초록 ▼

The permeability of the cement annulus surrounding a casing is measured by locating a tool inside the casing, placing a probe of the tool in contact with the cement annulus, measuring the change of pressure in the probe over time, where the change in pressure over time is a function of among other t

The permeability of the cement annulus surrounding a casing is measured by locating a tool inside the casing, placing a probe of the tool in contact with the cement annulus, measuring the change of pressure in the probe over time, where the change in pressure over time is a function of among other things, the initial probe pressure, the formation pressure, and the permeability, and using the measured change over time to determine an estimated permeability. The estimated permeability is useful in determining whether carbon dioxide can be effectively sequestered in the formation below or at the depth of measurement without significant leakage through the cement annulus.

대표청구항 ▼

What is claimed is: 1. A method of determining an estimate of the permeability of a cement annulus in a formation traversed by a well-bore using a tool having a hydraulic probe and a pressure sensor, comprising: locating the tool at a depth inside the well-bore with the hydraulic probe in hydraulic

What is claimed is: 1. A method of determining an estimate of the permeability of a cement annulus in a formation traversed by a well-bore using a tool having a hydraulic probe and a pressure sensor, comprising: locating the tool at a depth inside the well-bore with the hydraulic probe in hydraulic contact with the cement annulus; using the pressure sensor to measure the pressure in the hydraulic probe over a period of time in order to obtain pressure data; finding a relaxation time constant estimate of the pressure data by fitting the pressure data to an exponential curve which is a function of the relaxation time constant, and a difference between a starting pressure in the hydraulic probe and the formation pressure; and determining an estimate of the permeability of the cement annulus according to an equation which relates said permeability of the cement annulus to said relaxation time constant estimate. 2. A method according to claim 1, wherein: the well-bore has a casing around which the cement annulus is located, and said locating the tool inside the well-bore includes selecting a location in the well-bore and setting the tool at that location, and drilling a hole in the casing to expose the cement annulus. 3. A method according to claim 2, wherein: said drilling comprises monitoring torque on a drill bit, and terminating drilling based on a change of torque. 4. A method according to claim 3, wherein: said drilling further comprising monitoring depth of penetration on of the drill bit, and terminating drilling based on said change of torque if the drill bit has penetrated to a depth approaching the thickness of the casing. 5. A method according to claim 1, wherein: said relaxation time constant estimate is determined according to p p * = p f * + ( p w * - p f * ) ⁢ ⅇ - t τ  where pp* is the hydraulic probe pressure measured by the pressure sensor of the tool, p*f is the formation pressure, pw* is the initial pressure at which the hydraulic probe is set, t is time, and τ is said relaxation time constant estimate. 6. A method according to claim 1, wherein: said equation is k c = V t ⁢ c t ⁢ μ 4 ⁢ τ ⁢ ⁢ r p ⁡ [ 1 - 2 ⁢ ln ⁢ ⁢ 2 π ⁢ r p l c ] ,  where kc is said permeability estimate of said cement annulus, τ is said relaxation time constant estimate, lc is the thickness of said cement annulus, Vt is the fluid volume of the lines of the tool connected to the hydraulic probe, ct is the compressibility of the fluid in the tool, rp is the radius of the hydraulic probe, and μ is the viscosity of the fluid in the tool. 7. A method according to claim 6, further comprising: determining said compressibility of the fluid in the tool by imposing a known volume of expansion on the fixed amount of fluid in the system, sensing a resulting change in flow-line pressure, and calculating compressibility according to c t = - 1 V ⁢ ⁢ Δ ⁢ ⁢ V Δ ⁢ ⁢ p , where V is an initial volume of the flow-line, ΔV is the expansion volume added to the flow line, and Δp is the change in pressure. 8. A method according to claim 1, wherein: said fitting comprises permitting said relaxation time constant estimate, said pressure in the hydraulic probe and said formation pressure to be variables which are varied to find a best fit. 9. A method according to claim 1, wherein: said fitting comprises fixing at least one of said pressure in the hydraulic probe and said formation pressure in finding said relaxation time constant estimate. 10. A method according to claim 1, further comprising: comparing said determined permeability estimate to a threshold value for the purpose of determining the suitability of storing carbon dioxide in the formation at or below that depth. 11. A method according to claim 1, wherein: said period of time is less than said relaxation time constant estimate. 12. A method according to claim 1, further comprising: generating a viewable log or chart showing at least one permeability estimate or indication of suitability for storing carbon dioxide at or below at least one depth in the formation. 13. A system for determining an estimate of the permeability of a cement annulus in a formation traversed by a well-bore having a casing, comprising: a tool having a hydraulic probe, a pressure sensor in hydraulic contact with the hydraulic probe and sensing pressure in the hydraulic probe, a drill capable of drilling the casing, and means for hydraulically isolating said hydraulic probe in hydraulic contact with the cement annulus; and processing means coupled to said pressure sensor, said processing means for obtaining pressure measurement data obtained by said pressure sensor over a period of time while said hydraulic probe is hydraulically isolated in hydraulic contact with the cement annulus, for finding a relaxation time constant estimate of the pressure data by fitting the pressure data to an exponential curve which is a function of the relaxation time constant, and a difference between a starting pressure in the hydraulic probe and the formation pressure, and for determining an estimate of the permeability of the cement annulus according to an equation which relates said permeability of the cement annulus to said relaxation time constant estimate. 14. A system according to claim 13, wherein: said processing means is at least partially located separate from said tool. 15. A system according to claim 13, further comprising: means coupled to said processing means for generating a viewable log or table of at least one estimate of the permeability of the cement annulus as a function of depth in the well-bore or formation. 16. A system according to claim 13, wherein: said processing means for finding said relaxation time constant estimate finds said relaxation time constant according to p p * = p f * + ( p w * - p f * ) ⁢ ⅇ - t τ  where pp* is the hydraulic probe pressure measured by the pressure sensor of the tool, p*f is the formation pressure, pw* is the initial pressure at which the hydraulic probe is set, t is time, and τ is said relaxation time constant estimate. 17. A system according to claim 13, wherein: said equation is k c = V t ⁢ c t ⁢ μ 4 ⁢ τ ⁢ ⁢ r p ⁡ [ 1 - 2 ⁢ ln ⁢ ⁢ 2 π ⁢ r p l c ] ,  where kc is said permeability estimate of said cement annulus, τ is said relaxation time constant estimate, lc is the thickness of said cement annulus, Vt is the fluid volume of the lines of the tool connected to the hydraulic probe, ct is the compressibility of the fluid in the tool, rp is the radius of the hydraulic probe, and μ is the viscosity of the fluid in the tool.