Abstract:
A method and apparatus for processing nuclear magnetic resonance (NMR) information to differentiate between oil based mud filtrate (OBM), and connate oils, during subsurface exploration. An NMR experiment is performed on a sample of formation fluid, and the resulting data is then processed to differentiate between the relaxation time parameters characteristic of OBMs and those characteristic of connate oils. A simple arithmetic difference between relaxation times calculated for successive samples may be followed by a distributional analysis performed in either the time domain or relaxation spectrum domain. The process is repeated for other fluid samples and the data for successive fluid samples is compared so that the user may determine when the formation fluid consists almost entirely of connate oil.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the use of nuclear magnetic resonance (NMR) technology in subsurface exploration, and more particularly to a method and apparatus for processing NMR signals during formation sampling to differentiate between synthetic drilling fluids and connate oils. 
     BACKGROUND OF THE INVENTION 
     Performing measurements on fluid samples is desirable in many oil industry applications. In the prior art, such measurements are typically made by bringing samples to the surface using sealed containers, and sending the samples for laboratory measurements. A number of technical and practical limitations are associated with this approach. 
     The main concern usually is that the sample(s) taken to the surface may not be representative of the downhole geologic formation due to the fact that only a limited number of samples can be extracted and taken to the surface. In addition, because these samples frequently contain highly flammable hydrocarbon mixtures under pressure, handling of such test samples can be both hazardous and costly. 
     Nuclear magnetic resonance (NMR) technology has alleviated some of these problems by enabling a user to determine many properties of an in-situ formation fluid without extracting numerous samples. These properties include hydrogen density, self-diffusivity, and relaxation times, T 1  and T 2 . The use of NMR measurements to determine formation properties is known in the field. For example, in U.S. patent application Ser. No. 08/996,716, filed Dec. 23, 1997, by the inventor of the present invention, a method and apparatus for making direct downhole NMR measurements of formation fluids is disclosed. The apparatus of the referenced disclosure is a downhole formation tester capable of providing NMR measurements such as hydrogen density, self-diffusivity, and relaxation times. 
     One problem not solved by the prior art is borne from the use of oil-based drilling muds (OBMs). The OBM filtrate that invades a drilled formation consists almost entirely of the continuous phase of the OBM. This synthetic or highly refined base oil cannot be reliably differentiated from the connate oil stored in the formation by conventional measurements--resistance, dielectric constant, and viscosity. In certain cases it is impossible to determine at what point the fluid pumped out by a downhole formation tester changes from OBM filtrate to crude oil. Collecting a sample of OBM filtrate is at best useless and at worst misleading about the hydrocarbon contents of a formation. It is therefore apparent that there is a need for a reliable and efficient method for differentiating between OBM filtrate and connate oil, a method that will enable the user to minimize costly surface extractions by employing the existing downhole NMR measurement technology. 
     The present method is based, in part, on the use of the equipment, described briefly above, disclosed by T. Blades and M. Prammer in U.S. patent application Ser. No. 08/996,716, filed Dec. 23, 1997. The disclosed device is a nuclear magnetic resonance (NMR) module fitted onto a modular formation tester. It uses small samples of the pumped fluid to determine parameters such as the hydrogen index, spin-lattice relaxation time T 1 , spin-spin relaxation time T 2 , and/or self-diffusivity, the measurement of which is based on hydrogen NMR relaxometry. The content of this application is hereby incorporated by reference for all purposes. 
     One of the few differentiating features between OBM filtrate and crude oil is that OBMs are composed of base oils that are non-toxic, highly refined or synthesized hydrocarbons. Molecular weight and molecular structure are in general well controlled. Crude oil, on the other hand, is a random mix of variable-length and variable-structure hydrocarbons. From an NMR standpoint, many refined or synthesized base oils are characterized by simple, well defined monoexponential relaxation spectra (either T 1  or T 2 ). By contrast, the crude oil mix exhibits a spectrum of internal characteristic relaxation times and diffusivities. They manifest themselves as a spectrum of relaxation times. While this difference in relaxation time behavior between OBM filtrate and crude oil has been known in the field, see R. Kleinberg and H. Vinegar, &#34;NMR Properties of Reservoir Fluids,&#34; The Log Analyst, vol. 37, no. 6 (November-December 1996), pp.20-32, it has not been applied in the context of downhole testing to separate the fluid phases and to tell at what point a sample of connate oil has been obtained in a downhole fluid sampler. It should be pointed out that the relaxation time parameters are described above in general terms (i.e., without specific reference to spin-lattice relaxation time T 1  or spin-spin relaxation time T 2 ), because it is known in the art that, in bulk fluids, T 1  is roughly equivalent to T 2 , and any analysis based on differences in relaxation time behavior could involve either or both parameter(s). 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and apparatus for using nuclear magnetic resonance (NMR) measurements to differentiate between oil based mud filtrate (OBM) and connate oil, based upon differences in relaxation time parameter behavior, during subsurface exploration. 
     In a preferred embodiment, the method of the present invention comprises: providing a sample of formation fluid; performing an NMR experiment on a sample of formation fluid; and processing the NMR signals obtained in the experiment to differentiate OBM filtrate signal contribution from connate oil signal contribution. In a specific embodiment of the present invention, the method comprises the steps of: introducing a sample of formation fluid into an NMR measurement chamber; polarizing hydrogen nuclei of the fluid sample by applying a static magnetic field; inducing magnetic resonance at a resonance frequency proportional to the static magnetic field; and recording an echo-decay curve by means of a Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence. In this embodiment, hydrogen nuclei of the fluid sample are polarized by a static magnetic field of approximately 1000 Gauss and magnetic resonance is induced at a resonance frequency of approximately 4.2 MHZ. 
     In accordance with another aspect of the present invention, the method further comprises the steps of: repeating the steps of providing a sample, performing an NMR experiment on the sample and processing the NMR signals; comparing the processed data with that generated for previous fluid samples to determine when the reservoir fluid consists entirely of connate oil. In a specific embodiment the method further comprises the step of collecting a fluid sample for further analysis. In a specific embodiment, the method may also further comprise displaying the processing results to an operator. 
     In a preferred embodiment of the present invention, the step of processing NMR signals, described above, comprises determining an NMR relaxation time parameter for a current fluid sample. This parameter may be either the spin-lattice relaxation time, T 1 , or the spin-spin relaxation time, T 2 . It can be appreciated by one skilled in the art that, in bulk fluids, these two relaxation time parameters are roughly equivalent, so either or both may be employed in the method and apparatus of the present invention. In this embodiment, the step of &#34;comparing&#34; described above comprises taking the difference of relaxation time parameters calculated for successive fluid samples. A small difference between relaxation time parameters of successive fluid samples will be followed by a distributional analysis. It can be appreciated by one of ordinary skill in the art that this distributional analysis may be performed in either the time domain or the relaxation spectrum domain. The transform from time domain to relaxation spectrum domain is well known in the art and is described, for example, in U.S. Pat. No. 5,517,115, which is hereby incorporated by reference. It will also be obvious to one skilled in the art that, at some point during the analysis, one or more statistical parameters for a distribution (e.g. mean, variance, skew) may be determined for a distribution of relaxation time associated with a particular fluid sample. 
     In one aspect of the present invention, the distributional analysis described above is performed in the time domain and a uni-exponential model is fitted to the detected echo-decay curve. The average of squared differences between data points on the echo-decay curve and the uni-exponential model is computed and recorded as a fitting error (chi 2 ). An increase in fitting error denotes a shift in fluid sample content from OBM filtrate to connate oil. 
     In another aspect of the present invention, the processing of data is performed directly in the relaxation spectrum domain. In this embodiment, the half-amplitude of the distribution of relaxation times is measured and recorded. An increasingly broad half-amplitude distribution denotes a shift in fluid sample content from OBM filtrate to connate oil. 
     In a preferred embodiment, the apparatus of the present invention comprises: a vessel for containing fluid samples; an NMR testing module capable of performing an NMR experiment on a sample of formation fluid; an NMR signal processor to differentiate OBM filtrate signal contribution from connate oil signal contribution; and an output device for monitoring processor results. In accordance with a preferred embodiment of the invention, the NMR testing module capable of performing an NMR experiment further comprises: a means for polarizing hydrogen nuclei of a fluid sample by applying a static magnetic field; a means for inducing magnetic resonance at a frequency proportional to the static magnetic field; and a means for recording an echo decay curve by means of a CPMG pulse sequence. 
     In another preferred embodiment of the invention, the processor and output device of the apparatus is a computer system. In this embodiment the computer system comprises: a central processing unit (CPU), memory unit(s), one or more storage devices, and one or more input/output devices. A system bus is provided for communicating between the above elements. 
     The method and apparatus of the invention can thus signal the user as to when a sample of crude production can be collected and brought to the surface, thereby increasing the overall efficiency of both the testing and drilling processes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: 
     FIG. 1 is a plot of simulated CPMG data from OBM filtrate with T 2  =1000ms, best uni-exponential fit, and best multi-exponential fit; 
     FIG. 2 is a plot of simulated CPMG data from crude oil with T 2  &#39;s between 10 and 1000ms, best uni-exponential fit, and best multi-exponential fit; 
     FIG. 3 is a plot of the expected behavior of fitting error chi 2  as a function of time; 
     FIG. 4 is a plot of relaxation spectrum representative of all OBM base oils measured; and 
     FIG. 5 is a typical relaxation spectrum plot of a crude oil. 
     FIGS. 6A and 6B is a flowchart detailing the steps for practicing different preferred embodiments of the invention. 
     FIG. 7 is a schematic representation of an apparatus in accordance with the present invention. 
     FIG. 8 is a block diagram of a representative computer system which may serve as the processor and output device of the apparatus in a specific embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference is now made to FIG. 1 which illustrates a plot of simulated CPMG data from OBM filtrate with T 2  =1000ms. Data points 10 from a best uni-exponential fit and data points 12 from a best multi-exponential fit are plotted on the same set of axes. As is evident from FIG. 1, there is no significant difference between the two models; fitting the decay curve with a uni-exponential test function or a multi-exponential model essentially produces the same result. Collecting NMR data, constructing uni-exponential and multi-exponential models, and other NMR signal processing is known in the art and is described, for example, in U.S. Pat. Nos. 5,212,447; 5,280,243; 5,309,098; 5,412,320; 5,517,115; and 5,557,200 to the assignee of the present application, the relevant portions of which are hereby incorporated by reference. 
     Reference is now made to FIG. 2 which illustrates a plot of simulated CPMG data from crude oil with T 2  &#39;s between 10 and 1000ms. Again, data points 20 from a best uni-exponential fit and data points 22 from a best multi-exponential fit are plotted on the same coordinate system. As is evident from FIG. 2, and as can be appreciated by one skilled in the art, there is a strong difference between the two models. Again, it should be pointed out that while FIG. 1 and FIG. 2 are based upon measurements of the spin-spin relaxation time T 2 , of the fluid, in bulk fluids, either spin-lattice relaxation time, T 1 , or spin-spin relaxation time, T 2 , may be employed in the method and apparatus of the present invention. 
     Reference is now made to FIG. 3, which is a plot of the behavior of the fitting error, chi 2 , as a function of time. In a preferred embodiment of the present invention, such a plot would normally be created only after calculating a small difference between relaxation time parameters for successive fluid samples. Fitting error chi 2  is plotted along the y-axis 30 and time, measured in minutes to hours, is plotted along the x-axis 32. Segment 34 of the plot represents a period of low fitting error, when mostly OBM filtrate is produced from the formation. Segment 36 of the plot represents an increase in the fitting error chi 2 , and indicates an increase in the crude oil content of the sample. Segment 38 of the plot represents a time when the fitting error has risen and stabilized on a new plateau, characteristic for the T 2  spectrum of a crude oil. At this time, the formation fluid is determined to consist mostly of crude oil. 
     FIG. 1, FIG. 2, and FIG. 3 all represent an analysis performed in accordance with the present invention in the time domain. 
     Reference is now made to FIG. 4, which illustrates a plot of a relaxation spectrum representative of all OBM base oils measured. Once again, this type of plot would be created ordinarily when the calculated difference between relaxation time parameters for successive fluid samples is sufficiently small. Spin-spin relaxation time T 2 , measured in milliseconds, is plotted along the x-axis 50 and amplitude is plotted along the y-axis 52. Since the synthetic base oils measured are composed of simple C 16  /C 18  hydrocarbon chains with identical molecular correlation times, a single peak 54 is observed in the NMR T 2  relaxation spectrum. The position of peak 54 can change with temperature, but its characteristic shape does not. 
     Reference is now made to FIG. 5, which is a plot of a relaxation spectrum representative of a crude oil. Spin-spin relaxation time, T 2 , measured in milliseconds, is plotted along the x-axis 60 and amplitude is plotted along the y-axis 62. The long &#34;tail&#34; 64 towards short relaxation times is due to the presence of long hydrocarbon chains (e.g., asphaltenes) with long molecular correlation times. The position and particular shape of the distribution is of less importance than its width 66 compared to peak 54 in FIG. 4. 
     FIG. 4 and FIG. 5 represent an analysis performed in accordance with the present invention in the T 2  relaxation domain. Again, it is known in the art that, when dealing with bulk fluids, this type of analysis could also be performed in the spin-lattice (i.e., T 1 ) relaxation domain. 
     Reference is now made to FIGS. 6A and 6B, which are a flowchart detailing the steps for practicing different preferred embodiments of the present invention. A sample of formation fluid is introduced into a vessel (or NMR measurement chamber) in step 70. This aspect of the method is disclosed in detail in the co-pending application Ser. No. 08/996,716, which is incorporated herein by reference. The hydrogen nuclei of the fluid sample are polarized by application of a static magnetic field in step 72. In a preferred embodiment this field is approximately 1000 Gauss. In step 74, magnetic resonance is induced at a resonance frequency proportional to the static magnetic field of step 72, which in a preferred embodiment is approximately 4.2 MHz. An echo decay curve is recorded by means of a CPMG pulse sequence in step 76. The NMR signals are processed to differentiate OBM filtrate signal contribution from connate oil signal contribution in step 78. In a preferred embodiment, in step 80, this signal processing comprises determining an NMR relaxation time parameter for a current fluid sample. At this point, in step 82, if only one fluid sample has been taken, steps 70 through 80 are repeated for a another sample. In step 84, if multiple samples have been taken, and the simple difference between relaxation time parameters associated with successive fluid samples is small, a distributional analysis is performed. In the alternative, if the difference between relaxation time parameters for successive fluid samples is large, steps 70 through 84 are repeated for another fluid sample. 
     At this point, those skilled in the art will realize that the analysis can be performed either in the time domain or in the relaxation spectrum domain. An example of T 2  spectrum analysis is discussed in U.S. Pat. No. 5,517,115 which is incorporated herein for all purposes. 
     If time domain analysis is selected, in step 86, a uni-exponential model is fit to the detected echo decay curve. The average of squared differences between data points on the detected curve and the uni-exponential model is computed and recorded as the fitting error, chi 2 , in step 88. In step 90, the fitting error computed for the current fluid sample is compared to that computed for the previous fluid samples. In step 92, if the user is not satisfied that fitting error has stabilized, or if only one fluid sample has been taken, steps 70 through 90 are repeated for another fluid sample. If the user is satisfied that the fitting error has stabilized, he will proceed to step 100 and collect a fluid sample for further analysis. It will be appreciated that the observation of the fitting error can be automated in alternate embodiments and its stabilization can be determined, for example, by measurements of the slope of the fitting curve. Alternative embodiments will be readily recognized by those skilled in the art. 
     In the alternative, returning to step 84, if processing is selected in the relaxation spectrum domain, in step 94, the half-amplitude of the relaxation time distribution is measured and recorded. In step 96, the half-amplitude distribution width recorded for the current sample is compared to that for previous fluid samples. In step 98, if the user is not satisfied that the distribution width has stabilized, or if only one fluid sample has been taken, steps 70 through 84 and steps 94 and 96 are repeated for another fluid sample. If the user is satisfied that the fitting error has stabilized, he will proceed to step 100 and collect a fluid sample. The process can be automated to avoid human input, if necessary. 
     Reference is now made to FIG. 7, which illustrates, in a schematic form, an apparatus 110 in a preferred embodiment of the present invention. The apparatus comprises a vessel 112 for containing fluids, an NMR testing module 114 capable of performing an NMR experiment on the fluid sample contained within the vessel 112, a processor 116 to differentiate NMR signals, and an output device 118, such as a display, for monitoring the results. The design and operation of the vessel and the testing module are described in detail in U.S. patent application Ser. No. 08/996,716, filed Dec. 23, 1997, which is incorporated herein for all purposes. The operation of processor 116 is described above with reference to FIG. 6. It can be appreciated that the processor can be implemented in hardware or software, depending on the particular application. 
     Reference is now made to FIG. 8, which illustrates, in block diagram form, a representative computer system 120, which may serve as the processor and display elements of the apparatus of the present invention. Computer system 120 includes a central processing unit (CPU) 122, memory unit 124, one or more storage devices 126, and one or more input/output devices 128. In a preferred embodiment the system comprises a display 130, a communication interface 132, and a printer 134. A system bus 136 is provided for communicating between the above elements. 
     While the present invention has been described with reference to the preferred embodiments, those skilled in the art will recognize that numerous variations and modifications may be made without departing from the scope of the present invention. Accordingly, it should be understood that the embodiments of the invention described above are not intended as limitations on the scope of the invention, which is defined only by the following claims.