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And S-Wave Seismic Attenuation for Deep Natural Gas Exploration and Development DE-FC26-04NT42243

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And S-Wave Seismic Attenuation for Deep Natural Gas Exploration and Development DE-FC26-04NT42243 Novel Use of P- and S-wave Seismic Attenuation for Deep Natural Gas Exploration and Development DE-FC26-04NT42243 Final Report October 1, 2004 to September 30, 2006 Issued: October 2006 Contributors Dr. Joel Walls* Dr. M. T. Taner* Richard Uden* Scott Singleton* Naum Derzhi* Dr. Gary Mavko** Dr. Jack Dvorkin** *Principal Contractor: Rock Solid Images 2600 S. Gessner Suite 650 Houston, TX, 77036 **Subcontractor: Petrophysical Consulting Inc. 730 Glenmere Way Emerald Hills, CA, 94062 Novel Use of P-wave and S-wave Seismic Attenuation for Deep Natural Gas Exploration and Development, Final Report DE-FC26-04NT42243 DISCLAIMER This report was prepared as an account of work sponsored by the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. 2 Novel Use of P-wave and S-wave Seismic Attenuation for Deep Natural Gas Exploration and Development, Final Report DE-FC26-04NT42243 ABSTRACT Deeply buried gas reservoirs along the Gulf of Mexico shelf are an important future energy resource for the U.S. One of the greatest problems encountered by operators in this area is identifying commercially viable targets for drilling. Because of the great depth of the reservoirs (over 15,000 ft), the most common 3D seismic methods for direct hydrocarbon indication, such as AVO (amplitude versus offset), are not reliable. Many wells have been drilled on deep AVO anomalies, only to find that they contain noncommercial quantities of gas (the so called “fizz- water” problem). Other problems in detecting deep gas formations are caused by inadequate offset in the seismic data acquisition and by high fluid pressures, which tend to make gas look more like water in a seismic sense. In 2004, Rock Solid Images undertook a project to demonstrate novel and robust techniques for reducing hydrocarbon indicator risk in deep gas sands by exploiting an additional set of completely independent indicators – the rock inelastic properties. These inelastic properties of P- wave and S-wave energy from multicomponent seismic provide a crucial added dimension of discrimination for pore fluids and lithology. The objective of this project was to develop and test a new methodology for computing P- wave and S-wave attenuation from standard well log data, using the log-derived attenuation for generating P-wave synthetic seismic traces with and without attenuation effects, and extracting seismic attenuation attributes from multicomponent P-wave and S-wave seismic data and relating these to the presence of high GOR oil or natural gas. The objective was achieved, resulting in a new algorithm to compute both Qp and Qs from conventional well log data, an algorithm to create full offset, full waveform synthetics incorporating the effects of attenuation, and two algorithms to compute attenuation from seismic data. We found that attenuation in seismic data can be related to gas-bearing reservoirs, and can be used as a reconnaissance tool in exploration; that attenuation can have a substantial impact on seismic response, both post-stack and pre-stack, and cause significant changes in seismic amplitude with offset, especially at the bottom of a gas zone; and we concluded that attenuation should be used in conjunction with other seismic attributes such as elastic attributes and geologic context in order to reduce risk in the search for DHIs. However, attenuation alone can be a valuable tool in deep targets because AVO may fail in these environments 3 Novel Use of P-wave and S-wave Seismic Attenuation for Deep Natural Gas Exploration and Development, Final Report DE-FC26-04NT42243 TABLE OF CONTENTS DISCLAIMER ................................................................................................................................ 2 ABSTRACT.................................................................................................................................... 3 TABLE OF CONTENTS................................................................................................................ 4 EXECUTIVE SUMMARY ............................................................................................................ 7 PART 1: ROCK PHYSICS RELATIONS FOR QP AND QS....................................................... 8 P-WAVE ATTENUATION IN RESERVOIR AND NON-RESERVOIR ROCK .................. 8 DEFINITIONS AND BASICS........................................................................................... 9 MODULUS DISPERSION AND ATTENUATION AT PARTIAL SATURATION..... 13 MODULUS DISPERSION AND ATTENUATION IN WET ROCK............................. 22 EXAMPLES OF ATTENUATION CALCULATION..................................................... 25 COMPARISON TO DATA.............................................................................................. 28 A THEORETICAL ESTIMATE OF S-WAVE ATTENUATION IN SEDIMENT.............. 33 S-WAVE ATTENUATION DATA.................................................................................. 34 S-WAVE ATTENUATION THEORY ............................................................................ 38 COMPARING Qp/Qs WITH Qp/Qps .................................................................................... 42 A SIMPLE DERIVATION............................................................................................... 43 LABORATORY EXAMPLE ........................................................................................... 46 PART 1 SUMMARY.............................................................................................................. 47 PART 2: SYNTHETIC SEISMOGRAM MODELING WITH Q................................................ 49 MODELING ATTENUATION EFFECTS IN A GAS RESERVOIR................................... 49 SYNTHETIC SEISMIC TRACES AT NORMAL INCIDENCE .................................... 50 SYNTHETIC SEISMIC GATHERS ................................................................................ 52 AVO CURVES FROM GATHERS ................................................................................. 53 TUNING AND ATTENUATION .......................................................................................... 55 HALF-SPACE MODELING OF PP REFLECTIONS WITH ATTENUATION .................. 60 THE EARTH MODEL ..................................................................................................... 60 CALCULATING THE AVO RESPONSE....................................................................... 62 ACCOUNTING FOR ATTENUATION IN AVO ........................................................... 62 APPROXIMATIONS ....................................................................................................... 63 ATTENUATION EFFECT ON AVO – ANALYTICAL APPROXIMATION............... 64 IMPLEMENTATION....................................................................................................... 65 TESTING.......................................................................................................................... 66 HALF-SPACE MODELING OF PS REFLECTIONS WITH OFFSET ................................ 68 4 Novel Use of P-wave and S-wave Seismic Attenuation for Deep Natural Gas Exploration and Development, Final Report DE-FC26-04NT42243 MATLAB APPLET FOR MODELING THE EARTH BEYOND THE WELL ................... 73 PART 2 SUMMARY.............................................................................................................. 75 PART 3: PROPERTIES OF PORE FLUIDS AT HIGH PRESSURE AND TEMPERATURE.. 77 COMPARISON TO THE BATZLE-WANG (1992) EQUATIONS...................................... 77 EFFECT ON ELASTIC PROPERTIES OF SAND ............................................................... 79 EFFECT ON AVO RESPONSE............................................................................................. 80 FLUID PROPERTIES ............................................................................................................ 81 PART 3 SUMMARY.............................................................................................................. 81 PART 4: ATTENUATION AND DISPERSION FROM SEISMIC AND LINK TO WELL DATA ........................................................................................................................................... 83 GENERAL DEFINITIONS.................................................................................................... 83 “CLOSING THE LOOP” FOR Q........................................................................................... 84 SPECTRAL RATIO METHODS ........................................................................................... 85 SPECTRAL RATIO FOR STRATIGRAPHIC ATTENUATION................................... 86 SPECTRAL RATIO FOR INTRINSIC Q, PLUS STRATIGRAPHIC ATTENUATION ..........................................................................................................................................
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