Determining stress parameters of formations from multi-mode velocity data
AssigneeSchlumberger Technology Corporation
InventorBikash K. Sinha
Life of this patent
Estimating the term…
Abstract
A method for determining unknown stress parameters in earth formation measures velocities in four sonic transmissions modes (compression, fast shear, slow shear and Stoneley) at a series of depths. Relationships between measured velocities and other measured values, two independent linear constants, and three nonlinear constants associated with equations of motion for pre-stressed isotropic materials are expressed in a set of four or five velocity difference equations derived from non-linear continuum mechanics. The velocity difference equations are solved using inversion for useful stress parameters, including maximum horizontal stress, minimum horizontal stress, pore pressure, and change in pore pressure over time.
Claims15
A method for determining an unknown stress parameter of an earth formation proximate to a borehole in the formation at a given depth, the method comprising:
- a) measuring a first set of parameters of the formation within a selected depth interval of the borehole to produce a first set of measured values, the first set of parameters including compressional velocity, fast-shear velocity, slow-shear velocity and Stoneley velocity;
- b) estimating a second set of parameters of the formation to produce estimated values;
- and
- c) solving a set of first, second, third, and fourth velocity difference equations to determine the unknown stress parameter at a depth within the depth interval using the measured values and the estimated values.
A method for determining a difference in pore pressure at a given depth in an earth formation proximate to a borehole, before and after a production period, the method comprising:
- a) measuring a first set of parameters of the formation within a selected depth interval of the borehole before the production period to produce a first set of measured values measured within the depth interval, the set of parameters including one of a group of velocities, the group consisting of compressional velocity, fast-shear velocity and slow-shear velocity;
- b) measuring the first set of parameters within the selected depth interval after the production period to produce a second set of measured values measured within the depth interval;
- c) estimating a second set of parameters of the formation to produce estimated values;
- d) solving a set of first, second, third, and fourth velocity difference equations to determine unknown stress parameters at a depth within the depth interval using the first and second sets of measured values and the estimated values;
- and
- e) solving a fifth equation to determine difference in pore pressure.
A method for determining pore fluid overpressure of an overpressured shale formation at a given depth in an earth formation proximate to a borehole, the method comprising:
- a) measuring compressional velocity of the formation within a selected depth interval of the borehole to produce a set of measured values;
- b) estimating density and compression to produce estimated values;
- c) calculating pore fluid overpressure by evaluating an expression involving the measured values and the estimated values, and the vertical effective stress coefficient of compressional velocity.
Dependencies are inferred from the claim text.