Method for comprehensive evaluation of shale fracability under the geology-engineering “double-track” system

A method for evaluation of shale fracability under geology-engineering “double-track” system, comprising the following steps:

• S1:
• Divide a target horizontal fracturing interval into multiple sampling sections;
• S2:
• Establish a reservoir property evaluation factor of each sampling section, and calculate a geological evaluation index of the target horizontal fracturing interval according to the reservoir property evaluation factor of each sampling section;
• S3:
• Establish a brittleness factor, a natural fracture factor and a natural fracture opening factor of each sampling section, and then establish an engineering evaluation index of each sampling section according to the brittleness factor, the natural fracture factor and the natural fracture opening factor of each sampling section;
• The brittleness factor is calculated according to the following equations:
• C
• i
• =
• λ
• [
• 1
• -
• exp
• ⁡
• (
• M
• i
• E
• i
• )
• ]
• +
• η
• ⁢
• (
• σ
• pi
• -
• σ
• ci
• )
• σ
• pi
• λ
• +
• η
• =
• 1
• Where, Ci is the brittleness factor of the sampling section i based on a stress-strain curve, dimensionless;
• λ and η are standardized coefficients, dimensionless;
• Mi is a softening modulus of the sampling section i, in gigapascals (Gpa);
• Ei is an elasticity modulus of the sampling section i, in GPa;
• σpi is a peak strength obtained from a triaxial compression test at the sampling section i, in megapascals (Mpa);
• σci is a participating strength obtained from the triaxial compression test at the sampling section i, in MPa;
• The natural fracture factor is calculated according to the following equations:
• P
• Fi
• =
• P
• fi
• -
• P
• f
• ⁢
• min
• P
• f
• ⁢
• max
• -
• P
• f
• ⁢
• min
• P
• fi
• =
• 2
• ⁢
• E
• 2
• (
• K
• 2
• 1
• ⁢
• i
• +
• K
• 2
• 2
• ⁢
• i
• )
• ⁢
• υ
• i
• K
• 1
• ⁢
• i
• =
• 0.3172
• ρ
• i
• +
• 0.0457
• V
• c
• ⁢
• i
• +
• 0.2131
• ln
• ⁡
• (
• DT
• )
• ×
• 0.5041
• i
• K
• 2
• ⁢
• i
• =
• 2.1332
• ρ
• i
• +
• 0.0768
• V
• c
• ⁢
• i
• +
• 1.1886
• ln
• ⁡
• (
• DT
• )
• ×
• 9.1808
• i
• Where, PFi is the natural fracture factor of the sampling section i, dimensionless;
• Pfi is a representation number of natural fracture development degree in sampling section i, ×106 m−1;
• Pfmax and Pfmin are respectively maximum and minimum representation numbers of natural fracture development degree in all sampling sections, ×106 m−1;
• K1i and K2i are respectively Type I and Type II fracture toughness of the sampling section i, in MPa·m0.5;
• νi is an average static Poisson's ratio of the sampling section i, dimensionless;
• ρi is an average shale density of the sampling section i, in g/cm3;
• Vci is an average mud content of the sampling section i, in percentage (%);
• DTi is an average acoustic time difference of the sampling section i, in μs/m;
• The natural fracture opening factor is calculated according to the following equations:
• P
• Ti
• =
• P
• t
• ⁢
• max
• -
• P
• ti
• P
• t
• ⁢
• max
• -
• P
• t
• ⁢
• min
• (
• 12
• )
• P
• ti
• =
• σ
• xi
• ⁢
• l
• 1
• ⁢
• i
• 2
• +
• σ
• yi
• ⁢
• l
• 2
• ⁢
• i
• 2
• +
• σ
• zi
• ⁢
• l
• 3
• ⁢
• i
• 2
• +
• 2
• ⁢
• τ
• ⁢
• ll
• xyi
• ⁢
• 1
• ⁢
• i
• ⁢
• 2
• ⁢
• i
• +
• 2
• ⁢
• τ
• ⁢
• ll
• yzi
• ⁢
• 2
• ⁢
• i
• ⁢
• 3
• ⁢
• i
• +
• 2
• ⁢
• τ
• ⁢
• ll
• zxi
• ⁢
• 1
• ⁢
• i
• ⁢
• 3
• ⁢
• i
• l
• 1
• ⁢
• i
• =
• 1
• -
• l
• 3
• ⁢
• i
• 2
• 1
• +
• tan
• 2
• ⁢
• θ
• i
• l
• 2
• ⁢
• i
• =
• l
• 1
• ⁢
• i
• ⁢
• tan
• ⁢
• θ
• i
• l
• 3
• ⁢
• i
• =
• cos
• ⁢
• α
• i
• Where, PTi is the natural fracture opening factor of the sampling section i, dimensionless;
• Pti is a fluid pressure when a natural fracture of the sampling section i is opened, in MPa;
• Ptmax and Ptmin are respectively maximum and minimum fluid pressures in all sampling sections to meet the opening of the natural fracture, in MPa;
• σxi, σyi and σzi are respectively normal stress, tangential normal stress and vertical stress of a shaft in the sampling section i, in MPa;
• l1i, l2i and l3i are respectively cosine values of an included angle between the natural fracture and maximum horizontal principal stress, minimum horizontal principal stress and vertical stress in the sampling section i, dimensionless;
• τxyi, τyzi and τxzi are respectively shear stress components of the sampling section i, in MPa;
• θi is the included angle between the natural fracture and the maximum horizontal principal stress, the minimum horizontal principal stress and the vertical stress in the sampling section i, in degrees (°);
• α1 is a dip angle of the natural fracture in the sampling section i, in degrees (°);
• S4:
• Calculate an engineering evaluation index of the target horizontal fracturing interval according to the engineering evaluation index of each sampling section;
• S5:
• Evaluate the shale fracability of the target horizontal fracturing interval according to the geological evaluation index and the engineering evaluation index;
• S6:
• Establish a fracturing construction scheme for the target horizontal fracturing interval according to the geological evaluation index and the engineering evaluation index;
• wherein, in step S6, the fracturing construction scheme for the target horizontal fracturing interval is established according to the geological evaluation index and the engineering evaluation index, specifically as follows:
• when the geological evaluation index is within a range of [0,0.1] and the engineering evaluation index is within a range of [0,1.0], fracturing construction of the target horizontal fracturing interval will be abandoned
• when the geological evaluation index is within a range of [0.1,0.4] and the engineering evaluation index is within a range of [0,0.7], and when the geological evaluation index is within a range of [0.4,0.7] and the engineering evaluation index is within a range of [0.7,1.0], slickwater fracturing+fiber temporary plugging and diverting will be carried out for the target horizontal fracturing interval
• when the geological evaluation index is within the range of [0.1,0.4], and the engineering evaluation index is within the range of [0.7,1.0], slickwater fracturing will be carried out for the target horizontal fracturing interval;
• when the geological evaluation index is within the range of [0.4,0.7] and the engineering evaluation index is within a range of [0,0.3], and when the geological evaluation index is within a range of [0.7,1.0] and the engineering evaluation index is within a range of [0.3,0.7], large-scale fracturing+fiber temporary plugging and diverting+medium multi-clustered volume fracturing will be carried out for the target horizontal fracturing interval, the large-scale fracturing means that liquid intensity for fracturing is 150% of that for the slickwater fracturing
• when the geological evaluation index is within the range of [0.4,0.7] and the engineering evaluation index is within the range of [0.3,0.7], and when the geological evaluation index is within the range of [0.7,1.0] and the engineering evaluation index is within the range of [0.7,1.0], large-scale fracturing+fiber temporary plugging and diverting will be carried out for the target horizontal fracturing interval;
• when the geological evaluation index is within the range of [0.7,1.0] and the engineering evaluation index is within the range of [0,0.3], large-scale fracturing+fiber temporary plugging and diverting+multi-clustered volume fracturing will be carried out for the target horizontal fracturing interval.