Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2002-09-19
2004-12-21
Gutierrez, Diego (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
Reexamination Certificate
active
06833699
ABSTRACT:
FIELD OF THE INVENTION
This application is directed to nuclear magnetic resonance (NMR) logging, in particular to a method and system for in-situ calibration and even more particularly to calibrated transformations, which can be applied to NMR logs to quantify non-producible water in hydrocarbon-bearing reservoirs.
BACKGROUND OF THE INVENTION
Due to the environmental and economic factors, the oil and gas industry typically conducts comprehensive evaluation of underground hydrocarbon formations to predict their producibility. Formation evaluation, also know as logging, determines potential performance of a hydrocarbon reservoir at the early stages of its development thus minimizing both the environmental impact and financial investment associated with further reservoir development. Known subsurface geological evaluation techniques include sonic logging, gamma ray logging, and electric logging. Recently, however, progress in nuclear spectroscopy and borehole imaging resulted in the development of a nuclear magnetic resonance (“NMR”) well-logging technology, which ensures environmentally safe formation logging that is unaffected by variations in the matrix mineralogy.
The principle underlying the NMR logging is that an assembly of magnetic moments, such as those of hydrogen nuclei, when exposed to a static magnetic field, aligns along the direction of the magnetic field. Upon consequent application of an oscillating magnetic field, the direction of the magnetic moments is tipped into the transverse plane. Upon cessation of the oscillating magnetic field, the magnetic moments precess to their original alignment thus generating a magnetic echo. The alignment time of the magnetic moments in the static magnetic field, also known as longitudinal or spin-lattice relaxation time, is characterized by a time constant T
1
. The alignment time due to the loss of coherence of the magnetic moments in the oscillating magnetic field, also known as transverse or spin-spin relaxation time, is represented by a time constant T
2
. These relaxation parameters are generally used to estimate, inter alia, saturation, porosity, permeability, as well as the type and amount of fluids that will be produced from a well. NMR measurements of these and other parameters of the geologic formation can be done using, for example, the centralized MRIL™ tool made by NUMAR, a Halliburton company. The MRIL™ tool is described, for example, in U.S. Pat. No. 4,710,713 to Taicher et al. and in various other publications such G. R. Coates, L. Xiao, and M. G. Prammer, “NMR Logging Principles and Applications”, 2000, Butterworth-Heinemann. Details of the structure and the use of the MRIL™ tool, as well as the interpretation of various measurement parameters are also discussed in U.S. Pat. Nos. 4,717,876; 4,717,877; 4,717,878; 5,212,447; 5,280,243; 5,309,098; 5,412,320; 5,517,115, 5,557,200 and 5,696,448, all of which are commonly owned by the assignee of the present invention. The content of the above patents and publications is hereby expressly incorporated by reference.
One of the earliest and still the most widely used applications of NMR logging is estimating the bulk volume of irreducible water (BVI) of reservoir formations. It allows the user to partition porosity into static and dynamic quantities, those fluids that will be held to the rock and fluids that will be produced. BVI also provides information needed to compute permeability using a popular equation developed by Coates and Denno in “The Producibility Answer Product.” Current NMR methods used to determine BVI, such as cutoff-BVI and spectral-BVI, however, do not adequately incorporate capillary pressure, which is an essential feature of geological formation. An NMR bases method for determining BVI as a function of reservoir's capillary pressure would expand the scope of uses of NMR data, such as predicting free water levels, water block due to aqueous phase drilling and/or completion fluid retention, capillary pressure curves, more accurate determination of movable fluid and accurate determinations of hydrocarbon pore volume.
The cutoff BVI model (CBVI) is based on the observation made by Timur in “Pulsed nuclear magnetic resonance studies of porosity, movable fluid and permeability of sandstones,” that short relaxation times represent capillary bound fluids (BVI) and longer relaxation times represented free fluid index (FFI). Using a three component model and a “critical spin-lattice relaxation time” of 12 milliseconds, he achieved a good match to core derived irreducible saturation values using an air/brine displacement pressure of 50 psi. In 1990 Miller et al. in “Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination”, introduced a logging system that employed static and radio frequency magnetic fields capable of measuring spin-echo magnetic resonance to determine porosity. BVI was determined by fitting the spin-spin echo data using a bi-exponential equation constrained to a time gate of 21 milliseconds. Following the work of Timur the time gate method recognized that early echoes decayed rapidly due to high surface area pores that hold water to the rock's surface. The particular time selected was based on a best match to core derived irreducible saturations. However, the capillary pressure used to achieve this condition was not specified.
Recognizing that valuable information could be obtained with regard to pore size distribution and fluid types exponential fitting methods evolved into multi-exponential inversion. As a result, a relaxation time cutoff parameter of 25 to 46 milliseconds was implemented to obtain BVI. Following Timur, the cutoff value was selected based on comparisons to core measurements. Straley et al. in “NMR in Partially Saturated Rocks: Laboratory Insights on Free Fluid Index and Comparison With Borehole Logs” selected a T
1
cutoff of 46 millisecond for samples that had been centrifuged using an air/brine pressure of 100 psi. Dunn et al. in “On the Calculation and Interpretation of NMR Relaxation Time Distributions” concluded that a T
1
cutoff of 33 milliseconds compared best to samples de-saturated to an air/brine pressure of 400 psi. Morriss et al. in “Field Test of an Experimental Pulsed Nuclear Magnetism Tool” found that a 27-millisecond T
2
cutoff best approximated BVI when compared to core measured saturations centrifuged using an air/brine pressure of 50 psi. Prammer in “NMR Pore Size Distributions and Permeability at the Well Site” selected a 25 to 30 millisecond cutoff based on a best match between core measured brine permeability and computed NMR permeability using the free fluid model.
Subsequent studies report the relaxation time cutoff varied depending on lithology and mineral content. A notable comparison is the study of two carbonate formations one in west Texas described by Chang et al. in “Effective Porosity, Producible Fluid and Permeability in Carbonates from NMR Logging” and the other a Middle East carbonate described by Kenyon et al. in “A Laboratory Study of of Nuclear Magnetic Resonance Relaxation and its Relation to Depositional Texture and petrophycical Properties—Carbonate Thamama Group, Mubarraz Field, Abu Dhabi.” Chang et al. studied dolomitic carbonates and found that a 92-millisecond T
2
cutoff best fit samples centrifuged using an air/brine pressure of 100 psi. In contrast to this, Kenyon et al. found carbonate samples from the Mubarraz Field yielded a relaxation time cutoff of 190 milliseconds when de-saturated using an air/brine capillary pressure of 25 psi.
Coates et al. in the paper entitled: “A New Characterization of Bulk-Volume Irreducible Using Magnetic Resonance,” however, identified that CBVI model has several limitations. In particular, the model is susceptible to textural and lithological variations, because it relies on the assumption that smaller pores remain at 100% saturation and the cutoff value represents a threshold size of pore that limits drainage. As a result, Coates et al. developed a spectral BVI (SBVI) model that emulates films of water l
Galford James E.
Marschall David M.
Fetzner Tiffany A.
Gutierrez Diego
Halliburton Energy Service,s Inc.
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