Sub-reamer for bi-center type tools

1. A downhole drilling tool comprising a bit body which defines a rotational axis, a first end adapted to be detachably secured to a drill string, a pilot section on a second, opposite end of the bit body and an eccentric reamer section positioned intermediate the first and second end sections, said tool defining a maximum tool diameter and a pass-through diameter as measured from the pass-through axis, said tool further comprising:

a sub-reamer section disposed intermediate the reamer and pilot sections;

a plurality of cutting blades formed radially on said sub-reamer section about said rotational axis where said cutting blades collectively define an arc equal to or greater than 180 degrees;

said cutting blades each defining an endpoint; and

where the endpoints on said cutting blades being positioned a distance less than or equal to one-half of the maximum tool diameter, as measured from the rotational axis.

2. The downhole tool of claim 1 where said cutting blades formed on said sub-reamer section are substantially disposed about a radial arc of 360 degrees.

3. The downhole tool of claim 1 wherein the endpoints defined by the cutting blades formed on said sub-reamer section are of a dissimilar distance from the rotational axis.

4. The downhole tool of claim 1 wherein the endpoints defined by the cutting blades are of the same distance from the rotational axis.

5. A downhole drilling tool comprising a bit body which defines a rotational axis and a pass-through axis, a first end adapted to be detachably secured to a drill string, a pilot section on a second, opposite end of the bit body and an eccentric reamer section intermediate the first and second end, said reamer section describing a reamer drill diameter and including one or more cutting blades which describe a radial arc having a midpoint, said tool further defining a maximum tool diameter and a pass-through diameter, where said reamer drill diameter and said maximum tool diameter being measured from the rotational axis and said pass-through diameter being measured from the pass-through axis, said tool further comprising:

a sub-reamer section disposed intermediate the reamer and pilot sections;

a first region of the sub-reamer section being defined by the intersection of the pass-through diameter and the reamer drill diameter where said first region is located opposite the reamer section;

a second region of the sub-reamer being defined by the intersection of the pass-through diameter and the reamer drill diameter, where said second region is located opposite of and complementary to said first region;

said sub-reamer provided with one or more radially-extending cutting blades which each define endpoints;

said endpoints on said cutting blades provided on said sub-reamer and disposed in said first region extending no further than one-half of the pass-through diameter, as measured from the rotational axis; and

the endpoints on said cutting blades provided on said sub-reamer and disposed in said second region extending no further than one-half of the reamer drill diameter, as measured from the rotational axis.

6. The downhole tool of claim 5 where the cutting blades on the sub-reamer define a radial arc as measured about the rotational axis equal to or greater than 180 degrees.

7. The downhole tool of claim 5 wherein the endpoints defined by the cutting blades formed on the sub-reamer do not extend beyond one-half of the maximum tool diameter, as measured from the rotational axis.

8. The downhole tool of claim 5 where said bit body is formed of one or more detachable segments.

9. A downhole drilling tool comprising a bit body which defines a rotational axis, a first end adapted to be detachably secured to a drill string, a pilot section formed on a second, opposite end of the bit body and an eccentric reamer section intermediate the first and second end, said reamer section describing a reamer drill diameter or “RDD” and including one or more cutting blades which describe an arc having a midpoint Q, said tool defining a maximum tool diameter and a pass-through diameter, where the reamer drill diameter and the maximum tool diameter are measured from the rotational axis and the pass-through diameter is measured from a pass-through axis, said tool further comprising:

a sub-reamer section disposed intermediate the reamer and pilot sections;

a first region of the sub-reamer section defined by the intersection of the pass-through diameter and the reamer drill diameter, where said first region is formed opposite the reamer section;

a second region of the sub-reamer defined by the intersection of the pass-through diameter and the reamer drill diameter, where said second region is complementary to said first region;

said sub-reamer provided with one or more cutting blades which each define endpoints, where the endpoints on those cutting blades disposed in said first region extend no further than one-half of the pass-through diameter as measured from the rotational axis, and where the arc defined by said cutting blades is centered about a line drawn through the midpoint Q and said rotational axis.

10. The downhole tool of claim 9 where said bit body is formed of one or more detachable segments.

11. A bi-center bit having enhanced stability comprising:

a body defining a proximal end adapted for connection to a drill string and a distal end, where said distal end defines a pilot bit and an intermediate reamer section, where both the pilot bit and the reamer section include radially-extending upsets, each of which defines cutting surfaces, the upsets formed on the reamer section defining a leading cutting surface and one or more trailing cutting surfaces, where each of said upsets defines an endpoint;

said body defining a pass-through axis and a rotational axis;

a plurality of cutter assemblies being radially disposed about the cutting surfaces of the pilot bit and the reamer section;

the endpoint defined by the leading and trailing cutting surfaces of said reamer defining an arc having a midpoint;

said reamer section defining a reamer drill diameter as measured from the rotational axis;

a sub-reamer section disposed intermediate the pilot and the reamer sections;

first radially-extending upsets formed on said reamer section, where said first upsets are disposed in a first region defined by the intersection of the pass-through diameter and the reamer drill diameter, where the first region is opposite the mid-point of the reamer section;

second radially-extending upsets formed on said reamer section, where said first upsets are disposed in a second region formed complementary to said first region and oriented about the midpoint;

where the upsets formed in both the first and second regions define endpoints; and

the endpoints on these blades disposed on said sub-reamer section and located in said first region being spaced a selected distance from the rotational axis, where said distance is less than or equal to one-half of the pass-through diameter, where the endpoints on these blades disposed in the second region of said sub-reamer section extend a distance from the rotational axis one-half of the reamer drill diameter, as again measured from the rotational axis.