Patent Application
20110061863
This invention is generally related to evaluation of subterranean formations, and more particularly to fluid monitoring and flow characterization based on resistivity measurements in zones of individually controlled brine injection.
Reservoir multiphase transport properties such as relative permeability and capillary pressure are important parameters for reservoir characterization, management, forecasting, and performance analysis. It is known to use wireline logging tools to measure native formation resistivity in order to help estimate multiphase flow parameters. For example, co-owned U.S. Pat. No. 5,335,542 describes characterization of formation properties by combining probe pressure measurements with resistivity measurements from electrodes mounted on a pad in wireline formation tester. As fluid is withdrawn or injected into the formation at known rates, the fluid pressure of the formation and electromagnetic data are obtained. The electromagnetic and fluid pressure data can then be processed using various formation and tool models to obtain relative permeability information, endpoint permeability and wettability.
Drilling mud is usually weighted to maintain wellbore hydrostatic pressure above that of the formation in order to prevent the well from blowing out. This causes borehole fluids to enter the formation. Further, as the borehole fluids enter the formation, a mudcake is deposited on the borehole surface. The presence of a fluid-invaded region and mudcake around the borehole distorts the logs and can therefore make interpretation difficult. Conversely, the displacement of one fluid by another leads to a characteristic signature that may be used to infer multiphase flow properties, provided the underlying physics is taken into account, such as described in U.S. Pat. No. 5,497,321.
One problem with calculating multiphase transport properties based on measured resistivity is that aspects of intentional fluid introduction and resistivity measurement are difficult to control. For example, it is difficult to create timely and uniform changes in salinity within the borehole from which distinct fronts of contrasting salinity would be created. Also, electrical pathways within the borehole and along the borehole wall can affect formation resistivity measurement. This is described in U.S. Pat. No. 6,061,634.
In accordance with one embodiment of the invention apparatus for performing tests on a subterranean formation from a borehole comprises: hydraulic isolators which create a plurality of hydraulically distinct zones when actuated; at least one hydraulic conduit for introducing fluid to the hydraulically distinct zones; and a plurality of sensors for obtaining measurements of formation resistivity adjacent to ones of the hydraulically distinct zones as fluids of different conductivity are introduced to those hydraulically distinct zones via the at least one hydraulic conduit.
In accordance with another embodiment of the invention a method for performing tests on a subterranean formation from a borehole comprises: creating a plurality of hydraulically distinct zones; introducing fluids of different conductivity to at least one of the hydraulically distinct zones via the at least one hydraulic conduit; and obtaining measurements of formation resistivity adjacent to ones of the hydraulically distinct zones as the fluids of different conductivity are introduced.
Embodiments of the invention help to overcome some of the problems mentioned above. For example, the creation of hydraulically distinct zones enhances creation of timely and uniform changes in salinity within the borehole from which distinct fronts of contrasting salinity are created. Also, undesirable electrical pathways within the borehole and along the borehole wall that affect formation resistivity measurement can be mitigated by disposing sensors on the hydraulic isolators, e.g. on packers that are placed in contact with the borehole wall.
The tool can be used to create distinct zones and implement zone-specific testing. The sets of packers which abut the borehole wall when inflated are used to create hydraulically distinct zones 116, 118, 120 proximate to the tool. More particularly, a hydraulically distinct zone is defined within the borehole between adjacent sets of inflated packers. The zones are hydraulically distinct because the packers impede fluid flow within the borehole between different zones. The number and position of the packers may be configured for a particular borehole or formation. Once the zones have been created, the coiled tubing 104 in conjunction with flowline branches in the tool are used to displace fluid in the zones with a new fluid having a different characteristic electrical conductivity, e.g. injecting a brine solution to increase conductivity. In particular, a main valve 121 is connected between the coiled tubing and the tool and a branch line connected to the tubing via a valve 122 is used to introduce fluid supplied from a surface reservoir via a pump. In order to individually service each potential zone, individual branch lines may be connected to the tubes at each zone. A wireline disposed within the coiled tubing communicates commands to actuate the valves individually or in one or more groups. Any of various techniques known in the art, including but not limited to using borehole fluid or bypass fluid, can be used to control inflation and deflation of individual packers. Flow rate in each zone and total flow rate are monitored with flow meters. Consequently, controlled actuation of main valve 121 and individual valves 122 enables zone-specific control of fluid introduction so that fluid characteristic type and concentration can be independently changed and simultaneously different in different zones. A practical advantage of this feature is that each zone can simultaneously be subjected to a different salinity schedule. As described in published PCT patent application US2008/055719, by Ramakrishnan et al, entitled A METHOD FOR IMPROVING THE DETERMINATION OF EARTH FORMATION PROPERTIES, filed 3 Mar. 2008, which is incorporated by reference, injection of fluids of different salinity at different points in time creates a plurality of salinity fronts propagating into the formation, which improves the sensitivity of measurements to multiphase flow functional properties such as relative permeability and capillary pressure.
Although the use of multiple salinity fronts improves results, an inability to control inter-layer fluid flow rate also affects the ability to infer horizontal and vertical movement of fluid. The illustrated tool helps to overcome this problem. The location of the hydraulically distinct zones relative to boundary layers 130 may be adjusted by moving the tool within the borehole using the cable, selectively actuating sets of packers, and selectively actuating isolation valves. One or more of these techniques can be employed to configure the tool to communicate to the formation at intervals of choosing. For example, the tool may be configured such that the hydraulically distinct zones under test do not traverse boundary layers. The approximate location of boundary layers relative to the tool can be detected by various sensors, as known in the art. The adjacent packers which define a hydraulically distinct zone are then selected and actuated such that certain zones do not traverse boundary layers, e.g. zones 116, 120. Depending on the desired zone size and inter-packer distance relative to the distance between boundary layers it may be desirable to reposition the tool within the borehole before actuating the packers. It is of course recognized that the isolation provided by the packers is not absolute, but is rather sufficient for the measurements being made by the tool. Once the packers are actuated, the isolation valves are employed to inject fluid into different zones. Because creation of some hydraulically distinct zones that traverse formation layer boundaries may be unavoidable, it may be desirable to identify such zones and exclude them from testing. For example, boundary-traversing zone 118 defined between two non-traversing zones 116, 120 would not be subjected to changes in salinity or resistivity measurements.
The sensors can be implemented using various electrical and electromagnetic technologies. In one embodiment of the invention the sensors 108 are disposed on the packers. As an example, electrode segmented or overlapping ring sensors may be disposed on the packers. This advantageously enables the electrodes to be in contact with the formation as fluid salinity is changed. Further, by having large area sectors, a significant current may be injected. Alternatively, referring now to
It will be appreciated by those skilled in the art that the other sensors 109 are utilized to obtain other information to be used with information from the electrical or electromagnetic sensors 108 to calculate characteristics such as relative permeability, endpoint permeability and wettability. For example, a record of changes in the fluid pressure, fluid flow rate into the formation and fluid temperature for a particular zone would be used along with data indicative of resistivity to produce information of greater value to the operator in accordance with techniques generally known in the art.
Those skilled in the art will recognize that the illustrated tool may be used for various other tests. For example, flow rates can be adjusted using the valves to conduct fall-off tests. Fall-off pressure can also be acquired following a complete shutdown.
In an alternative embodiment the tool is adapted for CO2 sequestration injection. In this alternative embodiment, CO2 injection fluid is pumped via the coiled tubing. More particularly, non-conductive CO2 displaces the conductive brine. Because the presence of CO2 increases formation resistivity significantly, profiling measurements obtained in this manner are a good indicator of interval uptakes, and also may be used to measure downhole relative permeabilities. It is also possible to infer anisotropy of the formation from the inferred CO2 migration pathways.
Additional applications include injection of enhanced oil recovery (EOR) agents such as surfactants and polymers and combinations thereof for evaluating their potential for improving oil recovery. A simple example would be to quantify improved oil displacement as a result of fluid injection.
While the invention is described through the above exemplary embodiments, it will be understood by those of ordinary skill in the art that modification to and variation of the illustrated embodiments may be made without departing from the inventive concepts herein disclosed. Moreover, while the preferred embodiments are described in connection with various illustrative structures, one skilled in the art will recognize that the system may be embodied using a variety of specific structures. Accordingly, the invention should not be viewed as limited except by the scope and spirit of the appended claims.
