Method for estimating confined compressive strength for rock formations utilizing skempton theory

A method for estimating the CCS for a rock in the depth of cut zone of a subterranean formation which is to be drilled using a drill bit and a drilling fluid, the method comprising the steps of:

• a) determining the UCS for a rock in a depth of cut zone of a subterranean formation which is to be drilled using a drill bit and a drilling fluid;
• b) estimating the change in the strength of the rock based at least in part upon the change in the pore pressure (ΔPP) of the rock resulting from changes in the volume of the pores of the rock due to changes in confining stresses applied upon the rock due to drilling and due to fluid movement into and out of the pores of the rock in response to the drilling of the wellbore with a drill bit and drilling fluid;
• and
• c) estimating the CCS for the rock in the depth of cut zone by adding the estimated change in strength to the UCS.

A method for estimating the CCS for a rock in the depth of cut zone of a subterranean formation which is to be drilled using a drilling fluid, the method comprising the steps of:

• (a) estimating the CCS for substantially permeable rock in accordance with the following mathematical formula:
• CCSHP=UCS+f(DP);
• where:
• UCS=the unconfined compressive strength of the rock;
• DP=differential pressure acting upon the rock ;
• and
• f(DP)=a mathematical function of DP;
• (b) estimating the CCS for substantially impermeable rock in accordance with the following mathematical expression:
• CCSLP=UCS+f(DP);
• where:
• UCS=the unconfined compressive strength of the rock;
• DP=differential pressure acting upon the rock and is a function of the change in pore pressure □PP;
• and
• f(DP)=a mathematical function of DP
• (c) calculating an intermediate CCS (CCSMIX) for the rock based upon the estimated permeability of the rock and the confined compressive strengths CCSHP, CCSLP for substantially permeable and impermeable rocks.

A method for calculating ΔPP in a rock due to drilling, the method comprising:

• calculating a baseline ΔPP using a baseline mathematical formula;
• determining a ΔPP for the rock and drilling environment utilizing a computer model of the rock and drilling conditions based upon at least one characteristic of the rock, drilling conditions, and drill bit;
• determining a correction factor CF relating baseline ΔPP of the baseline ΔPP and the ΔPP of the computer model (CCS);
• and
• determining a ΔPP in another rock utilizing the baseline formula and the correction factor CF to enhance the estimate of the ΔPP.

A method of calculating corrected differential pressures across a rock in the depth of cut zone during drilling of a well bore in a subterranean wellbore, the method comprising the steps of:

• (a) calculating, utilizing a mathematical expression, a baseline DP across a rock in a depth of cut zone for a drill bit having a baseline profile under a baseline set of drilling conditions;
• (b) computing, using a computer model, the DP across the rock in the depth of cut zone for a drill bit having a first profile differing from that of the baseline profile under the baseline set of drilling conditions;
• (c) calculating a profile correction factor by comparing the baseline DP with the DP determined from the computer model;
• and
• (d) calculating a corrected DP, utilizing the mathematical expression and the profile correction factor, for a drill bit with the first profile. baseline set of drilling conditions.