Method for determining the pressure or stress of a geological formation from acoustic measurement

• US 5,142,471 A
• Filed: 04/05/1990
• Issued: 08/25/1992
• Est. Priority Date: 04/05/1990
• Status: Expired due to Fees

• Current Assignee: Board of Supervisors of Louisiana State University Agricultural & Mechanical College (Louisiana State University System)
• Original Assignee: Board of Supervisors of Louisiana State University Agricultural & Mechanical College (Louisiana State University System)
• Inventors: Desbrandes, Robert
• Primary Examiner(s): Hayes, Gail O.
• Application Number:
  US07/504,923
• Time in Patent Office: 873 Days
• Field of Search: 364/422, 175/48, 175/50, 73/155, 73/151
• US Class Current: 702/12
• CPC Class Codes: E21B 47/06 Measuring temperature or pressure
• CPC Class Codes: E21B 49/006 Measuring wall stresses in the borehole
• CPC Class Codes: G01V 1/50 Analysing data

Abstract

A method to determine stress, pore pressure, or other properties of a geological formation by measuring acoustic characteristics of the formation. The acoustic characteristics, which may be one or more of the following group of characteristics-compressional acoustic velocity, shear acoustic velocity, compressional acoustic attenuation, and shear acoustic attenuation-depend on the net stress of the formation. The net stress in turn, depends on the pore pressure. Measurements of one or more acoustic characteristics at several depths allows determination of the net stress, which in turn allows determination of the pore pressure and other properties of the formation.

Claims

1. A method for measuring and using the pressure in a geological formation, comprising the steps of:

(a) placing a sonde in a bore hole in the formation at each of a plurality of depths, sending acoustic signals into the formation, receiving the acoustic signals as reflected from the formation with said sonde, and determining at least one acoustic characteristic of the formation at each of said depths from the reflected acoustic signals received by said sonde;

(b) selecting estimated values for parameters determining a mathematical relationship, which relationship correlates values of the acoustic characteristic to values of stress;

(c) selecting an estimated value for the stress at each of said depths;

p1 (d) computing a calculated value for the acoustic characteristic at each of said depths through said mathematical relationship, said estimated parameters, and said estimated stress values;

(e) computing the difference, at each of said depths, between said calculated value of the acoustic characteristic and said measured value of the acoustic characteristic;

(f) computing a total error for all of said depths, from the values of each of said differences;

(g) changing at least one of said estimated parameters, or changing at least one of said estimated stress values, and repeating steps (d), (e), (f), and (g) until the value of said total error reaches an approximate minimum, whereby the estimated value of the stress at each of said depths becomes the measured value of the stress at each of said depths;

(h) selecting at least one of said depths;

(i) at each of said selected depths, subtracting the measured stress at said selected depth from the overburden pressure at said selected depth;

(j) at each of said selected depths, dividing the result of step (i) by the Biot constant of the formation at said selected depth;

(k) at each of said selected depths, adding to the result of step (j) the pressure, if any, due to any movable fluid which is in the formation and which movable fluid lies above said selected depth, whereby the result of this step (k) becomes the measured pressure of the formation at said selected depth; and

(l) performing at least one of the following steps (i), (ii), (iii), or (iv);

(i) pumping liquid from the formation to the surface if the measured pressure indicates that the liquid will not flow to the surface under the liquid'"'"'s own pressure;

(ii) causing liquid from the formation to flow to the surface under the liquid'"'"'s own pressure if the measured pressure indicates that the liquid will so flow;

(iii) depleting liquid or gas from the formation, and monitoring the pressure of the formation during said depletion;

or

(iv) estimating the quantity of gas reserves in the formation from the measured pressure, and producing gas from the formation if the estimated reserves indicate that such production is desirable.

2. A method for measuring and using the pressure in a geological formation as recited in claim 1, additionally comprising the steps of:

(a) identifying each of said depths for which said difference is less than a determined amount;

(b) computing a calculated value for the acoustic characteristic at each of said identified depths through said mathematical relationship, said estimated parameters, and said estimated stress values;

(c) computing the difference, at each of said identified depths, between said calculated value of the acoustic characteristic and said measured value of the acoustic characteristic;

(d) computing a total error for all of said identified depths, based on the values of each of said differences;

(e) changing at least one of said estimated parameters, or changing at least one of said estimated stress values, and repeating steps (b), (c), (d), and (e) until the value of said total error reaches an approximate minimum, whereby the estimated value of the stress at each of said identified depths becomes the measured value of the stress at each of said identified depths.

3. A method for measuring and using the pressure in a geological formation as recited in claim 1, wherein said geological formation comprises a plurality of layers, and wherein the steps of claim 6 are performed separately for each of said layers, and wherein the stress is constrained to vary continuously across the boundary between any two adjacent said layers whenever movable fluid may flow freely between said two adjacent layers.

4. A method for measuring and using the pressure in a geological formation as recited in claim 1, wherein said geological formation comprises a plurality of layers comprising differing fluids, and wherein the steps of claim 6 are performed separately for each of said layers;

and wherein the stress is constrained to vary continuously across the boundary between any two adjacent said layers.

5. A method for measuring and using the pressure in a geological formation as recited in claim 4, wherein the position of the boundary between any two said adjacent layers is a parameter which may vary in any repetition of steps (d), (e), (f), and (g) of claim 6.

6. A method for measuring and using the pressure in a geological formation as recited in claim 1, wherein the mathematical relationship has the form

7. A method for measuring and using the pressure in a geological formation as recited in claim 6, wherein f(n) has the form

8. A method for measuring and using the pressure in a geological formation as recited in claim 1, wherein the mathematical relationship has the form

9. A method for measuring and using the pressure in a geological formation as recited in claim 8, wherein f(n) has the form

10. A method for measuring and using the pressure in a geological formation, comprising the steps of:

(a) placing a sonde in a bore hole in the formation at each of a plurality of depths, sending acoustic signals into the formation, receiving the acoustic signals as reflected from the formation with said sonde, and determining a first acoustic characteristic of the formation at each of said depths from the reflected acoustic signals received by said sonde;

(b) measuring a second acoustic characteristic of the formation at each of a plurality of second depths;

wherein said first acoustic characteristic differs from said second acoustic characteristic; and

wherein said second depths optionally may be, but are not necessarily, identical to said first depths;

(c) selecting estimated values for first parameters determining a first mathematical relationship, which first relationship correlates values of the first acoustic characteristic to values of stress;

(d) selecting estimated values for second parameters determining a second mathematical relationship, which second relationship correlates values of the second acoustic characteristic to values of stress;

(e) selecting an estimated value for the stress at each of said first depths, and an estimated value for the stress at each of said second depths;

(f) computing a calculated value for the first acoustic characteristic at each of said first depths through said first mathematical relationship, said estimated first parameters, and said estimated stress values;

(g) computing a calculated value for the second acoustic characteristic at each of said second depths through said second mathematical relationship, said estimated second parameters, and said estimated stress values;

(h) computing first differences, at each of said first depths, between said calculated value of the first acoustic characteristic and said measured value of the first acoustic characteristic;

(i) computing second differences, at each of said second depths, between said calculated value of the second acoustic characteristic and said measured value of the second acoustic characteristic;

(j) computing a total error, for all of said depths, from the values of each of said first differences and each of said second differences;

(k) changing at least one of said estimated parameters, or at least one of said estimated stress values, and repeating steps (f), (g), (h), (i), (j), and (k) until the value of said total error reaches an approximate minimum, whereby the estimated value of the stress at each of said depths becomes the measured value of the stress at each of said depths;

(l) selecting at least one of said depths;

(m) at each of said selected depths, subtracting the measured stress at said selected depth from the overburden pressure at said selected depth;

(n) at each of said selected depths, dividing the result of step (m) by the Biot constant of the formation at said selected depth;

(o) at each of said selected depths, adding to the result of step (n) the pressure, if any, due to any movable fluid which is in the formation and which movable fluid lies above said selected depth, whereby the result of this step (o) becomes the measured pressure of the formation at said selected depth; and

(p) performing at least one of the following steps (i), (ii), (iii), or (iv);

(i) pumping liquid from the formation to the surface if the measured pressure indicates that the liquid will not flow to the surface under the liquid'"'"'s own pressure;

(ii) causing liquid from the formation to flow to the surface under the liquid'"'"'s own pressure if the measured pressure indicates that the liquid will so flow;

(iii) depleting liquid or gas from the formation, and monitoring the pressure of the formation during said depletion;

or

(iv) estimating the quantity of gas reserves in the formation from the measured pressure, and producing gas from the formation if the estimated reserves indicate that such production is desirable.

11. A method for measuring and using the pressure in a geological formation as recited in claim 10, additionally comprising the steps of:

(a) identifying each of said first depths for which said first difference is less than a determined amount, and identifying each of said second depths for which said second difference is less than a determined amount;

(b) computing a calculated value for the first acoustic characteristic at each of said first identified depths through said first mathematical relationship, said estimated first parameters, and said estimated stress values;

(c) computing a calculated value for the second acoustic characteristic at each of said second identified depths through said second mathematical relationship, said estimated second parameters, and said estimated stress values;

(d) computing first differences, at each of said first identified depths, between said calculated value of the first acoustic characteristic and said measured value of the first acoustic characteristic;

(e) computing second differences, at each of said second identified depths, between said calculated value of the second acoustic characteristic and said measured value of the second acoustic characteristic;

(f) computing a total error, for all of said identified depths, from the values of each of said first differences and each of said second differences;

(g) changing at least one of said estimated parameters, or at least one of said estimated stress values, and repeating steps (b), (c), (d), (e), (f), and (g) until the value of said total error reaches an approximate minimum, whereby the corresponding estimated value of the stress at each of said identified depths becomes the measured value of the stress at each of said identified depths.

12. A method for measuring and using the pressure in a geological formation as recited in claim 10, wherein said geological formation comprises a plurality of layers, and wherein the steps of claim 23 are performed separately for each of said layers, and wherein the stress is constrained to vary continuously across the boundary between any two adjacent said layers whenever movable fluid may flow freely between said two adjacent layers.

13. A method for measuring and using the pressure in a geological formation as recited in claim 10, wherein said geological formation comprises a plurality of layers, the layers comprising differing fluids, and wherein the steps of claim 23 are performed separately for each of said layers;

and wherein the stress is constrained to vary continuously across the boundary between any two adjacent said layers whenever movable fluid may flow freely between said two adjacent layers.

14. A method for measuring and using the pressure in a geological formation as recited in claim 13, wherein the position of the boundary between any two adjacent said layers is a parameter which may vary in any repetition of steps (f), (g), (h), (i), (j), and (k) of claim 23.

15. A method for measuring and using the pressure in a geological formation as recited in claim 10, wherein said first acoustic characteristic comprises compressional acoustic velocity and said second acoustic characteristic comprises shear acoustic velocity.

16. A method for measuring and using the pressure in a geological formation as recited in claim 10, wherein at least one of said mathematical relationships has the form

17. A method for measuring and using the pressure in a geological formation as recited in claim 16, wherein f(n) has the form

18. A method for measuring and using the pressure in a geological formation as recited in claim 10, wherein at least one of said mathematical relationships has the form

19. A method for measuring and using the pressure in a geological formation as recited in claim 18, wherein f(n) has the form f(n)=R log(n), wherein R is a linear coefficient.