended to be used. In step  212  the stability of the rock surrounding the wellbore is predicted based on the current parameters. Following the initialization step  210 , the stability calculations in step  212  would be based on the default parameters. 
     In step  214  the predicted stability of the rock surrounding the wellbore is displayed to the user using a 3-D display, preferably as described above with respect to FIG.  2 . As discussed above, the calculations underlying the predicted stability are performed and the predicted stability is displayed in real time in order to give the display a high degree of interactivity. In particular the delay time for recalculation (and preferably re-display) in real time based on a change in the orientation of the portion of the wellbore by the user is preferably less than 2 seconds, and even more preferably is less than 0.2 seconds. 
     In step  216  a determination is made as to whether the parameters being used give a suitable result in terms of stability of the rock surrounding the portion of the wellbore. This determination is preferably made by the user based on the viewing the stability information being displayed on the 3-D display and the current parameters. If the current parameters are not suitable, the user indicates this by entering in new parameters, step  220 , by moving the pointing device to change the orientation of the well or mud weight, and/or changing the parameter values in the data entry boxes. If the user determines that the current parameters are suitable, in step  218  the user proceeds with the remainder of the drilling process. The user preferably indicates the parameter acceptability to the computer program which then records and saves the current parameters for future use. Alternatively, the user can record the suitable parameters manually or electronically elsewhere on the computer. In practice, the user is often interested most in the mud weight and trajectory of the wellbore, given the parameters set by the drilling environment. 
     FIG. 4 is a flow chart showing steps of planning and drilling a well according to certain embodiments of the invention. In step  310  at least some of the parameters used by the interactive display are loaded from an existing earth model. In step  312  the user uses the interactive display. In this case, the parameters from the earth model are used as some or all of the initial parameters in step  210  of FIG.  3 . In step  314  the selected or preferred parameters, typically the orientation and/or mud weight are obtained from the interactive display. In step  318  the preferred orientation and/or mud weight are used to construct or modify a well plan. For example, in light of the preferred orientations obtained in step  314 , the planned trajectory is modified to incorporate one or more of the preferred orientations, or incorporate orientations approximating one or more of the preferred orientations, into an existing well plan. Finally, in step  320  a well is drilled using the constructed or modified well plan. 
     According to another embodiment of the invention, the interactive stability display is used during a drilling operation. During drilling, in step  322  the known orientation and a measured fluid pressure (mud pressure in this case) are entered as parameters in the interactive display. Other parameters may be used from an earth model (step  310 ). In step  312  the user uses the interactive display. In step  324 , the rock failure predictions from the interactive display are compared to information acquired from RAB logs or other imaging tools taken from the well during the drilling process. If an inconsistency is identified between the measured and predicted information, either the earth model can be updated, the well plan can be modified (e.g. with a new trajectory and/or mud weight), or both. In step  328 , the remainder of the well is drilled using the modified well plan. 
     According to another embodiment the interactive display can be used to predict rock stability in an open hole during production. According to this embodiment, in step  322  the known orientation for a portion of the open hole wellbore and the measured fluid pressure (in this case the pressure of the production fluid) is entered into the interactive display along with data from an earth model. In step  312  the interactive display is used. In step  330  rock stability predictions are obtained for the open hole wellbore. In light of the stability predictions a preferred or selected drawdown pressure is obtained, and in step  332 , the production is carried out using the preferred drawdown pressure. 
     Alternatively, according to another embodiment, in step  334 , the interactive stability display can be used to diagnose a problem encountered during drilling or during production. For example, if a rock failure is suspected in an open hole section of the wellbore during production, the interactive stability display can be used to aid in evaluating the likely location of the failure (in terms of both depth and circumferential position) and consequences (e.g. crushing of a screen, or disruption of gravel pack). 
     FIG. 5 is a diagram showing the implementation of an interactive stability display used to create a well plan and drill a well, according to a preferred embodiment of the invention. According to this embodiment, interactive stability display  100  is running on a laptop PC. The interactive stability display  100  obtains at least some of the parameters used in predicting the stability of the rock surrounding the wellbore from an earth model stored on storage system  412  of computer system  410 . Computer system  410  can be directly connected to the laptop PC via a network connection or dial-up connection, or it could be connected via a wireless connection. Furthermore, the connection between computer system  410  and the laptop PC can be permanent but is preferably temporarily established to load initial parameters and settings and to record and store output parameters such as orientation and/or mud weight. In some cases some data in the earth model can be updated in light of the results from the interactive stability display  100 . 
     The selected orientation and or mud weight is then used to construct or modify a well plan, as described above. The well plan may be on a separate computer  420  as shown in FIG. 5, the same laptop PC as display  100 , or it may be produced and used in hard-copy form. According to the invention, the well plan on computer  420  is then used to drill a well  424 . 
     FIG. 6 shows a portion of an interactive stability display according to another embodiment of the invention. In particular the window  510  is preferably used when planning the location and arrangement, or phasing, of perforations made during well completion in order to establish fluid communication between the surrounding reservoir rock and a conduit within the wellbore used to produced fluids. Many of the features of window  510  are as described with respect to FIG. 2 above. According to a preferred embodiment, the outer surface of the portion of the wellbore  124  is not shaded, but rather the perforations  520  are each shaded with colors according to the predicted stability of the rock surrounding the perforation. The surface of the wellbore  124  does not ordinarily need any color shading since the wellbore is normally cased at the time the perforations are shot. 
     The perforations  520  can also be arranged and relocated with respect to the portion of the wellbore preferably by clicking on the perforation and dragging the perforation to a new location. The user can also add new perforations through a menu or similar means. Other techniques could be used to add, delete, and move the location of perforations including: menus, radio buttons and the like. Another option for changing the arrangement of the perforations that is provided is for the user to rotate some or all of the perforations about the central axis of the portion of the wellbore. 
     According to the preferred embodiment, the perforations are always positioned perpendicular to the central axis of the portion of the wellbore because this is how most if not all perforations are commercially made. However, according to another embodiment of the invention the interactive display could allow for the changing of the inclination angle and orientation of a perforation relative to the central axis of the portion of the wellbore, which is initially set at 90 degrees. According to another embodiment, the length of a perforation can be changed from an initial value by right clicking on the perforation and entering a value in a pop up menu. According to another embodiment, a perforation can be selected by right clicking and then from a menu the user can choose to have a detailed view of the perforation in a format similar to that shown by the portion of the wellbore  124  and shaded area  126  in FIG. 2, except that the surface and shading represents the stability of the selected perforation instead of a wellbore. 
     FIG. 7 is a flow chart showing steps of making a completion plan and perforating a well according to an embodiment of the invention. In step  330  at least some of the parameters used by the interactive display for planning perforations are loaded from an existing earth model. In step  332  the interactive display for planning perforations is used by the user. In step  334  the selected or preferred parameters, typically the preferred location and direction for the perforations are obtained from the interactive display. In step  338  the preferred location and direction of the perforations are used to construct or modify a completion plan. Finally, in step  340  a well is perforated using the completion plan. It will be appreciated that the implementation shown in FIG.  5  and described above can be used with the embodiments for planning perforations for a well. 
     While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

Summary:
A method and system is disclosed for interactively displaying estimated stability of rock surrounding a wellbore. The display shows a three-dimensional representation of the orientation of a portion of the wellbore and the associated estimation of stability of the rock surrounding the wellbore. The user can alter the orientation of the portion of the wellbore, after which in real time the stability estimation is recalculated and redisplayed. The method and system can be used for planning or modifying a well plan, either before or during the drilling process. The method and system can also be used for diagnosis of stability problems. The method and system can also be used for displaying and analyzing the estimated stability of perforations surrounding a wellbore and for planning and arranging such perforations.