US 7,412,331 B2Granted

Method for predicting rate of penetration using bit-specific coefficient of sliding friction and mechanical efficiency as a function of confined compressive strength

AssigneeChevron U.S.A. Inc.

InventorsWilliam Malcolm Calhoun, Hector Ulpiano Caicedo, Russell Thomas Ewy

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Abstract

A method for predicting the rate of penetration (ROP) of a drill bit drilling a well bore through intervals of rock of a subterranean formation is provided. The method uses an equation based upon specific energy principles. A relationship is determined between a bit-specific coefficient of sliding friction μ and confined compressive strength CCS over a range of confined compressive strengths CCS. Similarly, another relationship for the drill bit is determined between mechanical efficiency EFFM and confined compressive strength CCS over a range of confined compressive strengths CCS. Confined compressive strength CCS is estimated for intervals of rock through which the drill bit is to be used to drill a well bore. The rate of penetration ROP is then calculated utilizing the estimates of confined compressive strength CCS of the intervals of rock to be drilled and those determined relationships between the bit-specific coefficient of sliding friction μ and the mechanical efficiency EFFM and the confined compressive strengths CCS, as well as using estimated drill bit speeds N (RPM) and weights on bit (WOB).

Claims15

Dependent claims

A method for predicting the rate of drilling of a well bore in a subterranean formation, the method comprising the steps of:

A) determining the rate of penetration (ROP) of a drill bit drilling a well bore through intervals of rock of a subterranean formation by:
a) determining for at least one type of drill bit a relationship between a bit-specific coefficient of sliding friction μ and confined compressive strength CCS over a range of confined compressive strengths CCS;
b) determining for the at least one type of drill bit a relationship between mechanical efficiency EFFM and confined compressive strength CCS over a range of confined compressive strengths CCS;
c) determining the confined compressive strength for intervals of rock through which the at least one type of drill bit is to be drilled to form a well bore;
and
d) calculating the rate of penetration ROP for the at least one type of drill bit drilling along the intervals of rock to create a well bore, the calculations utilizing the confined compressive strength of the intervals of rock being drilled and the relationships between the bit-specific coefficient of sliding friction μ and the mechanical efficiency EFFM and the confined compressive strengths CCS;
and
B) selecting a drill bit for drilling the well bore through the intervals of rock of the subterranean formation based on the calculated rate of penetration ROP;
wherein the confined compressive strength (CCS) of an interval of rock is determined at least in part based upon the unconfined compressive strength (UCS) of the interval of rock, the equivalent circulating density (ECD) of a drilling fluid being used to drill the interval of rock, the overburden stress (OB) removed from the interval of rock being drilled, the in situ pore pressure (PP) of pore fluids proximate the interval of rock being drilled, and the permeability of the interval of rock being drilled.

6 more elements

A method for predicting the rate of drilling of a well bore in a subterranean formation, the method comprising the steps:

A) back calculating confined compressive strength CCS of rock in an interval of a subterranean formation in which a well bore has been drilled using a type of drill bit and drilling fluids by:
a) measuring (i) the rate of penetration (ROP);
(ii) weight on bit (WOB);
(iii) bit torque T;
and (iv) the revolutions per minute (N) used during the drilling through an interval of rock in a subterranean formation by the type of drill bit;
b) estimating the coefficient of sliding friction μ during the drilling through the interval of rock;
and
c) selecting a value of CCS from a predetermined relationship between μ and CCS for the type of drill bit;
and
B) selecting a drill bit for further drilling of a well bore in an interval of rock of the subterranean formation based on the selected value of CCS;
and
C) back calculating the unconfined compressive strength UCS of the rock in the interval in accordance with the following mathematical expression:
CCS=UCS+DP+2DP sinFA/(1−sinFA)
where:
UCS=rock unconfined compressive strength;
DP=differential pressure (or confining stress) across the rock;
and
FA=internal angle of friction of the rock.

15 more elements

Dependencies are inferred from the claim text.