Telemetry transmitter optimization via inferred measured depth
A method whereby a downhole drilling transmission device that communicates to the surface automatically modifies its transmission parameters in order that it substantially improves its ability to adequately communicate with a surface receiver despite increasing signal attenuation between the two as the length of drillpipe increases. This utilizes a simple measure of localized downhole pressure that then relies upon a look-up table or similar that provides a correspondence between said pressure and measured depth. Such a look-up table or similar can be readily built by incorporating appropriate features of the planned well such as drilling fluid flow rate, drilling fluid density, drilling fluid viscosity, well profile, bottom hole assembly component geometry, drillpipe geometry, and indications as to whether the fluid is flowing or stationary. Upon determining the measured depth the tool then can attempt to modify or augment appropriate telemetry parameters in order to keep the signal received at surface within required parameters, thus offsetting the degradation due to increasing attenuation.
1. A method for enhancing downhole telemetry performance in a drill string comprising
(a) measuring downhole pressure at a specified location;
(b) inferring a measured depth from the measured downhole pressure; and
(c) modifying a downhole telemetry signal at one or more measured depths in order to offset signal-to-noise ratio reduction with increasing measured depth.
2. A method as claimed in claim 1 wherein the downhole pressure is hydrostatic pressure measured under static flow conditions.
3. A method as claimed in claim 1 wherein the downhole pressure is measured under moving flow conditions.
4. A method as claimed in claim 3 wherein the step of inferring comprises correlating the measured downhole pressure with the measured depth using a predicted equivalent circulating density at the specified location.
5. A method as claimed in claim 4 wherein the measured downhole pressure is selected from the group consisting of annulus pressure and bore pressure.
6. A method as claimed in claim 5 wherein the step of inferring a measured depth comprises associating a measured annulus pressure to a predicted annulus pressure then selecting a measured depth corresponding to the associated predicted annulus pressure.
7. A method as claimed in claim 5 comprising measuring a differential pressure between annulus and bore to determine downhole fluid flow velocity, then associated annulus pressure from the determined velocity.
8. A method as claimed in claim 1 wherein the method is performed in a drill string having a bottom hole assembly with no repeater, and the specified location is the location of the bottom hole assembly in a well bore.
9. A method as claimed in claim 1 wherein the method is performed in a drill string having a bottom hole assembly and at least one repeater, the specified location is the location of the repeater closest to the surface, and wherein the step of inferring measured depth comprises inferring a first measured depth between the specified location and the surface, determining a second measured depth between the specified location and the bottom hole assembly, then combining the first and second measured depths.
10. An apparatus for enhancing downhole telemetry performance comprising:
(a) a pressure sensor for measuring downhole pressure at a specified location;
(b) a telemetry signal transmitter;
(c) a processor with a memory having recorded thereon steps and instructions for
i. inferring a measured depth from the measured downhole pressure; and
ii. modifying a downhole telemetry signal of the transmitter at one or more measured depths in order to offset signal-to-noise ratio reduction with increasing measured depth.
11. An apparatus as claimed in claim 9 wherein the step of inferring comprises correlating the measured downhole pressure with the measured depth using a predicted equivalent circulating density at the specified location.
12. An apparatus as claimed in claim 11 wherein the pressure sensor is configured to measure annulus pressure or bore pressure or both.
13. An apparatus as claimed in claim 12 wherein the step of inferring a measured depth comprises associating a measured annulus pressure to a predicted annulus pressure then selecting a measured depth corresponding to the associated predicted annulus pressure.
14. An apparatus as claimed in claim 10 wherein the apparatus is part of a bottom hole assembly in a drill string with no repeater and the specified location is the location of the bottom hole assembly in a well bore.
15. An apparatus as claimed in claim 10 wherein the apparatus is part of a repeater in a drill string having a bottom hole assembly and at least one repeater and the specified location is the location of the repeater closest to the surface, and wherein the step of inferring measured depth comprises inferring a first measured depth between the specified location and the surface, determining a second measured depth between the specified location and the bottom hole assembly, then combining the first and second measured depths.