1. Field of the Invention
The invention is in the field of Nuclear Magnetic Resonance (xe2x80x9cNMRxe2x80x9d) tools and data processing methods. More specifically, the invention pertains to detecting borehole contamination and correcting NMR data for the effects of the contamination in well logging.
2. Description of the Related Art
Nuclear Magnetic Resonance has uses in many areas, including the fields of medicine, chemistry, non-destructive testing, and in well logging in the oil exploration industry. In the well logging industry, NMR is used in determining properties such as porosity of the formation, permeability, the bound liquid volume, the clay bound volume (CBW) and bulk volume irreducible (BVI), as well as other formation and reservoir fluid properties.
In a typical NMR device used in logging, a permanent magnet produces the static magnetic field and establishes the direction of orientation for the nuclear magnetic moments in the vicinity of the borehole. An RF field is applied in the plane perpendicular to the static magnetic field. Typically in the art, the static field B0 is a function of distance from the tool. Thus, at a given frequency, the NMR resonance condition
ƒ=xcex3B0/2xcfx80xe2x80x83xe2x80x83(1) 
where ƒ is the frequency of the RF field, and xcex3 is the gyromagnetic ratio must be satisfied. For a selected operating frequency, the location and size of the sensitive volume are determined by the magnetic field intensity, the field gradient and the effective bandwidth of the pulse. In the case of multi-frequency operations, a discrete number of closely spaced and substantially non-overlapping sensitive volumes are obtained. The union of these sensitive volumes is defined as the sensitive volume of a given tool with a given acquisition method.
Typically, drilling muds that are present within a borehole are oil or water based and hence have a large number of hydrogen nuclei: these are a strong source of contaminating NMR spin echo signals that may be stronger than the desired signals from the rock formation. To avoid receiving signals from within the borehole fluid, it is clearly desirable to contain the entire sensitive region within the rock formation and outside the borehole. In many tools, the sensitive region is a cylindrical shell which is coaxial to the permanent magnet, although other spatial arrangements can be used. Since the sensitive region lies close to the surface of the borehole cavity, geometric anomalies in the surface of the wellbore can cause portions of the sensitive region to lie inside the borehole cavity rather than inside the rock formation, causing NMR signals to be received from what is contained inside the borehole, usually drilling mud.
As one example of possible anomalies, the drilling tool can be off-axis with the borehole and additionally can be lying against one side of the borehole, revealing a portion of the sensitive region to the borehole cavity. In another example, the borehole might have an elliptical cross-section rather than a circular one. In yet a third possibility, there can be a significant amount of washout, where certain segments of the wall have separated and fallen away, leaving a cavity to one side of the borehole. Drilling muds typically contain 80% or more of fluids. This is much higher than the fluid content of the surrounding rock formation. Contamination of wellbore signals in NMR by mud signals spoils all critical petrophysical estimates including porosity, permeability, and T2 distribution.
The amount of contamination depends to a great deal on the depth of investigation of the measuring device. As an example, some methods, such as neutron porosity measurement, have a depth of investigation equal to 10xe2x80x3 to 20xe2x80x3, and consequently have very little if any contamination due to the presence of mud. However, current pad NMR tools have a depth of investigation of about 0.5xe2x80x3 to 1xe2x80x3 while centralized NMR tools have a 1xe2x80x3-3xe2x80x3 depth of investigation. When the sensitive region lies so close to the surface of the borehole, as is the case in NMR logging, the rugosity of the surface becomes important. With a high rugosity, there is ample opportunity for mud to enter into the sensitive volume and send anomalous signals to the receiver mechanism. Borehole rugosity is a valuable measure to know, since borehole rugosity allows the mud volume to infringe upon the measurements in the sensitive volume. Good knowledge of this value can alert the practitioner that the data needs to be flagged and corrected.
Although corrections for mud signals have been made in other logging methods, there is no similar method designed for use in NMR well logging. U.S. Pat. No. 3,321,625 (Wahl et al.) corrects for the effects of mud in gammaxe2x80x94gamma logging, using the knowledge of the mud density. U.S. Pat. No. 4,423,323 (Ellis et al.) addresses the problem in neutron logging.
Mechanical lever devices have been used in other logging processes to determine the amount of offset of the NMR tool. But these devices have not been used for the purpose of NMR measurements. Measuring devices can also be used to determine the borehole rugosity.
The present invention is a method of correction of data or results of processing data made by an NMR tool within a borehole for effects of borehole fluids. A fractional volume of the region of investigation of the NMR tool that lies within the borehole is determined. This is based upon knowledge of the geometry of the volume of investigation of the tool and its temperature dependence. Suitable averaging may be done within the vertical aperture of the NMR tool as well as averaging over a plurality of depths. The characteristics of the borehole fluids are either known or are measured within the borehole at a depth where the entire volume of investigation lies within the borehole. In one embodiment of the invention, a correction is made to the results obtained by processing the uncorrected data using prior art methods. In another embodiment of the invention, the NMR signals are corrected prior to processing.