Measuring focused through-casing resistivity using induction chokes and also using well casing as the formation contact electrodes

A petroleum well for producing petroleum products, comprising:

• a piping structure of said well extending within a formation;
• a first induction choke located downhole about said piping structure;
• a second induction choke located downhole about said piping structure, wherein said second induction choke is distally spaced from said first induction choke;
• a third induction choke located downhole about said piping structure, wherein said third induction choke is distally spaced from said first and second induction chokes;
• a downhole device for measuring the electrical properties in the petroleum well in the region of one or more of said induction chokes comprising:
• a first device terminal electrically connected to said piping structure on a one side of said first induction choke,
• a second device terminal electrically connected to said piping structure on another side of said first induction choke between said first and second induction chokes, such that said downhole device is electrically connected across an outside of said first induction choke,
• a first electrode terminal electrically connected to said piping structure between said second and third induction chokes, and
• a second electrode terminal electrically connected to said piping structure such that said third induction choke is located between the electrical connection location on said piping structure of said first and second electrode terminals.

A petroleum well for producing petroleum products where formation resistivity is measured, comprising:

• a well casing extending into a formation;
• a power source being adapted to output a time-varying current, and said power source having two power source terminals, a first of said power source terminals being electrically connected to said casing at the surface, and a second of said power source terminals being electrically connected to said formation;
• a surface modem having two surface modem terminals, a first of said surface modem terminals being electrically connected to said casing at the surface, and a second of said surface modem terminals being electrically connected to said;
• a first induction choke located downhole about said casing;
• a second induction choke located downhole about said casing, wherein said second induction choke is distally spaced from said first induction choke, and wherein said second induction choke is located farther downhole than said first induction choke;
• a third induction choke located downhole about said casing, wherein said third induction choke is distally spaced from said first and second induction chokes, and wherein said third induction choke is located farther downhole than said second induction choke;
• a downhole device for measuring and communicating said formation resistivity comprising:
• a first device terminal electrically connected to said casing on a source-side of said first induction choke,
• a second device terminal electrically connected to said casing on another side of said first induction choke between said first and second induction chokes, such that said downhole device is electrically connected across an outside of said first induction choke,
• a first electrode terminal electrically connected to said casing between said second and third induction chokes,
• a second electrode terminal electrically connected to said casing such that said third induction choke is located between the electrical connection location on said casing of said first and second electrode terminals,
• a downhole modem being communicably coupled to said device terminals, such that said downhole modem can send and receive communication signals along said casing via said device terminals, and
• a current sensor adapted to measure current flowing through at least one of said first and second electrode terminals as an indication of formation resistivity, and said current sensor being communicably coupled to said downhole modem, such that said downhole modem is adapted to receive measurement data related to said current flow measure from said current sensor and transmit said measurement data to said surface modem via said casing.

A method of measuring formation resistivity in a petroleum well, comprising the steps of:

• providing a piping structure of said well;
• providing a system for measuring formation resistivity in said well, said system comprising:
• a first induction choke located about said piping structure,
• a second induction choke located about said piping structure, wherein said first induction choke is distally spaced from said second induction choke, and
• a device located outside of said piping structure and having a terminal electrically connected to said piping structure between said first and second induction chokes;
• inducing a current flow through the formation;
• and
• measuring the current flow through the terminal as an indication of the resistivity of the formation.