NMR tracking of injected fluids
Formation testing systems and methods may inject fluids into a formation to initiate fractures and facilitate measurements of various formation properties. In accordance with certain disclosed embodiments, the injection tools are further provided with nuclear magnetic resonance (NMR) sensors to monitor the injected fluids and provide measurements of near-borehole fracture orientations and volumes. Contrast agents and/or magnetic resonance imaging (MRI) techniques may be employed. The fluid injection may occur via an extendible isolation pad, via a fracturing jet, or via an injection port in an isolated region of the borehole. The systems may employ pressure monitoring in conjunction with the NMR sensors to further enhance estimates of formation and fracture properties.
1. A logging system that comprises:
a logging tool that provides measurements indicative of nuclear magnetic resonance (NMR) signals in a sensing zone;
a fluid injector that, during said measurements, injects a fluid to open a fracture in or proximate to the sensing zone; and
a processor that determines, based at least in part on said measurements, NMR signal strength-versus-time values corresponding to different times during injection of the fluid, and wherein the processor determines a fracture volume based at least in part on the NMR signal strength-versus-time values.
2. The system of claim 1 , wherein the processor further derives a fracture orientation based at least in part on the NMR signal strength-versus-time values.
3. The system of claim 1 , wherein the logging tool collects magnetic resonance imaging (MRI) measurements.
4. The system of claim 1 , wherein the logging tool measures one or more relaxation time distributions.
5. The system of claim 1 , wherein the fluid includes an NMR contrast agent.
6. The system of claim 5 , wherein the contrast agent includes paramagnetic, ferrimagnetic, or ferromagnetic materials to reduce a relaxation time of the fluid.
7. The system of claim 5 , wherein the contrast agent includes nanoparticles that resist diffusion into a formation matrix.
8. The system of claim 5 , wherein the contrast agent is only added to the fluid after the fracture has been opened.
9. The system of claim 5 , wherein the contrast agent includes carbon-13 ( 13 C).
10. The system of claim 1 , wherein the fluid injector comprises a fracturing jet.
11. The system of claim 1 , wherein the fluid injector comprises an extendible isolation pad.
12. The system of claim 1 , wherein the processor determines the fracture volume by correlating NMR signal strength-versus-time values with pressure-versus-time values.
13. The system of claim 1 , wherein the processor identifies slope changes from the NMR signal strength-versus-time values, and wherein the processor associates different portions of the slope changes with fracture formation and increasing fracture volume.
14. A formation testing method that comprises:
injecting, by a fluid injector, a fluid into a formation to open a fracture through or near to a sensing zone;
collecting, by a downhole nuclear magnetic resonance (NMR) tool, NMR measurements from the sensing zone while injecting said fluid;
determining, by a processor, NMR signal strength-versus-time values corresponding to different times during injection of the fluid; and
determining, by the processor, an orientation of said fracture based at least in part on the NMR signal strength-versus-time values.
15. The method of claim 14 , further comprising estimating a volume of said fracture based at least in part on said NMR signal strength-versus-time values.
16. The method of claim 15 , further comprising displaying the fracture volume and orientation to a user.
17. The method of claim 14 , further comprising adding a contrast agent to said fluid after the fracture has opened.
18. The method of claim 14 , wherein said collecting includes adjusting the sensing zone to obtain NMR measurements as a function of at least azimuth and radial distance.
19. The method of claim 14 , wherein said injecting includes inflating one or more packers to isolate a borehole region near the sensing zone.
20. The method of claim 14 , wherein said injecting includes extending a probe to seat an isolation pad against a borehole wall near the sensing zone.
21. The method of claim 14 , wherein determining the orientation comprises correlating NMR signal strength-versus-time values with pressure-versus-time values.
22. The method of claim 14 , further comprising identifying slope changes from the NMR signal strength-versus-time values and associating different portions of the slope changes with fracture formation and increasing fracture volume.