Downhole electromagnetic sensing techniques

Abstract

An electromagnetic (EM) telemetry system includes an EM transmitter configured to transmit EM signals downhole and multiple sensors each configured to communicate with the EM transmitter and with another of the multiple sensors. Each sensor is placed a distance from another sensor along a length of a wellbore in the EM telemetry system. The EM telemetry system also includes a processor configured to select two or more sensors of the multiple sensors based on a signal to noise ratio (SNR) of an EM signal received from the two or more selected sensors, a depth of the EM transmitter, or both.

What is claimed is: 
     
         1 . An electromagnetic (EM) telemetry system, comprising:
 a measuring while drilling (MWD) display computer;   an EM transmitter in a first wellbore;   an EM receiver in a second wellbore remote from the first wellbore, the EM receiver being configured to receive signals from the EM transmitter in the first wellbore; and   a wireless transmitter configured to transmit signals received at the EM receiver in the second wellbore to the MWD display computer.   
     
     
         2 . The system of  claim 1 , wherein the EM transmitter is a first EM transmitter and the signals are first signals, and further comprising a second EM transmitter in a third wellbore, and wherein the EM receiver is configured to receive second signals from the second EM transmitter. 
     
     
         3 . The system of  claim 1 , wherein the second wellbore is offset from the first wellbore by between 1,000 meters and 3,000 meters. 
     
     
         4 . The system of  claim 1 , wherein the wireless transmitter is configured to transmit a downlink EM signal to the EM receiver. 
     
     
         5 . The system of  claim 1 , wherein the wireless transmitter is configured to transmit the signals to a remote data network. 
     
     
         6 . The system of  claim 1 , further comprising a downhole decoder connected to the EM receiver in the second wellbore, wherein the downhole decoder is configured to reduce noise of the received signals. 
     
     
         7 . The system of  claim 1 , wherein the EM receiver is configured to transmit a downhole signal to the EM transmitter. 
     
     
         8 . The system of  claim 1 , further comprising an insulated cable to transmit the signals received at the EM receiver to the wireless transmitter. 
     
     
         9 . The system of  claim 1 , wherein a depth of the EM receiver is greater than or equal to a depth of the EM transmitter. 
     
     
         10 . A method for downhole electromagnetic (EM) telemetry, comprising:
 transmitting an EM signal from an EM transmitter located in a first wellbore;   receiving the EM signal at an EM receiver located in a second wellbore remote from the first wellbore; and   wirelessly transmitting the EM signal to an MWD display computer from a surface wireless transmitter.   
     
     
         11 . The method of  claim 10 , wherein the EM signal is a first EM signal, and further comprising receiving a second EM signal from a third wellbore remote from the first wellbore and the second wellbore. 
     
     
         12 . The method of  claim 10 , further comprising wirelessly transmitting an EM downlink signal from the surface wireless transmitter to the EM receiver. 
     
     
         13 . The method of  claim 10 , wherein the EM signal is a first EM signal, and further comprising transmitting a second EM signal from the EM receiver to the EM transmitter. 
     
     
         14 . The method of  claim 10 , further comprising at least partially decoding the EM signal at a downhole decoder in the second wellbore. 
     
     
         15 . The method of  claim 10 , further comprising transmitting the EM signal from the EM receiver to the surface wireless transmitter with a wired connection. 
     
     
         16 . The method of  claim 10 , further comprising wirelessly transmitting the EM signal to a remote data processor. 
     
     
         17 . The method of  claim 10 , further comprising at least partially decoding the EM signal at the MWD display computer. 
     
     
         18 . A method for downhole electromagnetic (EM) telemetry, comprising:
 at an MWD display computer, receiving a signal from a wireless transmitter located at a surface location of a first wellbore, wherein:
 the wireless transmitter receives the signal from an EM receiver in the first wellbore; and 
 the EM receiver receives the signal from an EM transmitter located in a second wellbore remote from the first wellbore. 
   
     
     
         19 . The method of  claim 18 , wherein the signal includes survey information from the second wellbore. 
     
     
         20 . The method of  claim 18 , wherein the MWD display computer is a remote data processor.