Patent ID: 7958940

Claim:
A process to drill through a composite frac plug in an oil well, the composite frac plug including a resin body, and at least one material from a group consisting of a ceramic, cast iron, aluminum, and cloth, the process including the steps of (a) providing a mill including (i) a body having a generally cylindrical circumferential outer surface, and (ii) a cutting face at one end of said body generally normal to said outer surface, and including a center ( 50 ) and five spaced apart radially extending insert support structures ( 11 ), each of said insert support structures including (iii) an upstanding leg ( 11 A) with an outer edge ( 22 ) sloping downwardly and inwardly toward said center ( 50 ) at a selected angle (Y) in the range of twenty-one to twenty three degrees and canted back at an angle (Z) in the range of one to three degrees from a vertical axis (X) that is parallel to the axis of rotation of the mill, said leg 11 A including a front surface ( 24 ) and a rear surface ( 23 ), (iv) a first generally flat seat ( 27 ) normal to said front surface ( 24 ) and sloping downwardly toward said center ( 50 ) at said selected angle (Y), (v) a second generally flat seat ( 25 ) normal to said front surface ( 24 ), stepped upwardly from said first seat ( 27 ), and sloping downwardly toward said center ( 50 ) at said selected angle (Y), (vi) a plurality of carbide inserts affixed to each of said seats and extending above said ledge, said inserts shaped and dimensioned such that low areas are formed between adjacent carbide inserts affixed to one of said seats, one of said insert support structures extending past said center, and said carbide inserts on said seats of one of said insert support structures being offset from carbide inserts on the next succeeding insert support structure such that said low areas on said one of said insert support structures are offset from said low areas on said next succeeding insert support structure; (b) providing drill pipe having an exterior, a distal end and a proximate end; (c) attaching said mill to said distal end of said drill pipe; (d) inserting said mill and said distal end of said drill pipe in the oil well until said concave cutting face of said mill contacts the top of the composite frac plug; (e) engaging said proximate end of said drill pipe and (i) rotating said pipe and said mill at one hundred and forty to five hundred rpm, (ii) circulating drilling fluid such that the velocity of said fluid upwardly over said exterior of said drill pipe is in the range of three hundred to four hundred and seventy five feet per minute, and (iii) applying one thousand to three thousand pounds of slack off weight to cut through the frac plug.