The Pennsylvania State University. The Graduate School. Department of Electrical Engineering LOCATING PETROLEUM SOURCES USING DSP TECHNIQUES

Transcription

1 The Pennsylvania State University The Graduate School Department of Electrical Engineering LOCATING PETROLEUM SOURCES USING DSP TECHNIQUES A Thesis in Electrical Engineering by Wenxin Song 2015 Wenxin Song Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science May 2015

2 The thesis of Wenxin Song was reviewed and approved* by the following: Kenneth Jenkins Professor of Electrical Engineering Thesis Advisor Turgay Ertekin Professor of Petroleum and Natural Gas Engineering Undergraduate Program Chair of Petroleum and Natural Gas Engineering Head of John and Willie Leone Family Department of Energy and Mineral Engineering Qiming Zhang Distinguished Professor of Electrical Engineering Kultegin Aydin Professor of Electrical Engineering Head of the Department of Electrical Engineering *Signatures are on file in the Graduate School

3 iii ABSTRACT Over the last several decades, techniques used in detecting underground oil and gas have become mature. The most common way is using seismic technology which sends sound waves into the earth and uses geophones to receive the sound waves when they bounce back and analyzing seismic data to simulate stratum characteristics. Simulation is used to determine whether there are rocks containing oil and gas. This thesis devotes efforts to explain the whole detecting process as well as details in following steps such as acquiring data including, positioning/ surveying, seismic energy source, data recording, data interpretation, positioning at sea and on land, pre-data processing, data processing including deconvolution, stacking, reservoir imaging, reservoir characterization. Key Words: Data Acquiring, Data Processing, Stacking, Stacking, Reservoir Imaging

4 iv TABLE OF CONTENTS List of Figures... v Acknowledgements... vii Chapter 1 Introduction... 1 Chapter 2 Technique Introduction P Wave S Wave... 6 Chapter 3 Acquiring Data Introduction D Seismic Data Chapter 4 Acquiring Data At Land At Sea Energy Source Chapter 5 Data Processing Pre Data Processing Deconvolution Stacking Chapter 6 Reservoir Imaging Land Seismic Imaging Marine Seismic Imaging Chapter 7 Conclusion References... 32

5 v LIST OF FIGURES Figure 1-1. Subsurface [1] Figure 1-2. Underground oil and gas [2] Figure 1-3. Geologic cross section structure [4]... 3 Figure 2-1. Extraction of petroleum [5]... 4 Figure D P waves [6]... 5 Figure 2-3. Cross-section surface expression of P waves [6] Figure 2-4. S wave 3-D model [7] Figure 3-1. Sources and receivers Figure 3-2. Illustration of common depth point [9] Figure 3-3. Schematic of the seismic reflection method [9]... 9 Figure 3-4. Multichannel recordings for seismic reflection [9]... 9 Figure D seismic data [15] Figure D visualization [10] Figure D visualization [10] Figure 4-1. GAC broadband point-receiver [11] Figure 4-2. Receiver and source [11] Figure 4-3. Source ready [11] Figure 4-4. Check state [11] Figure 4-5. Validation state [11] Figure 4-6. Data acquisition ready [11] Figure 4-7. Positioning at sea [8] Figure 4-8. Energy source on land [8] Figure Seismic reflection at sea [12]

6 vi Figure Air gun [12] Figure 5-1. Data processing: Deconvolution [8] Figure 5-2. Data processing: Stacking [8] Figure 5-3. Data processing: Stacking 1 [8] Figure 5-4. Data processing: Stacking 4 [8] Figure 6-1. Land seismic imaging [14] Figure 6-2. Land seismic imaging [14] Figure D marine seismic imaging [15] Figure D marine seismic imaging [15] Figure D marine seismic imaging [15] Figure D marine seismic imaging [15] Figure 6-7. Subsurface reservoir [16]

7 vii ACKNOWLEDGEMENTS I would like to express my sincere gratitude to Professor Kenneth Jenkins for all his guidance and help throughout my thesis work. I also appreciate Professor Turgay Ertekin giving me precious advice on petroleum engineering which helps me a lot for my thesis. Furthermore, I wish to give my special thanks to my family and friends who always stand by me and give me great courage.

8 1 Chapter 1 Introduction Exploring oil and gas is essential since natural fuel is still the main source for industry as well as manufacturing. The formation of oil can be briefly described as the organism underground becomes kerogen and when the sediment temperature rises higher than 110 Celsius degree kerogen becomes oil [1]. If the temperature increases above than the boiling point, the oil then turns into gas. When pressure goes up, oil is squeezed to the upper ground which causes pores on the surface. While in most cases, there are rocks that prevent the oil from continuing coming up and they become seals and traps. Time after time the reservoirs are formed. The subsurface structure is shown in Figure 1-1. A simple model of underground oil and gas layers distribution is shown in Figure 1-2 below. Figure 1-1. Subsurface [1].

9 2 Figure 1-2. Underground oil and gas [2]. Similarly, Marcellus shale is also a type of sedimentary rock mostly in northeastern America, extending through Pennsylvania, New York, Ohio and West Virginia. It s common for certain underground area to be comprised of pre-existing rock fragments which contain silicon dioxide [2]. Being classified as black shale, Marcellus shale holds large amounts of natural gas which attracts many oil companies attention and makes them try every possible way to develop fracturing and drilling techniques to detect and drill Marcellus shale better and faster as much as possible. Two geoscience professors in United State think Marcellus shale can hold up to 500 trillion cubic feet (TFC s) of natural gas which shocked people nationwide [3]. Because if that is true, it means that it will be the biggest natural gas fields in America and have a great effect on the price of natural gas all over the world. Distribution of black shale, shale and siltstone, siltstone and shale, sandstone, red beds, chert and limestone is shown in the Figure 1-3. It s a cross section photo of geological structure and the areas of distribution from Tennessee to New York is also labeled according to each type of rocks.

10 3 Figure 1-3. Geologic cross section structure [4]. But the way to detect oil and gas underground is difficult, usually because they are contained in reservoir rocks, including sandstone which is especially suited to contain natural gas and oil. The tiny pore spaces can hold gas and oil onto molecules between each of grains of sand in rock. Additionally due to the complex formation process, it s not easy to find exactly where reservoirs are. Especially the drilling process is expensive and takes much energy and time. For this reason, detecting what kind of rocks contain oil and gas becomes the main objective for finding those energy resources. It includes their types and shapes as well as properties like gravitation and magnetism.

11 4 Chapter 2 Technique 2.1 Introduction Though it s impossible to predict whether there is natural oil and gas under the subsurface with a hundred percent accuracy, we still can gather data for as many clues as possible to image what exists underground. Acoustic survey is useful in getting the images of underground reservoirs. The most common way to find reservoirs is using seismic technology which sends sound waves into the earth and uses geophones to receive the sound waves when they bounce back [5] and measurements are estimated for the time taken to reflect back to earth. For example, air guns can generate acoustic pulses through underground rocks. When pulses meet substances, they will bounce back. Different sound speeds take different travelling time which helps us detect what kind of materials the sound waves go through. Because of different geological structures underground, the pulses we receive can help us understand what kind of layers beneath as well as the depths of each layer. Then we can create 3-D seismic images underground with software simulation. Modern machine used in drilling in shown in Figure 2-1. Figure 2-1. Extraction of petroleum [5].

12 5 2.2 P wave There are mainly two types of sound waves used in seismic signal processing techniques, named as P waves and S waves. A P wave, also known as primary wave, has highest velocity travelling through the earth among other seismic waves. Geologists use P waves primarily. P waves are known as compressional waves just the same as acoustic waves. The difference is that P waves move through solid and liquid media like ground which means that there are contraction and expansion zones. A 3-D model of P wave is shown in the Figure 2-2. Figure D P waves [6]. As shown in the Figure 2-2, P waves propagate longitudinally through solid earth. The vibration of wave is parallel to the energy wave s travelling direction [5]. When we look in the direction of right on P waves, it looks like sand grains on the surface of sandstone bed. Today, in exploring petroleum underground, usually we use data of two-way wave travel times of P waves to make up 3D seismic surveys and 2D seismic lines [6]. Figure 2-3. Cross-section surface expression of P waves [6].

13 6 When we describe the surface expression of P waves, we call them Rayleigh Surface Waves. They move up and down and propagate across the surface and can be examined in cross-section views. In the case of P wave sources like explosions, earthquakes and vibroseis trucks, there is response of Rayleigh waves which look like ripples travelling across the surface of earth. 2.3 S wave The other type of waves are Shear waves, referred as S waves previously. Shear waves travel through the earth rather than the surface. Differing from a P wave which parallels the wave, the S wave moves in the direction perpendicular to the wave propagation [7]. Its 3-D model is shown in the Figure 2-3. The cross section of 3-D version is shown in the Figure 2-4. Figure 2-4 S wave 3-D model [7].

14 7 Chapter 3 Acquiring data 3.1 Introduction In order to get representation of some portions of the earth s subsurface geologic structure graphically as accurate as possible, we need to do seismic exploring. Many companies want to evaluate a potential reservoir for oil or gas with lowest cost but highest accuracy. As a result, we have to produce seismic images appearing to their needs. A group of receivers and sources is shown in Figure 3-1. Figure 3-1. Sources and receivers. It s not as easy as it is supposed to be such straightforwardly for seismic imaging which needs leading-edge technology as well as human experience. First of all, we use sources of seismic energy which are being controlled and transmitted it into the earth. With these propagating waves, we can illustrate the region of interest (ROI). When the energy is reflected, diffracted, refracted, from geologic boundaries underground, it is recorded. Usually reflected waves go downward and bounce off a layer or object in the soil or rock and finally return to the earth surface. The refracted waves also go downward and turn at a geological boundary but they won t return back to the

15 8 surface until they go along the surface or rock layer. After that, we detect it on seismometers which are spread out along an array on earth. When we acquire data on land, we use geophones as seismometers. Because most of them measure the velocity s vertical component, sometimes we set multicomponent receivers so that we can deal with other situations for elastic wavefield. When we acquire seismic data in the ocean, we need to consider one situation that if the geophones move. Usually we use air guns or vibrators [8]. Air guns generate pulses like an explosion using air which is compressed. Vibrators can make compressional energy too. For the air guns, we set them in a linear or areal array under the sea, towing them to the vessel. As long as the energy reflects under the ocean, they can be detected because we set large amounts of hydrophones in streamers. Illustration of common depth point is shown in the Figure 3-2. S1 S2 represent sources and R1 R2 represent receivers. Figure 3-2. Illustration of common depth point [9]. There are mainly two types of seismic surveys, one which is a 2D survey and the other one is a 3D survey. Compared with 3D survey, 2D survey is much cheaper and display seismic data in a vertical when we slice the earth and get a cross-section plane. If the expenditure bound is not allowed, we can only use a 2D survey but we can obtain detailed exploration results. A single reflection model is shown in the Figure 3-3. Suppose there is only one source as well as receiver. The seismic wave goes through the surface of the earth and when it reaches

16 9 different layers of the subsurface, it has different angles of reflection respectively. They are all collected by the receiver on ground. Figure 3-3. Schematic of the seismic reflection method [9]. An example of multichannel recordings for seismic reflection is shown in Figure 3-4. In this case, there is still only one source but six receivers which collect reflections from different angles respectively. Figure 3-4. Multichannel recordings for seismic reflection [9].

17 D Seismic Data Several steps are essential for seismic data acquisition. First of all, the best location for the survey should be determined. Second energy sources like vibroseis on land and airguns in the ocean are set. After getting the recorded data, we need to process and interpret them for further research [8]. Seismic data is shown in Figure 3-4 and Figure 3-5. Figure D seismic data [15]. Figure 3-5. Data for seismic imaging [15].

18 11 3D seismic data are displayed as a three-dimensional cube that may be sliced into numerous planes or cross-sections. More expensive than 2D, 3D data produces spatially continuous results which reduce uncertainty in areas of structurally complex geology and/or small stratigraphic targets. Recently scientists use software simulation to process 3-D seismic data and simulate a 3-D model of subsurface geology structures [10]. A 3-D seismic data visualization is shown in Figure 3-6 and Figure 3-7. Figure D visualization [10]. Figure D visualization [10].

19 12 Chapter 4 Acquiring Data 4.1 At Land Thanks to advanced technology, we can acquire seismic data much faster at lower cost but with higher quality. We set bunches of point receiver sensors in array and transmit broadband data with minimum logical records [11]. An illustration of sets of broadband point-receivers is shown in Figure 4-1. Figure 4-1. GAC broadband point-receiver [11]. Where to position instruments on the surface of the ground is essential for accurate data acquisition. It also helps us get information about where we collect the data which is important for surveying. For this reason, we distribute the data receivers as well as seismic sources beforehand. With the exact location information, it s approachable to getting origin of these data with mathematical calculation. Thanks to the advanced satellite positioning system, we can get accurate positions of receivers and energy sources. The integrated point-receiver land seismic system can control every source point with each receiver lively [11], which is shown in Figure 4-2.

20 13 Figure 4-2. Receiver and source [11]. The acquisition system works and monitors the data continuously. A central acquisition system begins to check the related receiver s state whether the source has already prepared for data acquisition [11]. This step is shown in Figure 4-3. The central system checks whether the sources are ready for data acquisition is shown in Figure 4-4. Figure 4-3 Source ready [11]. Figure 4-4 Check state [11].

21 14 The acquisition system validates that everything is under control and within the spread quality rules. If there are any mistakes or a not ready state is detected, the system will show a failure signal for data acquisition. As long as there is one failure, the central system will command next available source group to take responsibility of acquiring data [9]. The status is shown in Figure 4-5. Figure 4-5 Validation state [11]. When the central system verifies that both the sources and receivers are ready to acquire data, it sends out signal information ok to sweep which is shown in Figure 4-6. Figure 4-6 Data acquisition ready [11].

22 At Sea The unstable undersea environment increases its difficulty to position in a real time series. All the equipment settled undersea would be motional continuously which means that we have to get the position information time after time. With these results, we can further get the accurate locations of source energy as well as hydrophones instantly with the movement [8]. A model of positioning at sea is shown in Figure 4-7. The system contains a navstar system, starfox satellite, GPS preference station with data link to muti uplink station, GPS connection signals from multiple preference stations, uplink station, and GPS preference station. Figure 4-7. Positioning at sea [8]. 4.3 Energy Source Generating energy into the earth surface is the first step for collecting data. A seismic vibrator is one type of energy source which can make energy signals which can cause refractions and reflections during the process. Seismic vibrators generate seismic waves in the way of continuous sweeps [8]. A 3-D reflection model of energy source on land is shown in Figure 4-8.

23 16 Figure 4-8. Energy source on land [8]. When under water, seismic vibrators are replaced by air guns. The compressed air is released and generates acoustic energy. In order to get the exact frequencies of sound, we arrange several air guns in a certain array under water. To collect the refracted and reflected energy, we use hydrophones which can transmit these energies to the system responsible for recording raw data. Usually the data is recorded in a couples of lines at the same time and then record them simultaneously [8]. Figure 4-9 shows a seismic wave used in the ocean. Figure 4-9. Seismic wave [16].

24 17 Figure Seismic reflection at sea [12]. Figure 4-10 illustrates how an energy source works at sea environment. Air guns are widely used as seismic energy sources in marine environment due to its multiple advantages. It can minimize oscillation caused by bubbles and optimize amplitudes between peak and peak [12]. The air gun sends out energy vertically downwards with frequencies in a broad band which can generate pulses having certain value of peak-to-peak amplitudes [11]. An air gun s interior structure is shown in Figure Figure 4-11 Air gun [12].

25 18 Usually air guns are placed in a linear array and the strength of the array is approximately proportional to the pressure value generated by the air gun array. To make the array signatures generated by air guns as close to ideal pulses as possible, we have to make sure the primary-tobubble ratio is high enough. At the land, we use geophones to receive the wave refractions as well as the reflections and then record the data by converting the signals to simple impulses. While at sea, hydrophones are used to detect the reflected energy source and then transmit it to the system for further recording.

26 19 Chapter 5 Data processing 5.1 Pre Data Processing In order to reduce the cost as well as danger of oil and gas exploration when working with complex unconventional reservoirs, we need to develop seismic data processing in highest quality. First, we want to make sure that all the seismic data is in correct imaging sequence so that the remainder of processing sequence can also be correct. During this pre-processing step, we have to take several problems into consideration, like de-bubble, statics correction and noise attenuation [23]. The role of seismic data processing is to remove the input pulse as well as noise, leaving only an earth model. Without the unwanted background noise as well as output pulse, the seismic data looks much more pure and good enough for research. There is existing data processing modules to remove the noise and make the data clean so that we can run the imaging algorithm well and the real data won t be affected by noise. We have to make sure that these models work reliably and accurately with fast speed as well as substantial computing power. Usually there are existing near-surface velocity anomalies as well as topography which can cause time shifts. Time shifts are annoying because they can reduce signal-to-noise ratios and distort reflecting horizon s geometry which are not easy for geologists to interpret data. As a result, we often apply a bulk shift to correct data and use module solutions for source refraction statics [23]. For the main three types of characteristics of seismic data, they are essential when we interpret the data. We remove multiples of the wave which is unwanted coherent energy and just leave the pure signal itself so that we can get accurate frequency, amplitude and phase estimates of the seismic data. Also we shape seismic wavelet for further data character interpretation. All these

27 20 geophysical processes as well as complicated math algorithms require large amounts of computing sources when applied to seismic data [21]. Part of the output pulses contained seismic energy which is reflected, as well as other noises come from the background. All of these outputs are combined together and we have to remove those undesirable noises to build up real model. As a result, accurate and reliable power with high speed is essential as well as complicated math calculations and related process in geophysics which results in requirement of high column storage space for seismic data processing [8]. There are some techniques for seismic data processing and other details we have to take into considerations. First of all, in order to decrease noise coming from the ground and other directions, vector filtering is useful and can thus increase energy recorded from a P-wave. Due to the fact that mode contamination is expected on ground. It is essential to separate s and p wave energy and horizontal parts of the p wave energy and take care of effects of amplitude variation with offset. Extract the directions of p and s waves separately for the faster ones, and slow ones and obtaining the attribute volumes as well as velocities [22]. Some problems come along with the traditional assumption. Because we can t always get the pure record of the energy. For example, when we record p wave energy, there are s wave parts contained in the record while when we try to record wave energy and there would also be some p wave parts including in it. Besides that, for the traditional way we measure the attribute amplitude as well as analyzing the amplitude variation with offset value with long offsets, the result is not correct enough due to the fact that usually the p wave is not as critically vertical as we suggest. For these reasons, it is more appropriate to acquire full wave [24] so that we can separate s and p wave energy efficiently and thus get their characteristics respectively. Furthermore, with wide azimuth technique [21], we can decrease the side effect of variations of velocity when processing seismic data which helps to achieve seismic images with higher definition. Also, it provides more

28 21 information such as gradients and velocities within certain distance as well as directions so that the more details about the underground structures we can predict [22]. 5.2 Deconvolution To best interpret geological structure using seismic processing, we need to convert seismic data recorded into a form which can help in description. Usually we use methods like seismic reflection to explore the hydrocarbons underground. It s is essential to improve image quality as well as the discrimination and identification ability for reflectors, which remains a great challenge for the reflection method. Also, the original reflection method is sensitive to prediction error which is calculated by actual values minus predicted values. As a result, we have to develop new approaches to processing and then filtering seismic data in order to improve the imaging fidelity. One way is that we use information from seismic trace s earlier parts and take deconvolution on the latter parts. What s called predictive deconvolution. During this process, we can predict signal s multiples and reverberation. The other advantage of predictive deconvolution is that we can predict trances with the information of traces from neighborhood in the case of multitrace sense [13]. Besides, it is also common to apply deconvolution on seismic data processing when we explore oil and gas underground. This is another type of deconvolution which is wavelet deconvolution. It helps us get better traces resolution and thus we can define the geology of subsurface much better with better thinner subsurface layers identification [25]. Because sometimes distortions and noise existing with primary signals when we are taking measurements, we use deconvolution methods to invert degradation which is quite common when doing image processing. When we are doing deconvolution, we can take an inverting step to the convolution process. But usually this process is not stable at all which causes noise amplification. As a result, removing the noise is essential in deconvolution process to make

29 22 it efficient. Using the wavelet method to solve the noise problem in deconvolution is a good choice. We create a wavelet to diagonalize the operator we have which could expand the exciting basis. The other appropriate way is that we use a wavelet basis which is fixed to inverse the convolution with Galerkin-type approach. We reduce the inversion to an operator which is truncated to some extent when the representation of operator looks like sparse in wavelets. The deconvolution process of seismic data is shown in Figure 5-1. Due to the background noise, the actual seismic response is contaminated but after the deconvolution and the noise canceled with each other and the signal is clearer for further research [8]. Figure 5-1. Data processing: Deconvolution [8]. 5.3 Stacking When measuring seismic reflections, we can get couples of seismic traces which have same midpoints for stacking according to specific geometry of energy source and receiver. The longer the distance between energy source and receiver, the more time it takes for seismic wave to travel back and forth. Then we record all the traces with each distance.

30 23 We add all the traces of seismic data which have all the same original points being reflected together to one trace and converge them in an order from left to right as shown in Figure 5-2. From the sum of stacking traces, we can get a higher percentage of signals among the total signals as well as noises than each seismic trace we have recorded. Coherent noises can be removed and other undesirable noises such as waves reflected by surface of the ground several times could be removed by stacking process. In this process, we can get rid of the noises automatically because they are canceled by summing process so that we can get a high percentage in pure signal according to the total signals with noises. In order to get images in high definition and more accurately, we stack as many traces as possible and insure a better output. An illustration of stacking is shown in Figure 5-2 [8]. Figure 5-2. Data processing: Stacking [8]. For every shot taken from the receivers, it contains the information of seismic data. The parallel traces group together as shown in the picture. The shorter distance between the receiver and seismic wave, the faster the wave reaches the receiver which is recorded earlier than other wave. Similarly, when the distance between the receiver and the seismic wave becomes longer, it takes longer in time series for wave to reach the receiver and the trace shows up lower in the stacking image. Those images we get are important clues for us to determine the characteristics of rocks

31 24 underground. For this reason, the clearer the images are, the more accurate analysis we can provide [8]. As a result, we differentiate noise from the signal itself as much possible so that we can remove it at the best level. As we continue stacking, the time domain filtering image gets more and more close to the geology characteristics underground. Then we can illustrate the events amplitudes as well as structures underground for further research. The seismic image after first stacking process taken is shown in Figure 5-3. After stacking four times the seismic image is much clearer in details in rock structure which is shown in Figure 5-4 [8]. Figure 5-3. Data processing: Stacking 1 [8]. Figure 5-4. Data Processing: Stacking 4 [8].

32 25 Chapter 6 Reservoir Imaging 6.1 Land Seismic Imaging Today, challenging circumstance is the one of the main roadblocks in our research for new sources of energy. Seismic imaging is the most widely used technology to find oil and gas below thousands of meters of rock. Until now the acquisition of seismic data has involved trucks, sending sound waves into the ground to insure no interference between the signals. Signals are reflected by the rocks below and picked up by multiple receivers which are arranged as grids on the ground. Usually we use geophones on land and analyze the reflected waves and produce an image about the prediction of the oil and gas locations [14] [17]. Recently with the new simultaneous source acquisition technology, we can significantly improve the quality of seismic data than we can acquire on land. One of these technologies is called independent simultaneous source (ISS). ISS technology allows trucks to operate independently. Furthermore the interference between signals can be removed by advanced processing process which significantly decreases the survey time by up to 80 percent of previous survey time. Besides, deployment of simultaneous source technology also helps in seismic imaging. With the help of all these high technologies, we can generate high quality 3-D images for customers and make better decisions about where to drill [14]. To image underground geological structures, we use seismic reflection techniques which set reflections at specific places and then interpret the corresponding impedance changes in the form of amplitudes. Impedances within reflecting boundaries are produced by seismic volume [14].

33 26 For seismic imaging, besides data acquisition, estimation of velocity is essential since data geometry has a great impact on the seismic imaging method. Furthermore, generating an accurate function for velocity simulation is fundamental for outputting clear seismic images [14] [17]. Basically there are two main types of algorithms used in seismic imaging. One is wavefield-continuation and the other is integral method. The integral method can output images more accurately for the underground geological structures [14]. A 3-D model of land seismic imaging is shown in Figure 6-1 and Figure 6-2. Figure 6-1. Land seismic imaging [14]. Figure 6-2. Land Seismic Imaging [14].

34 Marine Seismic Imaging Drilling becomes more difficult when we move into deeper horizons. Operating in the marine environment is challenging. Due to its natural characteristic, the environment can t be steady all the time. Besides that, there are lots of dangers when acquiring the seismic data undersea because of the weather like very high waves and strong winds. When drilling a well, its size is very important to us. It provides us the position information we need. Drilling process can be a risk if without enough seismic data. Maps and models are the eyes of sub surfaces. When we start from 2D data to identify the area of interests, the pictures can be used but there are lots of missing parts which we need to see. Shallows, mountains, volcanoes could have significant effects on the quality of the images we generate [15]. 2D imaging is like looking through a slice of blinds and we can t see the whole picture. 3D imaging can help us look trhough the blinds. Then we make 3D pictures which are shown in Figure 6-3, Figure 6-4, Figure 6-5. Figure D marine seismic imaging [15]. Full waveform version is a processing technique that we can apply on our existing data set. So we don t need to go out to acquire data which saves us much time and energy. When we start apply the full waveform version, there are some images of areas that we couldn t see before

35 suddenly come into focus and help us identify exactly where shallows and gas are from the 3-D survey [15]. 28 Figure D marine seismic imaging [15]. Figure D marine seismic imaging [15]. Recently a new technology has been developed, called 4-D survey. When we take 3-D pictures of sub surfaces and repeat the survey after certain period of time and look for differences between each survey. With 4-D differences, we could have a chance to explore environment changing within reservoirs that we haven t seen them before [15]. An illustration is shown in Figure 6-6. Figure D marine seismic imaging [15].

36 29 Risks would be high for a new project if without those advanced technologies. We need to get the right data at the right time. A lot of remaining reserves or existing reservoirs we find are self-explored and more are in the complex environments where there could have limits for drilling seismic technology. It means that we have to challenge our assumptions, break the old methods and develop the new ones [15]. All we can see on the surfaces is the ground and we don t really know what the shape of the things beneath the ground. The images can show layers of rock structures, shale which are folded. Sometimes there are mountains beneath the ground as well [15]. A 3-D model of subsurface reservoir is shown in Figure 6-7. Figure 6-7. Subsurface reservoir [15]. For this reason, the objective is to take pictures beneath the surface of the earth. We use sound wave to take those pictures. During the survey, we find that waves are distorted by a salt canopy in the Golf of Mexico. The data we get is in the form of binary digits and the processing people turn them into something useful for seismic imaging. All the computers are linked together with massive network [15]. Thousands of CPUs process algorithms to approach the seismic data and bring them all together in the end to find the final images. We collect the seismic data into the laboratory and slice them, map the data and basically produce the photography maps that actually reproduce the surface [15]. There are contours on the surface. They are actually sub

37 30 surfaces which can help us in placing wells and reservoirs. The benefit of advanced technique is that it can let us generate images at the same time the data being acquired. As we add more and more data, we can make comparisons with our previous results and we are seeing things we have never seen before. After the interpreters form seismic images, we can now see clearly about what we are drilling, where the faults are, where the reservoirs exist and we can estimate how much oil is down there. Thanks to the seismic data, costs can be reduced significantly which benefits all of us [15].

38 31 Chapter 7 Conclusion The thesis explores the signal processing techniques used in detecting underground oil and natural gas sources. It starts from the formation of oil and gas which provides geological background for technical research. A brief understanding of how the subsurface structure looks can provides a suggestion as to where to commence for the study. The simple principle behind it is that we send seismic wave through the earth and use receivers to acquire the signal reflected back. Energy source provides seismic waves. For the on land environment, we use seismic vibrators as energy source and geophones as data acquisition equipment. While for the marine environment, air guns provide seismic energy and hydrophones are used to acquire data. After we acquire seismic data we have to clean up the data before processing, which is known as pre data processing. Vector filtering is useful in eliminating background noise, improving signal to noise ratio. Wavelet deconvolution can efficiently invert degradation, remove distortions and noise from the primary signals. A stacking process is used to remove coherent noises and other undesirable noises by averaging numbers of traces of seismic data together. By repeating stacking, images get closer to the original subsurface so that we can better predict the geological structures underground. With the clear processed seismic data, we can further pursue our research in seismic imaging. We apply full waveform version which is a processing technique that is applied to the existing data. 3-D seismic imaging is the more widely used technique. It can simulate the reservoir underground at both land or at sea. There are still many challenges and optimization in technologies waiting for us to solve since oil is proved to never run out.

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40 33 [13] Robert E. Sheriff, What Is Deconvolution. Search and Discovery Article #40131 (2004). [14] Land Seismic imaging. Retrieved from [15] Marine Seismic imaging. Retrieved from [16] Fractured Reservoir Imaging Solutions. Retrieved from [17] Seismic Imaging and Inversion. Retrieved from [18] Abdulrahman Mohammad Saleh Al-Moqbel, Reservoir Characterization Using Seismic Reflectivity and Attributes. Massachusetts Institute of Technology. June 7, [19] John A. Scales, Theory of Seismic Imaging. Samizdat Press. New England Research. Jan 10, [20] Fred Hilterman, Tad W. Patzek, Advanced reservoir imaging using frequency-dependent seismic attributes. CFDA Title: Fossil Energy Research and Development. CFDA Code: [21] Signal Processing and Data Conditioning. Retrieved from s/signal.aspx. [22] Wide Azimuth. Retrieved from Brochure.pdf. [23] Products and Services. Retrieved from [24] Full-Wave Imaging. Retrieved from %20Sheets/Brochures/BR_GXT_Full-wave_Services.pdf.

41 34 [25] Robert E. Sheriff, What Is Deconvolution. Search and Discovery Article # AAPG Explore. April, 2004.