Automated rate control system for hydraulic fracturing

1. A method for hydraulically fracturing a subterranean formation, comprising:

preparing and sending a first command signal from a master controller to a plurality of pumps of a pump system, wherein the first command signal specifies a flow rate output for each pump of the plurality of pumps to achieve a first target flow rate for a fracturing fluid being injected into the subterranean formation;

injecting the fracturing fluid into the subterranean formation at the first target flow rate;

monitoring over time a pressure of the fracturing fluid injected into the subterranean formation at the first target flow rate;

based on the pressure of the fracturing fluid injected into the subterranean formation, determining with the master controller when to increase a flow rate of the fracturing fluid to a second target flow rate; wherein determining with the master controller when to increase the flow rate of the fracturing fluid to the second target flow rate comprises:

measuring a maximum pressure at the first target flow rate;

calculating a slope of the pressure versus time after reaching the maximum pressure; and

determining to increase the flow rate of the fracturing fluid to the second target flow rate upon establishing that the slope is negative,

preparing and sending a second command signal from the master controller to the plurality of pumps, wherein the second command signal specifies the flow rate output for each pump to achieve the second target flow rate; and

injecting the fracturing fluid into the subterranean formation at the second target flow rate.

2. The method of claim 1 , wherein determining with the master controller when to increase the flow rate of the fracturing fluid to the second target flow rate comprises:

measuring a maximum pressure at the first target flow rate; and

determining to increase the flow rate of the fracturing fluid to the second target flow rate upon expiration of a predetermined time period following measurement of the maximum pressure.

3. The method of claim 1 , wherein determining with the master controller when to increase the flow rate of the fracturing fluid to the second target flow rate comprises:

measuring a maximum pressure at the first target flow rate;

calculating a time period elapsed between sending the first command signal and when the maximum pressure is measured; and

increasing the flow rate of the fracturing fluid to the second target flow rate upon expiration of the time period after measuring the maximum pressure.

4. The method of claim 1 , further comprising increasing the flow rate of the fracturing fluid to the first and second target flow rates at a constant rate.

5. The method of claim 1 , further comprising increasing the flow rate of the fracturing fluid to at least one of the first and second target flow rates at a variable rate.

6. The method of claim 1 , further comprising increasing the flow rate of the fracturing fluid to the second target flow rate based on the pressure of the fracturing fluid at the first target flow rate as measured over time.

7. The method of claim 6 , further comprising:

measuring a slope of the pressure of the fracturing fluid at the first target flow rate as measured over time; and

increasing the flow rate of the fracturing fluid to the second target flow rate based on the slope.

8. The method of claim 1 , wherein each pump includes a local feedback loop, the method further comprising:

obtaining a measured flow rate of the fracturing fluid;

comparing the measured flow rate against the flow rate output specified by the first command signal with each local feedback loop; and

adjusting operation of the corresponding pump with each local feedback loop when a difference between the measured flow rate and first command signal is determined.

9. The method of claim 1 , wherein each pump includes a master feedback loop, the method further comprising:

providing operational feedback data to the master controller via each master feedback loop; and

modifying operation of one or more of the plurality of pumps based on the operational feedback data.

10. A fracturing control system, comprising:

a fluid system that mixes and dispenses a fracturing fluid;

a proppant system that conveys proppant to the fluid system to be included in the fracturing fluid;

a pump system including a plurality of pumps that receive and convey the fracturing fluid into a wellbore to hydraulically fracture a subterranean formation;

a master controller communicably coupled to and configured to operate the fluid system, the proppant system, and the pump system, wherein the master controller comprises a computer programmed to:

prepare and send a first command signal from a master controller to the plurality of pumps and thereby specify a flow rate output for each pump to achieve a first target flow rate for the fracturing fluid being injected into the subterranean formation;

monitor over time a pressure of the fracturing fluid injected into the subterranean formation at the first target flow rate;

determine when to increase a flow rate of the fracturing fluid to a second target flow rate based on the pressure of the fracturing fluid injected into the subterranean formation; wherein the flow rate of the fracturing fluid is increased to the second target flow rate after measuring a maximum pressure at the first target flow rate and upon expiration of a time period elapsed between sending the first command signal and when the maximum pressure is measured; and

prepare and send a second command signal to the plurality of pumps and thereby specify the flow rate output for each pump to achieve the second target flow rate.

11. The fracturing control system of claim 10 , further comprising a local feedback loop associated with each pump, wherein each local feedback loop monitors and controls an output of each corresponding pump.

12. The fracturing control system of claim 11 , wherein each local feedback loop compares a measured flow rate against the flow rate output specified by the first command signal and adjusts operation of the corresponding pump when a difference between the measured flow rate and first command signal is determined.

13. The fracturing control system of claim 11 , wherein the local feedback loop for each pump comprises a closed-loop control mechanism selected from the group consisting of a proportional controller, a differential controller, an integrative controller, and any combination thereof.

14. The fracturing control system of claim 10 , further comprising a master feedback loop associated with each pump to provide operational feedback data to the master controller from each corresponding pump.

15. The fracturing control system of claim 14 , wherein the operational feedback data is selected from the group consisting of real-time measured flow rate, real-time measured pressure, a currently-engaged pump gear, a commanded flow rate, a minimum flow rate capacity in the currently-engaged pump gear, a maximum flow rate capacity in the currently-engaged pump gear, a minimum and/or maximum flow rate capacity in an additional pump gear, a maximum pressure in the currently-engaged pump gear, a maximum pressure in the additional pump gear, and kick out pressure.

16. The fracturing control system of claim 10 , further comprising a target feedback loop communicably coupled to the master controller to provide the master controller with feedback data corresponding to real-time total flow rate and total pressure of the fracturing fluid injected into the subterranean formation.

17. The fracturing control system of claim 10 , wherein the flow rate of the fracturing fluid is increased to the second target flow rate based on a slope of the pressure versus time after reaching a maximum pressure at the first target flow rate.

18. The fracturing control system of claim 10 , wherein the flow rate of the fracturing fluid is increased to the second target flow rate upon expiration of a predetermined time period following measurement of a maximum pressure at the first target flow rate.

19. The method of claim 1 , wherein the subterranean formation is accessed offshore.

20. The system of claim 10 , wherein the fracturing control system is located on an offshore rig.