1. Field of the Invention
The invention is related to the field of electromagnetic induction well logging for determining the resistivity of earth formations penetrated by wellbores. More specifically, the invention addresses the problem of using multicomponent induction measurements in an anisotropic formation for reservoir navigation using determined distances to an interface in the earth formation.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, well boreholes are drilled by rotating a drill bit attached at a drill string end. The drill string may be a jointed rotatable pipe or a coiled tube. Boreholes may be drilled vertically, but directional drilling systems are often used for drilling boreholes deviated from vertical and/or horizontal boreholes to increase the hydrocarbon production. Modern directional drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at an end thereof that is rotated by a drill motor (mud motor) and/or the drill string. A number of downhole devices placed in close proximity to the drill bit measure certain downhole operating parameters associated with the drill string. Such devices typically include sensors for measuring downhole temperature and pressure, and tool azimuth and inclination. Also used are measuring devices such as a resistivity-measuring device to determine the presence of hydrocarbons and water. Additional downhole instruments, known as measurement-while-drilling (MWD) or logging-while-drilling (LWD) tools, are frequently attached to the drill string to determine formation geology and formation fluid conditions during the drilling operations.
Boreholes are usually drilled along predetermined paths and proceed through various formations. A drilling operator typically controls the surface-controlled drilling parameters during drilling operations. These parameters include weight on bit, drilling fluid flow through the drill pipe, drill string rotational speed (r.p.m. of the surface motor coupled to the drill pipe) and the density and viscosity of the drilling fluid. The downhole operating conditions continually change and the operator must react to such changes and adjust the surface-controlled parameters to properly control the drilling operations. For drilling a borehole in a virgin region, the operator typically relies on seismic survey plots, which provide a macro picture of the subsurface formations and a pre-planned borehole path. For drilling multiple boreholes in the same formation, the operator may also have information about the previously drilled boreholes in the same formation.
In development of reservoirs, it is common to drill boreholes at a specified distance from fluid contacts within the reservoir or from bed boundaries defining the top of a reservoir. In order to maximize the amount of recovered hydrocarbons from such a borehole, the boreholes are commonly drilled in a substantially horizontal orientation in close proximity to the oil water contact, but still within the oil zone. U.S. Pat. RE35386 to Wu et al., having the same assignee as the present application and the contents of which are fully incorporated herein by reference, teaches a method for detecting and sensing boundaries in a formation during directional drilling so that the drilling operation can be adjusted to maintain the drillstring within a selected stratum is presented.
The method comprises the initial drilling of an offset well from which resistivity of the formation with depth is determined. This resistivity information is then modeled to provide a modeled log indicative of the response of a resistivity tool within a selected stratum in a substantially horizontal direction. A directional (e.g., horizontal) well is thereafter drilled wherein resistivity is logged in real time and compared to that of the modeled horizontal resistivity to determine the location of the drill string and thereby the borehole in the substantially horizontal stratum. From this, the direction of drilling can be corrected or adjusted so that the borehole is maintained within the desired stratum. The resistivity sensor typically comprises a transmitter and a plurality of sensors. Measurements may be made with propagation sensors that operate in the 400 kHz and higher frequency range.
A limitation of the method and apparatus used by Wu is that resistivity sensors are responsive to oil/water contacts for relatively small distances, typically no more than 5 m; at larger distances, conventional propagation tools are not responsive to the resistivity contrast between water and oil. One solution that can be used in such a case is to use an induction logging tool that typically operates in frequencies between 10 kHz and 50 kHz. U.S. Pat. No. 6,308,136 to Tabarovsky et al. having the same assignee as the present application and the contents of which are fully incorporated herein by reference, teaches a method of interpretation of induction logs in near horizontal boreholes and determining distances to boundaries in proximity to the borehole.
U.S. Pat. No. 5,884,227, issued to Rabinovich et al., having the same assignee as the present invention, is a method of adjusting induction receiver signals for skin effect in an induction logging instrument including a plurality of spaced apart receivers and a transmitter generating alternating magnetic fields at a plurality of frequencies. The method includes the steps of extrapolating measured magnitudes of the receiver signals at the plurality of frequencies, detected in response to alternating magnetic fields induced in media surrounding the instrument, to zero frequency. A model of conductivity distribution of the media surrounding the instrument is generated by inversion processing the extrapolated magnitudes. Rabinovich works equally well under the assumption that the induction tool device has perfect conductivity or zero conductivity. In a measurement-while-drilling device, this assumption does not hold.
The Multi-frequency focusing (MFF) of Rabinovich is an efficient way of increasing depth of investigation for electromagnetic logging tools. It is being successfully used in wireline applications, for example, in processing and interpretation of induction data. MFF is based on specific assumptions regarding behavior of electromagnetic field in frequency domain. For MWD tools mounted on metal mandrels, those assumptions are not valid. Particularly, the composition of a mathematical series describing EM field at low frequencies changes when a very conductive body is placed in the vicinity of sensors. Only if the mandrel material were perfectly conducting, would MFF of Rabinovich be applicable.
U.S. patent application Ser. No. 10/295,969 of Tabarovsky et al. (“the Tabarovsky '969 application”), now U.S. Pat. No. 6,906,521, having the same assignee as the present invention and the contents of which are fully incorporated herein by reference, teaches a modification of the method of Rabinovich that applies MFF to induction measurements made with transmitters and receivers on a mandrel of finite conductivity. U.S. patent application Ser. No. 10/934,596 of Tabarovsky et al. (“the Tabarovsky '596 application), now U.S. Pat. No. 7,031,839 having the same assignee as the present application and the contents of which are fully incorporated herein by reference teaches methods for the optimum design of the MFF acquisition system for deep resistivity measurements in the earth. The frequencies at which the measurements are made are selected based on one or more criteria, such as reducing an error amplification resulting from the MFF, increasing an MFF signal voltage, or increasing an MFF focusing factor. In one embodiment of the invention, the tool has a portion with finite non-zero conductivity. The Tabarovsky '596 application teaches design of the MFF system for both wireline and MWD applications. In the case of MWD applications, the Tabarovsky 596 application also addresses the issue of reservoir navigation.
The teachings discussed above are all directed towards the use of conventional induction tools in which the transmitter and receiver coils are parallel to the tool axis. Such a tool may be referred to as the HDIL tool. U.S. patent application Ser. No. 10/373,365 of Merchant et al., published as U.S. 20030229449 having the same assignee as the present application and the contents of which are incorporated herein by reference teaches the use of multicomponent induction logging tools and measurements as an indicator of a distance to a bed boundary and altering the drilling direction based on such measurements. In conventional induction logging tools, the transmitter and receiver antenna coils have axes substantially parallel to the tool axis (and the borehole). The multicomponent tool of Merchant et al. has three transmitters and three receivers, with coils oriented in the x-, y- and z-directions and may be referred to hereafter as the 3DEX™ tool.
The teachings of Merchant are show that the 3DEX™ measurements are very useful in determination of distances to bed boundaries (and in reservoir navigation), Merchant discusses the reservoir navigation problem in terms of measurements made with the borehole in a substantially horizontal configuration (parallel to the bed boundary). This may not always be the case in field applications where the borehole is approaching the bed boundary at an angle. In a situation where the borehole is inclined, then the multicomponent measurements, particularly the cross-component measurements, are responsive to both the distance to the bed boundary and to the anisotropy in the formation. In anisotropic formations, determination of a relative dip angle between the borehole and the anisotropy direction may be used for navigation.
U.S. Pat. No. 6,643,589 to Zhang et al., having the same assignee as the present application and the contents of which are incorporated herein by reference, teaches the inversion of measurements made by a multicomponent logging tool in a borehole to obtain horizontal and vertical resistivities and formation dip and azimuth angles. The inversion is performed using a generalized Marquardt-Levenberg method. Knowledge of the relative dip angle could be used for reservoir navigation in anisotropic media. However, the method of Zhang, while extremely useful for wireline applications, may not be computationally fast enough to provide the angles in real time that are necessary for reservoir navigation.
U.S. application Ser. No. 10/186,927 of Xiao et al. (now U.S. Pat. No. 6,885,947), having the same assignee as the present invention and the contents of which are incorporated herein by reference, teaches the determination of a relative dip angle using HDIL and 3DEX™ measurements. Compared to Zhang, the method of Xiao has the added drawback of requiring HDIL measurements. In addition, it is not clear that the dip angles may be determined in real time for reservoir navigation.
There is a need for a method of processing multi-frequency data acquired with real MWD tools having finite non-zero conductivity to get estimates of relative dip angles that may be used in real time for reservoir navigation. The present invention satisfies this need.