Methods and systems of analyzing wellbore drilling operations

1. A method for analyzing a wellbore drilling operation, the method comprising:

acquiring sonic log data, gamma ray log data, and rate of penetration (ROP) data, wherein the sonic log data, the gamma ray log data, and the ROP data are associated with depth intervals of a wellbore;

generating mathematically averaged ROP data for the depth intervals of the wellbore;

determining a lithology of a rock formation:

using well log data limited to the sonic log data and the gamma ray log data and drilling data limited to the ROP data,

based on the sonic log data, the gamma ray log data, an upper gamma ray threshold, a lower gamma ray threshold, and one or more sonic data thresholds, and

by adjusting one or more of the upper gamma ray threshold, the lower gamma ray threshold, or the one or more sonic data thresholds such that the lithology of the rock formation correlates with the mathematically averaged ROP data;

determining unconfined compressive strength (UCS) of the rock formation associated with the wellbore using the determined lithology;

selecting drilling parameters for drilling a second wellbore using the determined UCS, wherein the drilling parameters include weight-on-bit and bit rotational speed; and

drilling the second wellbore using the selected drilling parameters.

2. The method of claim 1 , wherein generating the mathematically averaged ROP data for the depth intervals of the wellbore comprises generating a mathematically averaged ROP value corresponding to a particular depth interval of the wellbore based a first time period spent drilling a first depth interval and a second time period spent drilling a second depth interval, wherein the first depth interval is adjacently above the particular depth interval, and wherein the second depth interval is adjacently below the particular depth interval.

3. The method of claim 1 , wherein determining the lithology of the rock formation comprises determining a percentage of a particular depth interval of the wellbore occupied by a first rock and determining a percentage of the particular depth interval of the wellbore occupied by a second rock.

4. The method of claim 3 , wherein the first rock is clay and wherein the percentage of the first rock is determined based on the upper gamma ray threshold, the lower gamma ray threshold, and information from a mud log.

5. The method of claim 4 , wherein the percentage of the first rock at the particular depth interval is determined to be 100% if a gamma ray log data value corresponding to the particular depth interval exceeds the upper gamma ray threshold and wherein the percentage of the first rock is determined to be 0% if the gamma ray log data value is less than the lower gamma ray threshold.

6. The method of claim 3 , wherein, for the particular depth interval, the UCS of the rock formation is determined based on the percentage of the first rock at the particular depth interval and the percentage of the second rock at the particular depth interval.

7. The method of claim 6 , further comprising performing a depth adjustment of mathematically averaged ROP data such that the mathematically averaged ROP data correlates with the UCS of the rock formation with respect to individual depth intervals of the wellbore.

8. The method of claim 7 , further comprising determining abrasiveness of the rock formation based on the lithology of the rock formation and grinding factors of the first rock and the second rock.

9. The method of claim 8 , wherein determining the abrasiveness of the rock formation comprises determining abrasiveness of the rock formation at a particular depth interval based on the percentage of the first rock at the particular depth interval and the percentage of the second rock at the particular depth interval.

10. The method of claim 8 , further comprising determining total work done by a drill bit in drilling at least a segment of the wellbore based on the abrasiveness of the rock formation and the mathematically averaged ROP data.

11. The method of claim 10 , wherein determining the total work done by the drill bit comprises determining time spent drilling the particular depth interval by the drill bit based on a mathematically averaged ROP data value of the mathematically averaged ROP data, wherein the mathematically averaged ROP data value corresponds to the particular depth interval.

12. The method of claim 3 , wherein the one or more sonic data thresholds include a pair of sonic data thresholds defining a range of sonic transit time values, wherein a special mineral rock is determined as occupying a portion of a particular depth interval of the wellbore if the sonic transmit time for the particular depth interval is within the range.

13. The method of claim 12 , wherein the special mineral rock is one of anhydrite, gypsum, coal, salt, or igneous.