Seismic exploration and fracking

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1. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 1 LAND SEISMIC OIL EXPLORATION & HYDRAULIC FRACTURE TECHNIQUE AUTHOR: ENG. ALEJANDRO LEVY –…
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  • 1. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 1 LAND SEISMIC OIL EXPLORATION & HYDRAULIC FRACTURE TECHNIQUE AUTHOR: ENG. ALEJANDRO LEVY – QA/QC SESIMIC SUPERVISOR SEIS.ENG01@GMAIL.COM BOLIVIA 2015 ABSTRACT • We are walking trough an “hyper-information” age. At the same time we hear, so many things that are said which some of them are and not are true. The oil and gas, we liked or not, have been with us for more than 50 years in more products that we can imagine. The meaning of this lecture is to present the operations carried out in one of the first steps to the oil and gas production. Explain why is done, in which way, their environmental impact, and risks to human health. Finally the much controversial technique, hydraulics or stimulation fracturing of wells, with the aim of giving the participant of this course a technical understanding of how it's done and their risks.
  • 2. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 2 TABLE OF CONTENTS • SEISMIC INTRODUCTION • GEOLOGY BASICS • 2D – 3D & 4D SEISMIC PURPOSE • SEISMIC DATA • LAND SEISMIC SOURCES • SEISMIC LAND SENSORS “THE GEOPHONE” • THE SEISMIC LINE • THE RECORDING TRUCK “WHITE HOUSE” • SEISMIC DATA PROCESSING & INTERPRETATION • ENVIRONMENTAL CONCERNS FOR ONSHORE OIL & GAS EXPLORATION • FRACKING SEISMIC INTRODUCTION
  • 3. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 3 SEISMIC HISTORY • The first use of explosive to delineate structures under the earth was in the 1920’s and 1930’s in the Southern U.S. and South America • Digital processing and tape recording made a great improvement in the seismic techniques in the 50’s WHY, SEISMIC EXPLORATION • Seismic exploration is the search for commercially economic subsurface deposits of crude oil, natural gas, and minerals by the recording, processing, and interpretation of artificially induced shock waves in the earth. • Artificial seismic energy is generated on land by shallow borehole explosives such as dynamite, or surficial vibratory mechanisms known as vibrators. • Seismic waves reflect and refract off subsurface rock formations and travel back to acoustic receivers called geophones (on land). • The travel times (measured in milliseconds) of the returned seismic energy, integrated with existing borehole well information, aid geoscientists in estimating the structure (folding and faulting)and stratigraphy (rock type, depositional environment, and fluid content) of subsurface formations, and facilitate the location of prospective drilling targets.
  • 4. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 4 GEOLOGY BASICS
  • 5. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 5 ROCK TYPES • Rocks can be classified into three main types, depending on the chemistry of their formation a. Igneous Rocks: These rocks were formed by the cooling and subsequent solidification of a molten mass of rock material, know as magma. b. Metamorphic Rocks: Are those whose composition and texture has been altered by heat and pressure deep within the Earth’s crust. c. Sedimentary Rocks: Sedimentary rocks are the weathered debris derived by the slow processes of erosion of upland regions containing other rock types. ROCKS IGNEOUS METAMORPHICS SEDIMENTARY
  • 6. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 6 SEDIMENTARY BASINS • Sedimentary basins were formed over hundreds of millions of year by the action of the deposition of eroded material and the precipitation of chemicals and organic matter in the sea water. • External geological forces then distort and modify the layered strata.
  • 7. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 7 • The following sequences of pictures show (exaggerated) the formation of a typical basin. • Sediment collects on the sea-bed, the weight causing subsidence. • Different materials collected at the different times, so producing the regular “layering” of strata in the basin. • Volcanic action, or the movement of land masses, causes faults to appear in the basin.
  • 8. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 8 • These same forces cause rotation of the overall basin forming a new mount range • Erosion of the highlands, and additional subsidence forms yet another area of low-lying land that is filled with water forming another ancient sea. • Additional sedimentation takes place, causing an “unconformity” in the underlying strata.
  • 9. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 9 • Finally, land mass movement causes folding and distortion of the basin OIL AND GAS FORMATION • The temperature increases with depth within the Earth’s crust, so that sediments, and the organic material they contain, heat up as they become buried under younger sediments. • As the heat and pressure increase, the natural fats and oils present in buried algae, bacteria and other material link and form kerogen, an hydrocarbon that is the precursor of petroleum • As this source rock becomes hotter, chains of hydrogen and carbon atoms break away and form heavy oil. • At higher temperatures the chains become shorter and light oil or gas is formed. • Gas may also be directly formed from the decomposition of kerogen produced from the woody parts of plants • This woody material also generates coals seams within the strata • If the temperature and pressure gets too high, the kerogen becomes carbonized and dose not produce hydrocarbons.
  • 10. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 10 • The oil and gas produced by these processes may be in any combination and are almost always mixed with water • The minute particles of hydrocarbon are produced within the pores of PERMEABLE ROCKS (i.e.: sandstone) and, being lighter than the surroding material, move up through the rock until prevented from doing so by an IMPERMEABLE ROCK. • Although the initial source rock may only contain minute amount of hydrocarbon, as the particles of oil, gas and water move or MIGRATE, through the pore space within younger permeable rocks, they coalesce into large volumes • By the time this movement is stopped by the presence of a cap of impermeable rock (or when they reach the surface) the total hydrocarbon volume may be large enough to be a produce an oil or gas field that will be profitable to develop. • The ultimate profitability of such a field depends, of course, on external economic forces and world demand as much as on ease of extraction • As seismic exploration is concerned with the imaging of sub-surface structures, it is those structures that may indicate a potential hydrocarbon trap that are of most interest to the explorationist.
  • 11. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 11 OIL & GAS FORMATION AND TRAPS 2D – 3D & 4D SEISMIC PURPOSE
  • 12. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 12 2D – 3D & 4D SEISMIC • 2D Seismic shows a single slice of the earth. • 3D Seismic Shows a volume of earth. • 4D Seismic shows a 3D volume at different times in the life of an oil and/or gas field. • Seismic is the primary choice of data collection today for oil and gas exploration. 2D Seismic 3D Seismic
  • 13. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 13 SEISMIC DATA WHAT IS THE SEISMIC DATA? • Seismic data is an image of the earth below the surface of the ground. • Seismic data shows different rock formations as layers of reflectors. • Different rock types, and the fluids in the rocks, cause seismic reflection events. • Seismic data is collected in the field, processed in a computer center, and interpreted by a geophysicist.
  • 14. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 14 SEISMIC TRACE • The data recorded from one “shot” (one detonation of an explosive or implosive energy source) at one receive position is referred to as a seismic trace HOW THE SEISMIC DATA LOOKS LIKE
  • 15. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 15 • This seismic Trace is recorded as a function of time measured in milliseconds (the time since the shot was fired). • As this time represents the time taken for the energy to travel into the earth, reflect, and then return back to the surface. • During the processing sequence these traces are combined together in various ways, and modified by some fairly complex mathematical operation. SEISMIC PROFILE • The display of many traces side-by-side in their correct spatial positions produces the final “seismic section” or “seismic profile” • The seismic profile provides the geologist with a structural picture of the subsurface
  • 16. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 16 LAND SEISMIC SOURCES THE IDEAL SEISMIC SOURCE • Changes in the speed (velocity) of sound and the density within particular rocks causes reflection and refraction of the sound wave produced by a seismic source. • Specifically, variation of these parameters at an interface between two different rock types causes a reflection of some of the seismic energy back towards the surface. • It is the record of these reflections against time that produce our seismic section.
  • 17. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 17 • A seismic reflector can only reflect back to the surface an image of the energy pulse it receives. • If we send a complex pulse into the ground, that pulse will be superimposed on every reflector we record. • For this reason we wish to make the actual seismic source as close as possible to a single pulse of energy - a spike. • A spike of energy sent into the earth produces a set of clear reflections. • A more complex energy pule produces confused reflections
  • 18. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 18 • In practice and ideal spike is impossible to achieve. • As spike implies that an infinitely wide range of frequencies need to be present in the source, all released over an infinitesimally small time range. • The earliest seismic surveys used explosives as a seismic source with, for offshore exploration, up to 50 pounds (23 kg) of dynamite being exploded just below the surface of the water. • This is a very effective source, still used for onshore surveys, and for offshore the source use is the ‘airgun’. EXPLOSIVES • The Explosive source develop its power in a very short time (theoretically “cero”) VIBRATORS • Vibrational Sources (vibrators) distribute their power for a sustained period of time, usually several seconds
  • 19. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 19 EXPLOSIVES ADVANTAGES Dynamite is a high –power source of short duration As such, it creates a compact wavelet with a wide bandwidth Other advantages over vibrator trucks are its light weight, low cost, lack of required maintenance and capacity for deployment in rugged terrain unreachable by vehicles DISADVANTAGES  The process of drilling shot holes, burying the dynamite and cleaning up after the operation is labor intensive, and with this option the survey geometry cannot be changed without drilling new shot holes.  The input signal can be neither measured nor reliably repeated.  Explosive sources are subject to strict security regulations and permission for use and transportation may be difficult to obtain in some places.  The potential for causing damage prevents their use in populated areas. CHARACTERISTIC OF SEISMIC DYNAMITE • Explosive developed for seismic work use nitroglycerin and/or nitrocellulose as active ingredients. • The substances in their pure state are extremely dangerous and highly volatile. • However, when these highly explosive substances are absorbed by a pores material such as wood pulp, kieselguhr, powdered chalk, or roasted flour they are quite safe to transport, to store and use. • Currently the gelatin dynamites are the most widely used in seismic work world wide.
  • 20. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 20 • Some of the most important characteristics that seismic dynamite must possess are: High explosive power High detonation rate Great water resistance Effective detonation under great water pressure High density Freezing resistance Safety in handling • EMULSION HIGH EXPLOSIVE
  • 21. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 21
  • 22. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 22
  • 23. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 23 SEISMIC EXPLOSIVES • WesternGeco and Dyno Nobel developed dBX purpose-built seismic explosive, the first explosive specifically designed for seismic use. • The formulation offers significant geophysical benefits over conventional explosive, optimizing energy transfer to the earth and delivering higher S/N and greater bandwidth than dynamite. • A comparison test in Canada demonstrated the capability of the dBX source to improve imaging of deep reflectors
  • 24. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 24 VIBROSEIS • Vibrators are a Surface source • In a Vibroseis survey, specially designed vehicles lift their weight onto a large plate, in contact with the ground, which is then vibrated over a period of time (typically 8-20 seconds), with a sweep of frequencies. • Seismic vibrators are the predominant source used in land seismic exploration today • The performance of a seismic vibrator is dictated by its actuator, which is composed of a driven and a driving structure. • The main element of the driven structure is the baseplate which is pressed to the ground by weight of the truck • The main element of the driving structure is the heavy reaction mass. A piston inside the reaction mass is mounted above the baseplate with a hydraulic system to drive the mass up and down
  • 25. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 25 • During operation the vibrator moves into position and lowers the baseplate to the ground, where it applies a compression to the earth. By controlling hydraulic fluid flow around the piston inside the mass, the vehicle operator can make the piston and base plate assembly move up and down at specific frequencies, transmitting energy through the baseplate and into the ground. • The base plate is often coupled with a large fixed weight known as the hold-down weight • During those parts of the cycle in which the reaction mass is moving down and the base plate is moving up, the hold-down weight applies a compressive force to keep the base plate in contact with the ground HOW THE VIBRATOR WORKS HOW THE VIBRATOR WORKS (CONT.) • Harmonic distortions, or resonances, both in the vibrator and at the earth/baseplate interface, can have the effect of additional upward-directed force and must be considered in the selection of the desired vibrator output. • Increasing the hold-down weight on the vibrator adds stability to the system and helps establish optimal operating conditions. • For coupling (base plate/ground) stability the hold down weight limit should be between the 70-85%
  • 26. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 26
  • 27. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 27 • The energy developed in a sweep may or may not be sufficient to meet the proposed requirement (target depth), and therefore must be issued other sweeps (acting the vibrators in "fleet" or groups). • The fleet’s energy will be added in the Seismograph. VIBRATORS ADVANTAGES The energy spectrum can be controlled easily. The force applied to the ground can be monitors and adjusted in real time. Can be used in urban areas and can be equipped with special tires or track for deployment in environmentally sensitive areas, such as sand dunes or arctic snowpack. DISADVANTAGES The restriction of access in difficult terrains like swamps, mountains and coastal areas. Fleets of vibs are expensive and their maintenance as well. The input signal is not impulsive, so additional processing is required to extract interpretable data. A recorded trace is correlated with a reference trace to extract the reflected signal
  • 28. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 28 VIBS OPERATION VIBS OPERATION
  • 29. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 29 THE VIBROSEIS CONCEPT - SIGNALS USED IN VIBROSEIS OPERATIONS • One of the most important characteristic of the Vibroseis method is the limitation of the bandwidth of the source. • By this way, the Vibroseis technique allows us to generate only those frequencies we actually need whereas with an impulsive source like dynamite, some of the frequencies generated by the blast are ignored during the seismic acquisition. VIBROSEIS SYSTEM DESCRIPTION • The Sercel Vibroseis System is composed of : • A sweep generator, • A vibrator to emit the sweep into the earth, • A correlator to compress the long sweep into a short reflection pulse, • The correlator consists of a correlation process stage (FTP board in the Central Control Unit) that detects the reflected sweeps.
  • 30. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 30 HOW TO GENERATE A VIBROSEIS BAND-LIMITED SIGNAL • The signal that makes it possible to have a band-limited component amplitude spectrum, through Fourier Transform, is represented in time like that shown in Fig. 1 - c. • Unfortunately, the shape of that signal is not suited for the Vibroseis technique that requires a long, low-power rather than short, high-power signal. • To describe the signal used in the Vibroseis technique, we have to change the short, high-power signal (c) into a long, low-power signal while preserving the limited bandwidth of the component amplitude spectrum. • This signal is virtually a sine wave, called sweep in the Vibroseis terminology
  • 31. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 31 • To expand a short pulse of high peak amplitude into a long sweep of low peak amplitude you need to apply some frequency-dependent delays. The energy in both forms of the signal (i. e. pulse or sweep form) is the same. That’s why Vibroseis is not a low energy system but a low power system.
  • 32. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 32 • Naturally, in real-world situations we have to deal with multiple reflectors, hence multiple reflections. If the reflection time is shorter than the duration of the sweep, this causes the signals picked up by the geophones to overlap: • Where • Trace (a) shows the sweep reflected from the first reflector, • Trace (b) shows the same from the second reflector, • Trace (c) is the signal detected by the geophone, i. e. the sum of traces (a), and (b). • Trace (c) is passed through the correlator to generate trace (d). • The correlator boosts the signal and leaves the noise unchanged
  • 33. Author: Eng. Alejandro Levy - QA/QC Marine/Land Supervisor 17/12/2015 33 • It should also be noted that for a given sweep amplitude in Vibroseis the way of increasing the energy in the sweep is to increase its duration or/and to increase the number of vibrators. The fact is, it is the long duration of the sweep that allows us to get the necessary energy into the ground. So, the peak amplitude of the correlator output improves with the duration of the sweep. • The side lobes of the auto-correlation function of a sweep can be reduced by tapering the start/ends of the sweep. • It is important to consider that the ground can be mathematically
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