System, apparatus, and method of conducting borehole resistivity measurements

A method of measuring a resistivity of borehole fluid in a wellbore during drilling of the wellbore, said method comprising the steps of:

• lowering a measurement apparatus into the wellbore along with a drilling apparatus, wherein the measurement apparatus includes
• a generally longitudinally extending circumferential surface,
• a current emitting electrode disposed on the circumferential surface, and
• a current receiving electrode disposed on the circumferential surface and spaced apart from the current emitting electrode, the current receiving electrode being adapted to receive current propagated from the current emitting electrode, wherein an electrode array including the current electrodes occupies an arcuate section of the circumferential surface;
• during drilling, emitting a current from the current emitting electrode into a target region spaced laterally between the wall of the wellbore and the circumferential surface;
• during drilling, receiving, at the current receiving electrode, current emitted by the current emitting electrode into the target region, whereby the target region traversed by the emitted current extends less than 360° about said central axis;
• during drilling, measuring a voltage difference that is reflective of the resistivity of borehole fluid in the target region;
• and
• rotating the measurement apparatus in the wellbore through a range of 360°, whereby the emitting receiving and measuring steps are repeated a plurality of times during the 360° rotation to generate measurements corresponding to a plurality of angular tool positions;
• and
• wherein during said rotating step, the drilling apparatus is eccentered in the wellbore, and said repeated emitting, receiving, and measuring steps include performing said steps at a first angular position whereby the arcuate section is spaced from the borehole wall a minimum distance and at a second angular position wherein the actuate section is spaced a maximum distance from the borehole wall.

A measurement apparatus for measuring a resistivity of borehole fluid in a target region of a wellbore, said measurement apparatus comprising:

• a tool body having a central axis disposed in generally parallel relation with a longitudinal axis of the wellbore when said tool body is lowered into the wellbore, said tool body having an external, circumferential surface spaced radially outward from said central axis;
• a current emitting electrode disposed on said circumferential surface, said current emitting electrode being adapted to emit current into a target region spaced laterally between said circumferential surface and the wall of the wellbore;
• and
• a current receiving electrode disposed on said circumferential surface and spaced apart from said current emitting electrode, said current receiving electrode being adapted to receive current propagated from said current emitting electrode into the target region, wherein said electrodes are configured such that a circumferential span between said electrodes in the direction of a current path generated between the two electrodes extends less than about 180° about the central axis.