Patent ID: 6968274

Claim:
A method for predicting fracture gradient from cuttings while drilling which comprises: Obtaining a velocity measurement, V p , of a cutting by: a) Preparing a sample with two opposite, parallel surfaces separated by a distance L; b) Placing sample between P- or S-wave transducers; c) Sweeping a sinusoidal signal continuously through a proper frequency range; d) Extracting and registering the envelope of the received amplitude modulated signal; and e) Measuring Δf, the difference in frequency between two consecutive resonances, for each amplitude peak, and inserting Δf into the equation V p =2LΔf; f) Using the velocity measurement obtained to determine porosity from velocity by the Acoustic Formation Factor method using the equation: φ=1−( V pmeasured /V pmatrix ) β where β is 0.625 an V pmatrix is 18000 ft/sec for sand, respectively; Predicting pore pressure from the recorded CWT velocity by: g) Calculating total vertical stress (overburden stress) using density integration: S v =∫ρ( z ) gdz where z is depth, ρ(z)represents the formation bulk density as a function of depth, d is density, and g is the gravitational constant; h) Obtaining effective vertical stress using the Holbrook correlation σ v =σ max (1−φ) α where σ max and α are lithology dependent constant parameters defined by Holbrook; i) Determining pore pressure using Terzaghi's law: P p =(S v −σ v )/γ where P p is the pore pressure, S v is the total overburden stress calculated in step g, σ v is effective vertical stress calculated in step h, and γ=Biot coefficient=1−(K β /K γ ), with K β is bulk compressibility, and K γ is the grain compressibility; and Predicting fracture gradient by a modified Holbrook method using CWT drill cutting velocity in the equation: S hmin =δ[P p +σ v (1−φ)] where S hmin is minimum horizontal stress that is the lower limit of the fracture gradient, φ is porosity obtained in step f, P p is pore pressure obtained in step i, σ v is effective Stress calculated in step h, and δ is a local strength coefficient.