Method of analyzing a subterranean formation and method of producing a mineral hydrocarbon fluid from the formation

Abstract

Method of analyzing a subterranean formation traversed by a wellbore. The method uses a tool comprising a transmitter antenna and a receiver antenna, the subterranean formation comprising one or more formation layers. The tool is suspended inside the wellbore, and one or more electromagnetic fields are induced in the formation. One or more time-dependent transient response signals are detected and analyzed. Electromagnetic anisotropy of at least one of the formation layers is detectable. Geosteering cues may be derived from the time-dependent transient response signals, for continued drilling of the well bore until a hydrocarbon reservoir is reached. The hydrocarbon may then be produced.

1 . A method of analyzing a subterranean formation traversed by a wellbore, using a tool comprising a transmitter antenna and a receiver antenna, the subterranean formation comprising one or more formation layers and the method comprising:
 suspending the tool inside the wellbore;   inducing one or more electromagnetic fields in the formation;   detecting one or more time-dependent transient response signals;   analyzing the one or more time-dependent transient response signals taking into account electromagnetic anisotropy of at least one of the formation layers.   
   
   
       2 . The method of  claim 1 , wherein the at least one formation layer comprises three or more sub-layers. 
   
   
       3 . The method of  claim 2 , wherein one of the three or more sub-layers has a first resistivity or conductivity that is different from a second resistivity or conductivity of another one of the three or more sub-layers. 
   
   
       4 . The method of  claim 2 , wherein the sub-layers that are not individually resolved in the transient response signals jointly are approximated as one anisotropic formation layer. 
   
   
       5 . The method of  claim 1 , wherein analyzing the one or more time-dependent transient response signals taking into account electromagnetic anisotropy includes deriving an anisotropy parameter of the at least one formation layer from the detected one or more time-dependent transient response signals. 
   
   
       6 . The method of  claim 5 , wherein the anisotropy parameter comprises at least one from a group of parameters comprising anisotropy ratio, anisotropic factor, conductivity along a principal anisotropy axis, resistivity along the principal anisotropy axis, conductivity in a plane perpendicular to the principal anisotropy axis, resistivity in a plane perpendicular to the principal anisotropy axis; tool axis angle relative to the principal anisotropy axis. 
   
   
       7 . The method of  claim 1 , wherein analyzing the one or more time-dependent transient response signals comprises combining multi-axial transient measurements to derive an anisotropy parameter. 
   
   
       8 . The method of  claim 1 , wherein analyzing the one or more time-dependent transient response signals taking into account electromagnetic anisotropy comprises deriving at least one of time-dependent apparent conductivity, time dependent apparent resistivity, time-dependent dip angle, and time-dependent azimuth angle from the time dependence of the transient response signals. 
   
   
       9 . The method of  claim 1 , wherein one of the formation layers comprises an anomaly, and wherein analyzing the one or more time-dependent transient response signals comprises determining at least one of a distance and a direction between the tool and the anomaly from the one or more time-dependent transient response signals. 
   
   
       10 . The method of  claim 1 , wherein inducing one or more electromagnetic fields in the formation comprises generating a transmission and terminating the transmission, and detecting one or more time-dependent transient response signals comprises measuring a receiver response as a function of time following the terminating the transmission. 
   
   
       11 . A method of producing a mineral hydrocarbon fluid from an earth formation, the method comprising steps of:
 suspending a drill string in the earth formation, the drill string comprising at least a drill bit and measurement sub comprising a transmitter antenna and a receiver antenna;   drilling a well bore in the earth formation;   inducing an electromagnetic field in the earth formation employing the transmitter antenna;   detecting one or more time-dependent transient electromagnetic response signals from the electromagnetic field, employing the receiver antenna;   deriving a geosteering cue from the electromagnetic response;   continue drilling the well bore in accordance with the geosteering cue until a reservoir containing the hydrocarbon fluid is reached;   producing the hydrocarbon fluid.   
   
   
       12 . The method of  claim 11 , wherein drilling the well bore comprises operating a steerable drilling system in the earth formation. 
   
   
       13 . The method of  claim 11 , wherein inducing the electromagnetic field in the earth formation comprises generating a transmission and terminating the transmission, and detecting one or more time-dependent transient response signals comprises measuring a receiver response as a function of time following the terminating the transmission. 
   
   
       14 . The method of  claim 11 , wherein deriving the geosteering cue comprises analyzing the one or more transient response signals taking into account electromagnetic anisotropy of at least one of the formation layers. 
   
   
       15 . The method of  claim 11 , wherein deriving the geosteering cue comprises locating an electromagnetic anomaly in the earth formation based on the one or more time-dependent transient response signals. 
   
   
       16 . The method of  claim 15 , wherein locating the electromagnetic anomaly comprises determining at least one of a distance from the measurement sub to the anomaly and a direction from the measurement sub to the anomaly. 
   
   
       17 . The method of  claim 16 , wherein determining the distance comprises determining a time in which one of apparent conductivity and apparent resistivity begins to deviate from the corresponding one of conductivity and resistivity of formation in which the device is located. 
   
   
       18 . The method of  claim 16 , wherein determining the distance comprises determining a time in which one of apparent dip and apparent azimuth reaches an asymptotic value. 
   
   
       19 . The method of  claim 16 , wherein determining the distance comprises determining a time in which one of apparent dip and apparent azimuth and cross-component response, reaches a non-zero value. 
   
   
       20 . The method of  claim 15 , wherein locating the electromagnetic anomaly comprises deriving at least one of time-dependent apparent conductivity, time dependent apparent resistivity, time-dependent dip angle, and time-dependent azimuth angle from the time dependence of the transient response. 
   
   
       21 . The method of  claim 11 , wherein deriving the geosteering cue comprises deriving at least one of time-dependent apparent conductivity, time dependent apparent resistivity, time-dependent dip angle, and time-dependent azimuth angle from the time dependence of the transient response. 
   
   
       22 . A computer readable medium storing computer readable instructions that analyze one or more detected time-dependent transient electromagnetic response signals that have been detected by a tool suspended inside a wellbore traversing a subterranean formation after inducing one or more electromagnetic fields in the formation, wherein the computer readable instructions take into account electromagnetic anisotropy of at least one formation layer in the subterranean formation.