Method for evaluating rock compressive strength based on cuttings mineral composition, nano-indentation, and friction tests

A method for evaluating rock compressive strength based on cuttings mineral composition, nano-indentation, and friction tests, comprising the following steps:

• step 1.1:
• collecting fresh cuttings from a studied formation to prepare dry rock powder samples with a particle size of less than 75 Wm, and determining rock mineral composition and relative content of each rock mineral by using an X-ray diffractometer;
• step 1.2:
• collecting fresh cuttings from the studied formation, selecting blocky cuttings with a longest axis particle size of 0.8-1.2 cm, and grinding and polishing the blocky cuttings to prepare rock nano-indentation specimens;
• performing experiments on the rock nano-indentation specimens using a nano-indenter to obtain nano-indentation load-displacement curves of each rock mineral;
• observing residual nano-indentation impressions using a metallographic optical microscope, measuring nano-indentation crack lengths of rock, and calculating a mean nano-indentation crack length of rock;
• step 1.3:
• based on the nano-indentation load-displacement curves of each rock mineral, calculating elastic modulus and plane-strain fracture toughness of each rock mineral;
• step 1.4:
• based on the relative content of each rock mineral, the mean nano-indentation crack length of rock, and the plane-strain fracture toughness of each rock mineral, calculating a rock fracture toughness characteristic parameter by integrating parameters of each mineral according to a proportion of each mineral using a weighted superposition method;
• step 1.5:
• collecting fresh cuttings from the studied formation and performing cuttings friction testing to obtain a rock friction coefficient;
• step 1.6:
• based on the rock fracture toughness characteristic parameter and the rock friction coefficient, calculating the rock compressive strength according to a rock strength calculation model;
• wherein:
• in the step 1.4, the rock fracture toughness characteristic parameter is calculated according to the following formula:
• K
• CS
• =
• ∑
• 1
• r
• f
• i
• ⁢
• K
• IC
• ⁡
• (
• i
• )
• π
• ⁢
• c
• wherein KCS represents the rock fracture toughness characteristic parameter, in MPa, which is an equivalent parameter obtained by weighted integration of the plane-strain fracture toughness of each rock mineral;
• r represents a number of rock mineral types, which is an integer;
• fi represents a relative content of an i-th mineral, in %;
• KIC(i) represents the plane-strain fracture toughness of the i-th rock mineral, in MPa·m0.5;
• π represents pi, dimensionless;
• c represents the mean nano-indentation crack length of rock, in μm;
• the specific procedures of the step 1.6 are as follows:
• step 1.6.1:
• based on the rock fracture toughness characteristic parameter and the rock friction coefficient, calculating uniaxial compressive strength of rock according to the following formula:
• σ
• c
• =
• K
• CS
• [
• 3
• 1
• +
• μ
• 2
• -
• μ
• ]
• wherein σc represents the uniaxial compressive strength of rock, in MPa;
• μ represents the rock friction coefficient, dimensionless;
• step 1.6.2:
• based on the rock friction coefficient, the rock fracture toughness characteristic parameter, and the uniaxial compressive strength of rock, calculating triaxial compressive strength of rock according to the following formula:
• σ
• =
• σ
• p
• +
• μ
• ⁢
• σ
• c
• K
• CS
• ⁢
• σ
• c
• ⁢
• σ
• p
• +
• σ
• c
• 2
• wherein σp represents a confining pressure, in MPa;
• and σ represents the triaxial compressive strength of rock under the confining pressure of σp, in MPa.