Logging while drilling apparatus with multiple depth of resistivity investigation

A method of measuring resistivity while drilling comprising the steps of:

• (a) with a drill stem having a drill bit at the lower end and coil means supported on a tubular member thereabove, drilling a well borehole through a formation wherein drilling is accompanied by circulating drilling fluid through the well borehole to lubricate the drill bit;
• (b) forming an electromagnetic field from a coil means extending into the formation penetrated by the drill bit so that a first measurement indicative of formation resistivity without fluid invasion is obtained;
• (c) forming an electromagnetic field from a coil means extending into the formation penetrated by the drill bit so that a second measurement indicative of formation resistivity with fluid invasion is obtained wherein the electromagnetic fields are formed at audio and radio frequencies;
• and
• (d) sending the first and second measurements to the surface while drilling.

A method of dynamically logging while drilling a well borehole with a drill bit below a mandrel serially connected in a drill stem comprising the steps of:

• (a) drilling the well borehole with the drill bit;
• (b) flowing drilling fluid through the drill stem and drill bit during drilling to remove cuttings from the drill bit while fluid is circulated along the borehole and invades formations adjacent to the borehole;
• (c) during drilling, forming a field at audio and radio frequencies around the mandrel in formations adjacent to the borehole so that multiple resistivity measurements are obtained from the formations wherein first measurements are from formations prior to drilling fluid invasion and later measurements are from formations after drilling fluid invasion;
• and
• (d) transmitting while drilling the measurements to the surface.

A method of dynamically logging while advancing a well borehole with a drill bit connected below a coil supporting mandrel wherein drilling is accomplished by continuously flowing a drilling fluid through a drill stem including the mandrel and wherein the mandrel supports coils thereon for making resistivity measurements, the method comprising the steps of:

• (a) alternately transmitting upwardly and downwardly relative to the well borehole electromagnetic signals at audio and radio frequencies along the drill stem and into the formations adjacent to the well borehole wherein the transmission extends a specified depth radially in to the formations adjacent to the well borehole;
• (b) receiving at two spaced locations along the mandrel the transmitted signals;
• (c) measuring the phase shift between the two received signals;
• (d) as a function of phase shift, determining formation resistivity;
• and
• (e) transmitting formation resistivity to the surface during continued drilling.

A method of dynamically logging while advancing a well borehole with a drill bit connected below a coil supporting mandrel wherein the well borehole intercepts subsurface formations adjacent to the well borehole and the formations have a relative dip angle and the method comprises the steps of:

• (a) while drilling the well borehole with a drill bit, forming electromagnetic audio and radio frequency fields extending radially outwardly therefrom into formations around the well borehole around the mandrel wherein the field enables resistivity measurements, and periodically forming resistivity measurements;
• (b) serially passing the resistivity measurements through a filter means wherein the number of stages of the filter is N (a positive integer) and increasing N as the relative dip angle of the formation with respect to the well borehole increases;
• and
• (c) transmitting formation resistivity to the surface during continued drilling.

A method of measuring resistivity while drilling comprising the steps of:

• (a) with a drill stem having a drill bit at the lower end and coil means supported on a tubular member thereabove, drilling a well borehole through a formation wherein drilling is accompanied by circulating drilling fluid through the well borehole to lubricate the drill bit;
• (b) forming an electromagnetic field from a coil means extending into the formation penetrated by the drill bit so that an earlier measurement indicative of formation resistivity without fluid invasion is obtained;
• (c) forming an electromagnetic field from a coil means extending into the formation penetrated by the drill bit so that a later measurement indicative of formation resistivity with fluid invasion is obtained and wherein the fields of steps (b) and (c) are audio and radio frequency fields;
• and
• (d) sending the earlier and later measurements to the surface while drilling.