PTRT 1473: Exploration and Production II. Chapter 3: Exploration

Transcription

1 PTRT 1473: Exploration and Production II Chapter 3: Exploration

2 Hydrocarbon Accumulators Four conditions for hydrocarbon accumulation 1.A source rock rich in kerogen (waxy organic matter formed from buried plants and animals); 2.Kerogen buried deep enough for ground heat and pressure to cook it into oil in source rock (maturation); 3.There must be a porous and permeable reservoir rock to migrate and accumulate it; 4.a cap rock (seal) that prevents the hydrocarbon from escaping to the surface.

3 Source Rock Source rock: generates natural gas or crude oil Oil and gas are formed from ancient organic matter (dead plants and animals) preserved in sedimentary rocks Organic matter, mineral grains (sands and mud) are mixed and deposited together. They are not going to decay like those left on surface of land. When ocean organic matter (algae, spores, pollen, bacteria) is buried as kerogen, it is transformed into crude oil. Shales make up about 75% of all sedimentary rocks; shale is the most common source rock; about 90% of kerogen is found in shales; Black shale: 1-20% organic matter; Green shale: about 0.5% Dark limestones are also source rocks (N. Africa, Middle East) Dark limestone

4 Generation Temperature is the most important factor in generation of crude oil from organic matter in sedimentary rocks Minimum 150 F; maximum about 300 F for oil generation Biogenic gas formed by bacteria action at shallow depth where T< 150; Biogenic gas (mostly methane) also known as swamp gas is not trapped Thermogenic gas is generated and trapped at depth where T>300F; when T>300F, crude oil is converted to graphite (C) and natural gas Oil window is usually from about 7000 to ft deep; can extend to ft in areas such as Gulf of Mexico where thermal gradient is low Oil is originally generated as good with API gravity of Bacteria and physical and chemical processes degrade good oil to form heavy oil Maturity is the degree of oil generation in source rock; 30-60% of buried organic matter generates oil

5 Maturation It is the process of biological, physical and chemical alteration of kerogen into petroleum; Source rocks that experience the right conditions for these processes and can generate petroleum are termed mature. Maturation begins with a series of low-temperature reactions that involve anaerobic bacteria reduce the oxygen, nitrogen and sulphur in the kerogen, leading to an increased concentration of hydrocarbon compounds; this stage continues until the source rock reaches about 50 C. Significant amounts of petroleum only begin to form at temperatures over 50 C (122 F) and the largest quantity of petroleum is formed as the kerogen is heated to temperatures between 65 and 150 C ( F). At still higher temperatures oil becomes thermally unstable and breaks down to natural gas. Even after maturation, some of the kerogen still remains unaltered as a carbon-rich residue.

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7 100 o F Crude Oil Natural Gas Biogenic Bacteria 150 o F 200 o F 250 o F Heavy Oil Oil Window 7000 ft 300 o F Light Oil wet Heat ft 350 o F 400 o F Thermogenic dry

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9 Depth (ft) Temperature Gradient of Earth s Crust Temperature (F)

10 Migration Volume is increased dramatically when solid organic matter has been transformed into liquid crude oil or nature gas Large expansion of volume brings stresses which fracture the rock Hydrocarbons escape through the fractures; fractures close after Oil and gas flow upward along faults, fractures and permeable rock Carrier beds: very permeable rock layers that transmit fluids. If there is a trap along the migration route, the hydrocarbon can accumulate in the trap and form a reservoir. Only 10% of hydrocarbon formed is trapped; the rest stayed in the source rock, was lost during migration, or seeped into the earth s surface. Thermogenic gas cannot be found at shallow depths; Exp. Oil has migrated 200 miles horizontally out from the source area in the Williston basin of Montana

11 Primary and Secondary Migration Primary Migration (1): movement out from source rock to reservoir rock Secondary Migration (2): from reservoir to accumulation

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13 Accumulation Once gas and oil migrate into the trap, they separate according to density; gas goes to the top, oil goes to the middle, saltwater goes to the bottom Saturated pool: three phases coexist. Gas on top is saturated, reaches equilibrium state with oil phase; the oil phase has dissolved the maximum natural gas it can hold and is saturated Unsaturated pool: only oil exist in reservoir, it dissolved all the gas and can hold more, therefore is unsaturated Gas reservoir: only gas exist in reservoir

14 Reservoir Rock Sandstone Sandstone is sand grains (made up mostly of quartz) cemented together; quartz (silica) is one of the most abundant minerals on earth ; The grain size of sandstone is 1/16 to 2 mm in diameter; Sandstones make up about 11% of all sedimentary rocks It is the most common reservoir rock; porous and more permeable than carbonate reservoirs Well sorted sandstone (clean sands) are usually in light color. Carbonates Carbonates are made primarily of calcium carbonate; the most common source of carbonates is the shells of marine organisms Limestone is the most common carbonate rock; Carbonates comprise of about 13% of all sedimentary rocks Carbonates are brittle; they are often fractured; Fractured carbonates are prolific reservoir rocks with high permeability Carbonates like dark limestones can as well be good source rocks

15 Traps Structural Trap Traps created due to deformation (folds and faults) of sedimentary rock layers; there must be an impermeable caprock Stratigraphic Trap Traps created due to deposition irregularities, such as unconformities and facies; in this facie, the shale prevents the oil from escaping the trap

16 Angular unconformities Secondary Stratigraphic Traps

17 Sealing fault Anticline and domes are often cut by faults Fault can form a barrier to flow - seal If fault is a sealing fault, elevation of oil must be different in both reservoirs Many faults in deltas and coastal areas are sealing faults Domes Individual salt domes often have multiple traps Example: Bay Marchand salt dome in New Orleans has over 125 producing reservoirs

18 Geophysical Petroleum Exploration

19 Geophysical Methods of Exploration Gravity Surveys Gravity data provide a better understanding of the subsurface geology It s a relatively cheap, non-invasive, non-destructive remote sensing method No need of energy to operate in order to acquire data The objective in exploration work is to associate variations with differences in the distribution of densities and hence rock types Variations in gravity on the surface of the Earth are very small; so, units for gravity surveys are generally in milligals (mgal) where 1 mgal is one thousandth of 1cm/s 2 Standard gravity ( g n or g 0 ) is taken as the freefall acceleration of an object at sea level at a latitude of 45.5 and is cm/s 2 (or equivalently m/s 2 ); standard gravity is therefore Gal or mgal

20 Gravity Exploration

21 Magnetic Surveys The magnetic survey measures the variation in the earth s magnetic field mainly caused by the properties of rocks. Basement and igneous rocks are relatively highly magnetic ; also ferrous mineral deposits or buried iron and steel objects can be detected by magnetic survey Magnetic survey is airborne (plane or satellite); permits rapid survey and mapping with good coverage areas Magnetic survey is employed at the beginning of exploration

22 Controlled source electromagnetic (CSEM) surveying CSEM survey is a remote sensing technique that uses very low electromagnetic signals for measuring resistivity of subsurface objects in the marine environment; it is also known as Sea Bed Logging (SBL) and Remote Reservoir Resistivity Mapping CSEM survey is to a large extent complimentary to seismic, as it detects contrasts in electrical conductivity whereas seismic methods detects contrasts in acoustic impedance An electromagnetic field is generated in the substrate using a controlled source (usually towed to about ft above the sea bed) and measured at different offset locations using surface/seabed receivers Receivers are placed on sea floor and locations are based on seismic indication of hydrocarbon

23 What makes it possible to use EM methods for hydrocarbon exploration is that oil and gas have significantly lower electrical conductivity than salt water, so that a porous rock that is saturated with hydrocarbons will have a smaller conductivity (or greater resistance, hence the use of resistivity logs in well logging) than one that is saturated with salt water. The contrast could be as much as a factor of 100. So, when a transmitter (the controlled source) produces an electric current through the sea floor, the response measured at some other position will be affected if hydrocarbons are present.

24 Seismic Exploration When an earthquake or explosion happens, shock waves, also called seismic waves, travel through the ground and reflect off rocks in the subsurface the same way that ripples in a pond reflect off a boat in the water; Because boundaries between different rocks often reflect seismic waves, geophysiscists use these waves to generate pictures of what the subsurface looks like Seismic data can offer us a 2D and 3D image of subsurface before well is drilled; Deepwater wells cost sometimes 100 million; a hand of fore-knowing is crucial; It takes sometimes years before decision to drill is made Seismic data involves four steps: Acquisition Processing Display interpretation

25 Seismic Exploration 4 Stages 1. Acquisition To produce a seismic image of the subsurface, a seismic source must be generated and the resulting reflection data recorded by a field crew seismic source of energy: - dynamite - specialized air - vibrator (Vibroseis) 2. Processing Next the data must be processed; the raw data go through many complex procedures using powerful computers and finally a seismic section is produced

26 3. Display Seismic sections are created from the raw data recorded by the field crew, identifying and mapping geological structures that can act as oil traps 4. Interpretation If the results of the interpretation seem favorable, then an exploration borehole will be drilled. A well in a previously unexplored area is called a wildcat

27 Seismic Exploration Environments Land covers almost every type of terrain, such as jungle, desert, forest, urban settings, mountain regions and savannah, that exists on Earth, each with its own logistical problems. Transition Zone (TZ) area where the land meets the sea, such as river deltas, swamps and marshes, coral reefs, beach tidal areas and the surf zone; water is too shallow for large seismic vessels but too deep for the use of land traditional methods of acquisition Marine zone is either in shallow water areas (water depths of less than 30 to 40m for 3D marine seismic operations) or in the deep water areas, such as the Gulf of Mexico

28 Principles of seismic Reflection and Refraction Various layers of subsurface have different acoustical properties. Sound wave travels in different speed in different density rock, like light reflection between air and water boundary. The deeper the layer, the longer the echo takes to reach a hydrophone. where d is the depth of the reflector and V is the wave velocity in the rock

29 Acoustic Impedance Changes in acoustic impedance (AI) give rise to reflected seismic waves j k AI 1 = AI 2 j k AI 1 AI 2 r 1 v 1 =r 2 v 2 r 1 v 1 r 2 v 2 where r is the density and v is the wave velocity in the rock

30 Reflection coefficient AI w = r w v w j AI 1 = r 1 v 1 k AI 2 =r 2 v 2 If r 1 v 1 =r 2 v 2 no reflection Reflection coefficient can be 1 only if V 2 =0; this means the incident wave can not penetrate through the layer 2

31 Assignment In seismic surveys, the fraction of incident energy reflected from interface is called reflection coefficient; it is dependent on acoustic impedance (AI) contrast across interface. Hypothetical Rock Properties: Rock VP, km/s ρ, kg/m3 AI Granite Basalt Limestone Sandstone Shale Determine the reflection coefficient for these rock interfaces: 1) limestone-granite 2) limestone-sandstone 3) limestone-shale, 4) sandstone-shale Which of the interfaces is a poorest reflector? Which of the interfaces is the strongest reflector?

32 Seismic Data Acquisition Land survey acquisition Sound source: vibrator truck or dynamite Shock wave transmitted Subsurface Reflections (echoes) recorded at the surface via receivers called geophones at different angle Boundaries between various layers of seabed Marine survey acquisition Seismic source: air gun Subsurface Reflections recorded at the surface via receivers called hydrophones at different angle

33 Energy source signal Amount of energy released determines depth of penetration Frequency generated determines vertical resolution; frequency is the inverse of wavelength

34 A group of hydrophones in one stream create one 2D seismic image. 3D seismic image needs to be created by many streams of hydrophones. Different approaches have been applied to overlap many times to avoid interruption from deep water and salt. Narrow azimuth (NAZ) Multi azimuth (MAZ) Wide azimuth (WAZ) Rich azimuth (RAZ) NAZ Seismic wave pass through thick layer of salt often result in distorted data due to well crystallized materials has large refraction effect on wave or light. Multi-azimuth, wide azimuth and rich azimuth seismic has helped improve seismic data in these areas.

35 Definitions Two Way Time A seismic wave is often represented in diagrams as a straight line to show it as a ray; Two way time is the time taken for a surface-generated seismic wave to reach a subsurface rock layer and return to the surface. It is usually measured in milliseconds (1 sec = 1,000 milliseconds)

36 Peaks and Troughs Seismic wave is a series of peaks and troughs, a wiggle trace which shows it is composed of peaks and troughs with a zero crossing between the two Seismic interpreters differentiate between a zero crossing from a peak to a trough (positive to negative) and from a trough to a peak (negative to positive). Black wiggly lines Colored coded cross sections Intensity (amplitude) of each line (seismic trace) indicates the strength of the reflected signal. Interval between the strong amplitudes lines measures the time from one received signal to the next.

37 Seismic Velocity There is a wide range of values of velocity as the rocks are very variable. Things that affect the velocity at which the seismic wave can travel through a rock may be: degree of compaction, the presence of fluid, the type of fluid. TWT = two way time Material Typical seismic velocity (m/s) Air 330 Water Sandstone Limestone Clay Common MidPoint (CMP) Number The numbering system used across the top of a seismic profile is the CMP number. Each CMP is a vertical wiggle trace and has its own sequential number and x,y co-ordinate so that it can be located on an ordinance survey map. The distance between each CMP on a given seismic profile is the same, and a tradition has arisen in the seismic industry where 12.5m, 25m and 50m are the most common intervals.

38 Seismic Data Processing Seismic data can be initial processed on recording vessel and further processed and analyzed in computing center. Processing usually passes through three stages: Deconvolution (Filtering) Initial processing firstly eliminate bad records and correct for unwanted shallow surface effects Multiple reflections: multiple reflections (rays bouncing back and forth between layers before reflecting back to the surface) are common in marine seismic data, and are suppressed by seismic processing. Noises are also cleaned up Airwave travels directly from the source to the receiver and is an example of coherent noise; it travels at a speed of 330 m/s, the speed of sound in air Rayleigh wave propagates along a free surface of a solid Refraction/head wave refracts at an interface, travelling along it, within the lower medium and produces oscillatory motion parallel to the interface Cultural noise includes noise from planes, helicopters and electrical pylons and all of these can be detected by the receivers

39 Stacking Over 100 recordings at different locations are combined to form one seismic trace. The goal is to reduce noise ratio and multiple reflections within a single layer. Migration Migration is used to correct the seismic signal reflected from dipped surface. Sound need to be submitted underground such as in a testing well. Velocity of sound in each layer of rock need to be measured in seismic logs. Migration can be performed in time or depth. Migration of seismic section in time domain is time migration which gives the accurate measure of reflection points in constant velocity.

40 Seismic Data Display Simple 2D vertical slices provide first looks at the geology. Horizontal slices (time slices) can also be displayed. 3D cube can be rotated to get different views of the image of the subsurface

41 Seismic Data Interpretation Goal of all previous works is to make correct interpretation and then economic decisions. Knowledge from geophysicists, geologist, petrophysicist and reservoir engineers are needed as they search for the source rock, reservoir and trap for direct indicators of hydrocarbon presence. Direct hydrocarbon indicators (DHI) is an information obtained from seismic analysis that can calibrate other hydrocarbon reservoir information to predict new accumulations. Geophysicists look for DHI Most common DHI is bright spot ; if sandstone is replaced by hydrocarbon, which is low acoustic impedance. Acoustic signal travel through the boundary will have high contrast (large change). This increasing contrast is bright spot Quality of seismic data influences DHI interpretation Amplitude variationwith offset (AVO) is an anomaly also used to predict presence of hydrocarbons; amplitude of seismic signals are affected by: Offset angle Acoustic properties (velocity and density) of reservoir and cap rocks Content of pay zone (oil, gas, water)

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43 Seismic maps The main purpose of interpretation is to obtain depth map (structural map) of the surveyed area. These maps are given to the geologist to locate potential: 1- Exploration wells 2- Appraisal wells 3- Development wells Type of seismic maps: 1- Isochrones (Time) map 2- Velocities map 3- Depth map Construction of seismic maps: The following tools are required: a base map and seismic section

44 Isochrone map of reflector-1

45 Average velocity map of reflector-1

46 Depth map of reflector-1

47 1- Base map It consists of the following elements: a- Seismic lines b- Names and number of the seismic lines c- Shot point number d- Location of a wells e- Scale and north symbol A Base map of an area BA-1 Seismic Line BA-3 Shot point BA-2 BA-4 BA-6 BA-8 BA-5 BA-10 BA-12 BA-14 BA-16 N BA-7 BA-9 Well No.1 BA-11 1 cm=100m

48 Two way Time (msec) 2- Seismic sections It is a product of a final stage of data processing Shot point Seismic section of the line BA-3shows subsurface layers

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50 More Examples

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