Method for drilling directional wells

Abstract

A method for detecting and sensing boundaries between strata in a formation during directional drilling so that the drilling operation can be adjusted to maintain the drillstring within a selected stratum is presented. The method comprises the initial drilling of an offset well from which resistivity of the formation with depth is determined. This resistivity information is then modeled to provide a modeled log indicative of the response of a resistivity tool within a selected stratum in a substantially horizontal direction. A directional (e.g., horizontal) well is thereafter drilled wherein resistivity is logged in real time and compared to that of the modeled horizontal resistivity to determine the location of the drill string and thereby the borehole in the substantially horizontal stratum. From this, the direction of drilling can be corrected or adjusted so that the borehole is maintained within the desired stratum.

What is claimed is: 
     
       1. A method of locating a substantially horizontal bed of interest in a formation and maintaining a drillstring therein during the drilling operation, said drillstring including a measurement-while-drilling (MWD) electromagnetic propagation resistivity sensor, comprising the steps of: drilling a substantially vertical offset well in a formation having at least one selected substantially horizontal bed therein;   measuring resistivity in the formation at the offset well to provide a first resistivity log as a function of depth;   modeling the substantially horizontal bed to provide a modeled resistivity log indicative of the resistivity taken along the substantially horizontal bed, said modeling being based on said first resistivity log;   drilling a directional well in said formation near said offset well, a portion of said directional well being disposed in said substantially horizontal bed;   measuring resistivity in said directional well using the MWD electromagnetic propagation resistivity sensor to provide a second log of resistivity taken substantially horizontally;   comparing said second log to said modeled log to determine the location of said directional well; and   adjusting the directional drilling operation so as to maintain said drillstring within said substantially horizontal bed during the drilling of said directional well in response to said comparing step.   
     
     
       2. The method of claim 1 wherein said second log and said modeled log comprises: at least one resistivity horn being indicative of a boundary between said bed and an adjacent bed.   
     
     
       3. The method of claim 2 wherein said horn is a phase difference resistivity horn. 
     
     
       4. The method of claim 2 wherein said horn is an amplitude ratio resistivity horn. 
     
     
       5. The method of claim 1 wherein said second log and said modeled log are phase difference resistivity logs. 
     
     
       6. The method of claim 1 wherein said second log and said modeled resistivity log are amplitude ratio resistivity logs. 
     
     
       7. The method of claim 1 wherein said adjusting step further includes: maintaining said drillstring within the upper portion of said bed during the drilling of said directional well in response to said comparing.   
     
     
       8. A method of locating a substantially horizontal bed of interest in a formation and maintaining a drillstring therein during the drilling operation, said drillstring including a measurement-while-drilling (MWD) electromagnetic propagation resistivity sensor, comprising the steps of: drilling a substantially vertical offset well in a formation having at least one selected substantially horizontal bed therein;   measuring resistivity in the formation at the well to provide a first resistivity log as a function of depth;   modeling the substantially horizontal bed to provide a modeled resistivity log indicative of the resistivity taken along the substantially horizontal bed, said modeling being based on said first resistivity log;   drilling a substantially horizontal well within said substantially horizontal bed;   measuring resistivity in said horizontal well using the MWD electromagnetic propagation resistivity sensor to provide a second log of resistivity taken substantially horizontally;   comparing said second log to said modeled log to determine the location of said horizontal well; and   adjusting the horizontal drilling operation so as to maintain said drillstring within said substantially horizontal bed during the drilling of said well in response to said comparing step.   
     
     
       9. A method of locating a substantially horizontal bed of interest in a formation and maintaining a drillstring therein during the drilling operation, said drillstring including a measurement-while-drilling (MWD) electromagnetic propagation resistivity sensor, comprising the steps of: generating a first resistivity log for a substantially horizontal bed using assumed resistivity values for a desired course of drilling in a preselected dipping bed model to provide a modeled resistivity log indicative of the resistivity taken along the substantially horizontal bed in a formation;   drilling a directional well in said formation, a portion drilling a directional well in said formation, a portion of said directional well being disposed in said substantially horizontal bed;   measuring resistivity in said directional well using the MWD electromagnetic propagation resistivity sensor to provide a second log of resistivity taken substantially horizontally;   comparing said second log to said modeled first log to determine the location of said directional well; and   adjusting the directional drilling operation so as to maintain said drillstring within said substantially horizontal bed during the drilling of said directional well in response to said comparing step.   
     
     
       10. The method of claim 9 wherein said second log and said modeled log comprises: at least one resistivity horn being indicative of a boundary between said bed and an adjacent bed.   
     
     
       11. The method of claim 10 wherein said horn is a phase difference resistivity horn. 
     
     
       12. The method of claim 10 wherein said horn is an amplitude ratio resistivity horn. 
     
     
       13. The method of claim 9 wherein said second log and said modeled log are phase difference resistivity logs. 
     
     
       14. The method of claim 9 wherein said second log and said modeled resistivity log are amplitude ratio resistivity logs. 
     
     
       15. The method of claim 9 wherein said adjusting step further includes: maintaining said drillstring within the upper portion of said bed during the drilling of said directional well in response to said comparing.   
     
     
       16. A method of locating a substantially horizontal bed of interest in a formation and maintaining a drillstring therein during the drilling operation, said drillstring including a measurement-while-drilling (MWD) electromagnetic propagation resistivity sensor, comprising the steps of: generating a group of dipping bed model logs for various expected downhole conditions;   drilling a directional well in a formation, a portion of said directional well being disposed in a substantially horizontal bed;   measuring resistivity in said directional well using the MWD electromagnetic propagation resistivity sensor to provide a real time log of resistivity taken substantially horizontally;   comparing said real time log to at least one of the model logs in said group of dipping bed model logs until one of said model logs substantially matches said real time log defining a matched model log;   using input parameters from said matched model log to determine the location of said directional well; and   adjusting the directional drilling operation so as to maintain said drillstring within said substantially horizontal bed during the drilling of said directional well in response to said comparing step.   
     
     
       17. A method of locating a substantially horizontal bed of interest in a formation and maintaining a drillstring therein during the drilling operation, said drillstring including a measurement-while-drilling (MWD) electromagnetic propagation resistivity sensor, comprising the steps of: drilling a directional well in a formation, a portion of said directional well being disposed in a substantially horizontal bed;   measuring resistivity in said directional well using the MWD electromagnetic propagation resistivity sensor to provide a real time log of resistivity taken substantially horizontally;   generating a model resistivity log from a preselected dipping bed model for a substantially horizontal bed using input values selected so that the model resistivity log will substantially match the real time log defining a matched model log;   using input parameters from said matched model log to determine the location of said directional well; and   adjusting the directional drilling operation so as to maintain said drillstring within said substantially horizontal bed during the drilling of said directional well in response to said comparing step. .Iadd.   
     
     
       18.  A method of locating a bed of interest in a formation and maintaining a drillstring therein during the drilling operation, said drillstring including a measurement-while-drilling (MWD) resistivity sensor, comprising the steps of: (1) modeling at least one substantially horizontal bed to provide a modeled resistivity log indicative of the resistivity taken along the substantially horizontal bed, said modeling being based on information related to said substantially horizontal bed;   (2) drilling a directional well, a portion of said directional well intended to be disposed in said substantially horizontal bed;   (3) measuring resistivity in said directional well using the MWD resistivity sensor to provide a measured log of resistivity; and   (4) comparing the measured log to said modeled log to determine the location or position of said directional well with respect to said substantially horizontal bed. .Iaddend..Iadd.19. The method of claim 18 including the step of:   adjusting the directional drilling operation so as to maintain said drillstring within said substantially horizontal bed during the drilling of said directional well in response to said comparing step. .Iaddend..Iadd.20. The method of claim 18 wherein:   said information is selected from at least one of (a) data from an offset well which intersects with said horizontal bed, (b) seismic data, (c) assumptions of the horizontal bed, and (d) prior geological knowledge of   
     
     
        the horizontal bed. .Iaddend..Iadd.21.  The method of claim 18 including the steps of: identifying at least one resistivity horn in said measured log; and   using said resistivity horn as an indicator of a boundary between said substantially horizontal bed and an adjacent bed. .Iaddend..Iadd.22. The method of claim 21 wherein said comparing step further includes:   comparing said measured resistivity horn to said modeled resistivity log to determine the location or position of said directional well. .Iaddend..Iadd.23. The method of claim 21 wherein said horn is a phase difference resistivity horn. .Iaddend..Iadd.24. The method of claim 21 wherein said horn is an amplitude ratio resistivity horn. .Iaddend..Iadd.25. The method of claim 18 wherein said measured log and said modeled log are phase difference resistivity logs. .Iaddend..Iadd.26. The method of claim 18 wherein said measured log and said modeled log are amplitude ratio resistivity logs. .Iaddend..Iadd.27. The method of claim 18 wherein said measured log and said modeled log include both phase difference resistivity logs and amplitude ratio resistivity logs.   
     
     
        .Iaddend..Iadd.28.  The method of claim 19 wherein said adjusting step further includes: maintaining said drillstring within the upper portion of said bed during the drilling of said directional well. .Iaddend..Iadd.29. The method of claim 18 wherein said measured log includes resistivity from phase difference and resistivity from amplitude ratio and including the steps of:   identifying separations between said phase difference and amplitude ratio in said measured log; and   using said separations as an indicator of a boundary between said substantially horizontal bed and an adjacent bed. .Iaddend..Iadd.30. The method of claim 18 including the steps of:   identifying resistivity drops in said measured log; and   using said resistivity drops as an indicator of a boundary between said bed and an adjacent bed. .Iaddend..Iadd.31. The method of claim 18 wherein:   said MWD resistivity sensor comprises an electromagnetic propagation resistivity tool. .Iaddend..Iadd.32. The method of claim 18 wherein:   said measured log and said modeled log are derived from at least one of phase and amplitude measurements of an electromagnetic propagation   
     
     
        resistivity tool. .Iaddend..Iadd.33.  The method of claim 18 wherein: said measured log and said modeled log are derived from at least one of phase and amplitude measurements of an electromagnetic propagation   
     
     
        resistivity tool. .Iaddend..Iadd.34.  A method of locating a bed of interest in a formation and maintaining a drillstring therein during the drilling operation, said drillstring including a measurement-while-drilling (MWD) resistivity sensor, comprising the steps of: measuring resistivity in the formation at an offset well to provide a first resistivity log as a function of depth, the offset well having at least one selected substantially horizontal bed therein;   modeling the substantially horizontal bed to provide a modeled resistivity log indicative of the resistivity taken along the substantially horizontal bed, said modeling being based on said first resistivity log;   drilling a directional well in said formation near said offset well, a portion of said directional well intended to be disposed in said substantially horizontal bed;   measuring resistivity in said directional well using the MWD resistivity sensor to provide a second log of resistivity taken substantially horizontally; and   comparing said second log to said modeled log to determine the location or position of said directional well with respect to said substantially horizontal bed. .Iaddend..Iadd.35. The method of claim 34 including the step of:   adjusting the directional drilling operation so as to maintain said drillstring within said substantially horizontal bed during the drilling of said directional well in response to said comparing step.   
     
     
        .Iaddend..Iadd.36.  The method of claim 34 wherein said adjusting step further includes: maintaining said drillstring within the upper portion of said bed during   
     
     
        the drilling of said horizontal well. .Iaddend..Iadd.37.  A method of locating a bed of interest in a formation and maintaining a drillstring therein during the drilling operation, said drillstring including a measurement-while-drilling (MWD) resistivity sensor, comprising the steps of: (1) modeling at least one substantially horizontal bed to provide a modeled resistivity log indicative of the resistivity taken along the substantially horizontal bed, said modeling being based on information related to said substantially horizontal bed;   (2) drilling a substantially horizontal well within said substantially horizontal bed, a portion of said horizontal well intended to be disposed in said substantially horizontal bed;   (3) measuring resistivity in said horizontal well using the MWD resistivity sensor to provide a measured log of resistivity taken substantially horizontally; and   (4) comparing the measured log to said modeled log to determine the location or position of said horizontal well with respect to said substantially horizontal bed. .Iaddend..Iadd.38. The method of claim 37 including the step of:   adjusting the directional drilling operation so as to maintain said drillstring within said substantially horizontal bed during the drilling of said horizontal well in response to said comparing step. .Iaddend..Iadd.39. The method of claim 37 wherein:   said information is selected from at least one of (a) data from an offset well which bed, and (d) prior geological knowledge of the horizontal bed.   
     
     
        .Iaddend..Iadd.40.  The method of claim 37 including the steps of: identifying at least one resistivity horn in said measured log; and   using said resistivity horn as an indicator of a boundary between said substantially horizontal bed and an adjacent bed. .Iaddend..Iadd.41. The method of claim 40 wherein said comparing step further includes:   comparing said measured resistivity horn to said modeled resistivity log to determine the location or position of said directional well. .Iaddend..Iadd.42. The method of claim 40 wherein said horn is a phase difference resistivity horn. .Iaddend..Iadd.43. The method of claim 40 wherein said horn is an amplitude ratio resistivity horn. .Iaddend..Iadd.44. The method of claim 37 wherein said measured log and said modeled log are phase difference resistivity logs. .Iaddend..Iadd.45. The method of claim 37 wherein said measured log and said modeled log are amplitude ratio resistivity logs. .Iaddend..Iadd.46. The method of claim 37 wherein said measured log and said modeled log include both phase difference resistivity logs and amplitude ratio resistivity logs.   
     
     
        .Iaddend..Iadd.47.  The method of claim 37 wherein said measured log includes resistivity from phase difference and resistivity from amplitude ratio and including the steps of: identifying separations between said phase difference and amplitude ratio in said measured log; and   using said separations as an indicator of a boundary between said bed and an adjacent bed. .Iaddend..Iadd.48. The method of claim 37 including steps of:   identifying resistivity drops in said measured log; and   using said resistivity drops as an indicator of a boundary between said bed   
     
     
        and an adjacent bed. .Iaddend..Iadd.49.  A method of locating a substantially horizontal bed of interest in a formation and maintaining a drillstring therein during the drilling operation, said drillstring including a measurement-while-drilling (MWD) resistivity sensor, comprising the steps of: generating a modeled resistivity log for a substantially horizontal bed using assumed resistivity values for a desired course of drilling from a preselected dipping bed model, said modeled resistivity log being indicative of the resistivity taken along the substantially horizontal bed in a formation;   drilling a directional well in said formation, a portion of said directional well intended to be disposed in said substantially horizontal bed;   measuring resistivity in said directional well using the MWD resistivity sensor to provide a log of resistivity taken substantially horizontally; and   comparing said measured log to said modeled log to determine the location or position of said directional well with respect to said substantially horizontal bed. .Iaddend..Iadd.50. The method of claim 49 including the steps of:   adjusting the directional drilling operation so as to maintain said drillstring within said substantially horizontal bed during the drilling of said directional well in response to said comparing step.   
     
     
        .Iaddend..Iadd.51.  A method of locating a substantially horizontal bed of interest in a formation and maintaining a drillstring therein during the drilling operation, said drillstring including a measurement-while-drilling (MWD) resistivity sensor, comprising the steps of: generating a group of dipping bed model logs for various expected downhole conditions;   drilling a directional well in a formation, a portion of said directional well intended to be disposed in a substantially horizontal bed;   measuring resistivity in said directional well using the MWD resistivity sensor to provide a real time log of resistivity taken substantially horizontally;   comparing said real time log to at least one of the model logs in said group of dipping bed model logs until one of said model logs substantially matches said real time log defining a matched model log; and   using input parameters from said matched model log to determine the location or position of said directional well with respect to said substantially horizontal bed. .Iaddend..Iadd.52. The method of claim 51 including the step of:   adjusting the directional drilling operation so as to maintain said drillstring within said substantially horizontal bed during the drilling of said directional well in response to said comparing step.   
     
     
        .Iaddend..Iadd.53.  A method of locating a substantially horizontal bed of interest in a formation and maintaining a drillstring therein during the drilling operation, said drillstring including a measurement-while-drilling (MWD) resistivity sensor, comprising the steps of: drilling a directional well in a formation, a portion of said directional well being disposed in a substantially horizontal bed;   measuring resistivity in said directional well using the MWD resistivity sensor to provide a real time log of resistivity taken substantially horizontally;   generating a model resistivity log from a preselected dipping bed model for a substantially horizontal bed using input values selected so that the model resistivity log will substantially match the real time log defining a matched model log; and   using input parameters from said matched model log to determine the location or position of said directional well with respect to said substantially horizontal bed. .Iaddend..Iadd.54. The method of claim 53 including the step of:   adjusting the directional drilling operation so as to maintain said drillstring within said substantially horizontal bed during the drilling of said directional well in response to said comparing step.   
     
     
        .Iaddend..Iadd.55.  For use in connection with an earth borehole drilling apparatus that includes a drilling rig, a drillstring operating from said drilling rig for drilling an earth borehole, said drillstring including a bottom hole arrangement comprising a drill bit, a downhole resistivity measuring subsystem for measuring downhole formation resistivity near said bit by propagating electromagnetic energy into earth formations near said bit, receiving electromagnetic energy that has propagated through the formations and producing measurement signals that depend on the received signals; a method for directing the drilling of a well bore with respect to a geological bed boundary in said earth formations, comprising the steps of: producing from said measurement signals a recording of downhole formation resistivity as a function of borehole depth, said formation resistivity being in the form of a log depicting phase difference and amplitude ratio;   identifying separations between said phase difference and amplitude ratio in said log; and   implementing a change in the drilling direction of said drill bit in response to said identification of any separations between said phase   
     
     
        difference and amplitude ratio. .Iaddend..Iadd.56.  For use in conjunction with an earth borehole directional drilling apparatus that includes a drilling rig, a drillstring operating from said drilling rig for drilling an earth borehole, said drillstring including a bottom hole arrangement comprising a drill bit, a downhole resistivity measuring subsystem for measuring downhole earth formation resistivity near said bit by propagating electromagnetic energy into earth formations near said bit, receiving electromagnetic energy that has propagated through the formations and producing measurement signals that depend on the received signals and downhole telemetry means for transmitting the measurement signals; wherein said apparatus has initiated drilling a well bore within a first geological bed in said formations and approximately adjacent a geological bed boundary between said first bed and a second geological bed in said formations; a method for maintaining drilling of the well bore in said first bed, comprising the steps of: deriving from said measurement signals, downhole formation resistivity, said formation resistivity being in the form of a log depicting phase difference and amplitude ratio;   monitoring said resistivity, as a function of the borehole depth of the drill bit position;   identifying separations between said phase difference and amplitude ratio in said log; and   implementing changes in the drilling direction of said drill bit in response to the identification of any separations between said phase   
     
     
        difference and amplitude ratio. .Iaddend..Iadd.57.  For use in conjunction with an earth borehole drilling apparatus that includes a drilling rig, a drillstring operating from said drilling rig for drilling an earth borehole, said drillstring including a bottom hole arrangement comprising a drill bit, a downhole resistivity measuring subsystem for measuring downhole formation resistivity near said bit, receiving electromagnetic energy that has propagated through the formations and producing measurement signal that depend on the received signals; a method for directing the drilling of a well bore with respect to a geological bed boundary in said earth formations, comprising the steps of: producing from said measurement signals a recording of downhole formation resistivity as a function of borehole depth;   identifying resistivity drops in said resistivity recording; and   implementing a change in the drilling direction of said drill bit in response to said identification of any resistivity drops.     
     
     
        .Iaddend..Iadd.  .  For use in conjunction with an earth borehole directional drilling apparatus that includes a drilling rig, a drillstring operating from said drilling rig for drilling an earth borehole, said drillstring including a bottom hole arrangement comprising a drill bit, a downhole resistivity measuring subsystem for measuring downhole earth formation resistivity near said bit by propagating electromagnetic energy into earth formations near said bit, receiving electromagnetic energy that has propagated through the formations and producing measurement signals that depend on the received signals, and downhole telemetry means for transmitting the measurement signals uphole and uphole telemetry means for receiving the measurement signals; wherein said apparatus has initiated drilling a well bore with a first geological bed in said formations and approximately adjacent a geological bed boundary between said first bed and a second geological bed in said formations; a method for maintaining drilling of the well bore ins aid first bed, comprising the steps of: deriving from said measurement signals, downhole formation resistivity;   monitoring said resistivity as a function of the borehole depth of the drill bit position to detect any resistivity drops; and   implementing changes in the drilling direction of said drill bit in   
     
     
        response to the detection of any resistivity drops. .Iaddend..Iadd.59.  A method of locating a bed of interest in a formation and maintaining a drillstring therein during the drilling operation, said drillstring including a measurement-while-drilling (MWD) sensor, comprising the steps of: (1) modeling at least one substantially horizontal bed to provide a modeled log of a set of parameters indicative of the response taken as the MWD sensor traverses or moves within the bed, said modeling being based on information related to said substantially horizontal bed and the path of the MWD sensor;   (2) drilling a directional well, a portion of said directional well intended to be disposed in said substantially horizontal bed;   (3) measuring and recording MWD sensor output parameters indicative of the formation parameters in said directional well using the MWD sensor to provide a measured log of said output parameters; and   (4) comparing the measured log to said modeled log to determine the location or position of said directional well with respect to said substantially horizontal bed. .Iaddend..Iadd.60. The method of claim 59 including the step of:   adjusting the directional drilling operation so as to maintain said drillstring within said substantially horizontal bed during the drilling of said directional well in response to said comparing step. .Iaddend..Iadd.61. The method of claim 59 wherein:   said information is selected from at least one of (a) data from an offset well which intersects with said horizontal bed, (b) seismic data, (c) assumptions of the horizontal bed, and (d) prior geological knowledge of the horizontal bed. .Iaddend.