Dual-pump formation fracturing

1. A method, comprising:

conveying a downhole tool within a wellbore penetrating a subterranean formation, wherein the downhole tool comprises a first pump and a second pump both disposed within the downhole tool, and wherein the second pump has a larger displacement than the first pump;

isolating a portion of the wellbore comprising inflating a pair of external packers of the downhole tool positioned on opposing sides of an outlet of a probe;

initiating a fracture in the subterranean formation after isolating the portion of the wellbore by pumping fluid into the formation using the first pump via the probe, wherein initiating the fracture using the first pump comprises pumping fluid into the isolated portion of the wellbore;

stopping operation of the first pump in response to detecting initiation of the fracture;

propagating the fracture in the subterranean formation by pumping fluid into the formation using the second pump via the probe, wherein propagating the fracture using the second pump comprises pumping fluid into the isolated portion of the wellbore;

measuring a fracture pressure of the formation after initiating the fracture but before propagating the fracture; and

measuring a closure pressure of the formation after propagating the fracture.

2. The method of claim 1 wherein initiating the fracture using the first pump comprises operating the first pump at a first pressure, wherein propagating the fracture using the second pump comprises operating the second pump at a second pressure, and wherein the first pressure is substantially greater than the second pressure.

3. The method of claim 1 wherein initiating the fracture using the first pump comprises operating the first pump at a first flow rate, wherein propagating the fracture using the second pump comprises operating the second pump at a second flow rate, and wherein the second flow rate is substantially greater than the first flow rate.

4. The method of claim 1 further comprising pumping wellbore fluids out of the isolated portion of the wellbore using at least one of the first and second pumps before initiating the fracture.

5. The method of claim 1 further comprising further conveying the downhole tool within the wellbore and repeating the initiating and propagating.

6. The method of claim 1 wherein the maximum flow rate of the first pump is less than a minimum flow rate of the second pump.

7. The method of claim 1 wherein the downhole tool further comprises:

a reservoir containing hydraulic fluid;

a hydraulically actuatable device configured to receive pressurized hydraulic fluid, wherein the hydraulically actuatable device comprises a displacement unit including an actuation chamber for one of traversing formation fluid into and out of the downhole tool; and

means for selectively flowing hydraulic fluid from at least one of the first and second pumps to the hydraulically actuatable device.

8. The method of claim 1 wherein the downhole tool further comprises at least one motor operatively coupled to the first and second pumps, and wherein initiating and propagating the fracture each comprise operating the at least one motor.

9. The method of claim 8 wherein the second pump when actuated in a first direction is to flow fluid and when actuated in a second direction is to substantially not flow fluid, wherein the means for selectively flowing hydraulic fluid include at least one shaft coupling the at least one motor to the first pump and the second pump, and wherein the at least one motor is to rotate in a selective one of the first and the second directions.

10. The method of claim 1 wherein one of the first pump and the second pump is a variable-displacement pump, and wherein the other of the first pump and the second pump is a fixed-displacement pump.

11. The method of claim 1 wherein stopping operation of the pump comprises switching pumping operations from the first pump to the second pump.

12. The method of claim 1 , wherein the probe comprises an extendable probe configured to extend from the downhole tool to contact the subterranean formation or a non-extendable probe configured to contact the subterranean formation via extension of a backup piston of the downhole tool.

13. A method, comprising:

conveying a downhole tool to a first depth within a wellbore penetrating a subterranean formation, wherein the downhole tool comprises a first pump and a second pump both disposed within the downhole tool, and wherein the second pump has a larger displacement than the first pump; and

without further conveying the downhole tool within the wellbore:

isolating a portion of the wellbore;

pumping fluid into the isolated portion of the wellbore with the first pump via a probe utilizing a first flow rate and a first pressure to initiate a fracture in the subterranean formation after isolating the portion of the wellbore;

stopping operation of the first pump and enabling operation of the second pump in response to detecting initiation of the fracture;

pumping fluid into the isolated portion of the wellbore with at least the second pump via the probe utilizing a second flow rate and a second pressure to propagate the fracture;

measuring a fracture pressure of the formation after initiating the fracture but before propagating the fracture; and

measuring a closure pressure of the formation after propagating the fracture.

14. The method of claim 13 wherein the first flow rate is substantially less than the second flow rate.

15. The method of claim 13 wherein the first pressure is substantially greater than the second pressure.

16. The method of claim 13 wherein the downhole tool comprises a motor operably coupled to the first and second pumps, wherein pumping fluid into the subterranean formation with the first pump comprises operating the motor in a first rotational direction, and wherein pumping fluid into the subterranean formation with at least the second pump comprises operating the motor in a second rotational direction substantially opposite to the first rotational direction.