Marine EM, the Past, the Present, and the Future. Steven Constable Scripps Institution of Oceanography

Transcription

1 Marine EM, the Past, the Present, and the Future Steven Constable Scripps Institution of Oceanography

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6 Marine EM, the Past, the Present, and the Future Steven Constable Scripps Institution of Oceanography

7 On Tuesday 17th August, 2004 the front page of the Wall Street Journal had a lead article about a new geophysical technique. Marine CSEM was suddenly in the news

8 On Tuesday 17th August, 2004 the front page of the Wall Street Journal had a lead article about a new geophysical technique. Marine CSEM was suddenly in the news except is wasn t new. Over 23 years earlier an oceanographer called Charles Cox had suggested marine CSEM for hydrocarbon exploration

9 On Tuesday 17th August, 2004 the front page of the Wall Street Journal had a lead article about a new geophysical technique. Marine CSEM was suddenly in the news except is wasn t new. Over 23 years earlier an oceanographer called Charles Cox had proposed using marine CSEM for hydrocarbon exploration to Exxon.

10 Outline: Basics of the EM methods Early history of marine EM Commercialization 10 things you need to know The future

11 What is CSEM? Controlled Source ElectroMagnetic sounding has been used since the 1930 s to map sub-surface geology through the proxy of electrical conductivity. On land it is mainly used for mining exploration (shallower, conductive ore bodies), but has been used for map geological structure for oil exploration. Transmitters Receivers Bz I grounded wire (E) By Bx I horizontal loop (B) Ex V Ey V

12 What is MT? CSEM is related to another EM technique called MagnetoTelluric (MT) sounding, which uses similar receivers to measure Earth s natural magnetic field variations and the induced electric currents. MT signals are either part of a useful complementary method or a source of noise for CSEM. Natural-source Magnetotelluric fields Transmitters Receivers Bz I grounded wire (E) By Bx I horizontal loop (B) Ex V Ey V

13 Both can be used in the marine environment: Marine CSEM more sensitive to resistive rocks Marine MT more sensitive to conductive rocks But both methods can detect contrasts Air (resistive) Natural-source Magnetotelluric fields CSEM Transmitter Seawater (very conductive) m dipole, amps m Oil, Gas (resistive) Seafloor (variable conductivity) Electric and magnetic field recorders Salt, carbonate, volcanics (resistive)

14 Both MT and CSEM sounding use electromagnetic induction, which describes what happens around a time-varying primary magnetic field: primary magnetic field

15 Faraday s Law says that a time varying (or moving) magnetic field will induce electric fields: E dl = d C dt ( is magnetic flux). primary magnetic field electric field

16 Faraday s Law says that a time varying (or moving) magnetic field will induce electric fields: C E dl = d dt ( is magnetic flux). primary magnetic field electric field and current Ohm s Law says that E will generate a current J in a conductor: J = E

17 Faraday s Law says that a time varying (or moving) magnetic field will induce electric fields: C E dl = d dt ( is magnetic flux). Ohm s Law says that E will generate a current J in a conductor: J = E Ampere s Law says that the current will generate a secondary magnetic field: primary magnetic field secondary magnetic field electric field and current C B dl = µi

18 The secondary field opposes the changes in the primary field. The consequence of this is that conductive rocks absorb variations in EM fields more than resistive rocks. Depth > Half-space

19 The secondary field opposes the changes in the primary field. The consequence of this is that conductive rocks absorb variations in EM fields more than resistive rocks. This absorption is exponential: E(z) =E o e z/z s The rate of absorption is given by the skin depth, which depends on rock resistivity and period: Depth > High resistivity, long periods = large skin depths, greater penetration. Low resistivity, short periods = small skin depths, greater attenuation. Half-space

20 CRIPPS INSTITUTION OF OCEANOGRAPHY UCSD Secondary fields are different in amplitude, phase, and direction than the primary magnetic and electric fields. Electric fields can also be distorted galvanically, as in DC resistivity methods. Secondary fields Primary fields Induced magnetic fields Galvanic distortion of electric currents

21 MT and CSEM are methods to measure electrical conductivity. Conductivity varies over 5 orders of magnitude in common Earth materials, and provides the ideal mechanism for studying fluids, water, and geology, including hydrocarbons. Conductivity, S/m water 20 C 0 C seawater fresh water sediments mantle marine lower mantle land 1400 C 1200 C peridotite 1100 C basalt, salt, carbonates basaltic melt 10% 3% 1% partial melt ice, hydrate hydrocarbons Resistivity, Ωm 100 1,000 10,000

22 The Past

23 Interestingly, Cagniard proposed adaption to the marine environment in the 1953 paper that first presented the MT method. GEOPHYSICS, VOL 18, NO. 3 (JULY 1953), P

24 Charles (Chip) Cox and Jean Filloux of Scripps Institution of Oceanography made the first marine MT measurements in 1965, in 4300 m water offshore California. Charles Cox Tucson mag Jean Filloux seafloor mag Tucson mag seafloor E Cox, Filloux, and Larsen, 1968

25 Theory pre-dates practice, but even so marine CSEM is a young field. The earliest marine CSEM work was carried out by the British and US navies. This 1968 paper out of the US Navy Underwater Sound Lab appears to be the first proposal for marine CSEM as we now know it.

26 The same Chip Cox ( ) developed practical deepwater CSEM in the early 1980 s, partly under funding by DARPA to improve understanding of seafloor communications, and ONR for submarine noise studies.

27 In 1979 Chip carried out an experiment in nearly 3,000m water with transmissions of 80 amps on an 800 m antenna. Frequencies of Hz were detected 19 km away. His target was a mid-ocean ridge. Tow cable Transmitter Antenna Receiver Electrodes Young and Cox, 1981 Cox et al, 1981

28 Remarkably, within 2 years, in March 1981, Chip proposed the CSEM method for oil exploration to Exxon:

29 In the proposal, Chip explicitly described the direct detection of a hydrocarbon reservoir. Fields measured 2500 m from a 10 4 Am CSEM transmitter

30 I checked Chip s calculations and they are basically correct. However, the proposal was declined - Chip was too far ahead of the times Amplitude (nv/m) Hz Fields measured 2500 m from a 10 4 Am CSEM transmitter

31 Meanwhile, in the late 1980 s Martin Sinha of Cambridge (later Southampton) developed a UK CSEM capability based on Scripps, but with an flown transmitter capable of working over mid-ocean ridges. (From Sinha et al, 1990)

32 1 km 2 km thin resistive layer overhang Current industry funding for marine EM started in 1995 in order to develop MT imaging of salt in the Gulf of Mexico. Clearly, the price of oil was not a factor (adjusted for inflation, 1998 was the lowest price in history). 4 km 6 km SW NW Resistive (4 Ohm-m) Conductive (0.3 Ohm-m) What drove interest in such an expensive, nonseismic method?

33 m 1k overhang thin resistive layer m km 2k 6 km 4 SW Resistive (4 Ohm-m) NW Conductive (0.3 Ohm-m) Current industry funding for marine EM started in 1995 in order to develop MT imaging of salt in the Gulf of Mexico. Clearly, the price of oil was not a factor (adjusted for inflation, 1998 was the lowest price in history). What drove interest in such an expensive, nonseismic method? First, the seismic method was having great difficultly imaging beneath salt

34 1 km 2 km thin resistive layer overhang Current industry funding for marine EM started in 1995 in order to develop MT imaging of salt in the Gulf of Mexico. Clearly, the price of oil was not a factor (adjusted for inflation, 1998 was the lowest price in history). 4 km 6 km SW NW Resistive (4 Ohm-m) Conductive (0.3 Ohm-m) What drove interest in such an expensive, nonseismic method? But probably the biggest factor was the development of the tension leg platform, which enabled production of oil and gas from deepwater prospects but deepwater drilling is very expensive.

35 Wells drilled Oil price, nominal $ Gulf of Mexico, wells >1,000 m water depth and oil price Wells; Oil price First Scripps industry funding starts First Scripps industry funding stops Current industry funding starts Year First oilfield CSEM surveys, Angola Commercial marine MT was offered starting in The first oilfield CSEM tests were done in late early 2002, by Statoil and ExxonMobil. They both used Scripps receivers and the Southampton transmitter. Again, the driver was the high cost of drilling combined with limitations in the seismic method.

36 The seismic method has the problem that small gas saturations produce big velocity changes. Seawater/gas in 50% porous sandstone 1.6 Archie s with fizz-gas P-wave velocity, km/s Velocity Fizz-gas Gas saturation

37 However, electrical resistivity does not change until the gas saturation gets large. This means that marine CSEM can be used to assess targets prior to drilling. Seawater/gas in 50% porous sandstone Archie s with fizz-gas P-wave velocity, km/s Velocity Fizz-gas Least resistivity contrast easily detected with EM Archie s Resistivity, Ωm, log scale HS lower bound Gas saturation

38 Oil and gas reservoirs are too thin to be sensed with MT - now we need to use a man-made source of energy. Again, EM energy propagates best through the more resistive rocks. In this movie, the electric fields are absorbed more rapidly in seawater: Air Sea Sediment movie Kerry Key, SIO.

39 CRIPPS INSTITUTION OF OCEANOGRAPHY UCSD If we add a thin resistive layer to the model (an oil or gas reservoir?), then energy propagates even better through this layer than the surrounding sediments. Then we add a transmitter and receivers to actually collect these data. Air Sea Oil layer Sediment movie Kerry Key, SIO.

40 The effect can be up to an order of magnitude for a large oil or gas reservoir. This is what I call the canonical model, and is similar to the target chosen by Statoil for the first test of the marine CSEM method, and very similar to the model that Chip used in his proposal. In-line electric field, V/m/(Am) E-field amplitudes larger over hydrocarbon reservoirs no oil, gas 1000 m, 0.3 Ωm 1000 m, 1 Ωm 100 m, 100 Ωm oil, gas Source-receiver offset, km

41 Statoil s tests were made in November 2000 over the Girasoll oil field off Angola: tow A Distance North, (km) Radial Electric Field, (V/m)/(source dipole moment) Infinite Reservoir Oil Reservoir No Reservoir Terminating Reservoir V approximate extent of reservoir Distance East (km) N Source-Receiver Offset, m Constable and Srnka,

42 Similar studies were carried out by ExxonMobil in Jan 2002, also off Angola. These studies used 30 new instruments designed and built by Scripps for ExxonMobil, operated by AOA Geophysics. Distance (km) receiver 21 Salt 3 seismic O/W contact 6 Well B 9 Distance (km) Over a known discovery Well A tow Normalized field (V/(Am 2) ) Prior to drilling noise level reservoir limits Distance along line (km) Inline E Field, 0.25 Hz Receiver 21, Tow Line 2 2 min. Stacked Data With Reservoir Backgound Distance North (km) Salt Oligocene tow 3 Salt Miocene well receiver 21 Radial Electric Field, (V/(Am 2 )) noise level Rotated Inline E Field Receiver 21, Tow Line Hz Data Mio + Olig HC Filled Oligocene HC Filled Brine Filled Distance (km) Distance East (km) Constable and Srnka, 2007

43 Then things got interesting... By the end of 2002 three contractors were formed to offer marine MT/CSEM as a commercial product. At least 3 main contractors offering marine EM data collection and interpretation services Several special-purpose ships have been built since then. Much based on early academic science and technology.

44 Since then more companies have joined the party

45 The Present 10 things you should know about marine CSEM that are not likely to change

46 1. EM does not map oil or gas, or even rock type. We cannot tell the difference between an evaporite, a basalt, or an oil sand based on resistivity alone. Resistivity of Rocks and Minerals (Data from Telford et al.) basalts/lavas limestones wet OIL SANDS clays sandstones, quartzites dry pyrrhotite graphite seawater evaporites groundwater intrusive igneous rocks wet dry quartz Log 10 (Resistivity, Ωm) Thus, to manage false positives you need additional data - seismics, geology, magnetics, gravity, Integration with other data sets is perhaps the only safe way to use marine EM data.

47 2. The resolution of EM induction is between wave propagation and potential fields: High frequency (megahertz) Radar Wave equation: Resolution ~ wavelength 2 E = µ E t + µ 2 E t 2 Seismics r u c 2 Mid frequency ( Hz) Inductive EM Diffusion equation: Resolution ~ size/depth 2 E = µ E t Zero frequency DC Resistivity Laplace equation: Resolution ~ bounds only 2 E =0 Gravity/ Magnetism r 2 U =0 µ = electrical conductivity ~ = magnetic permeability ~ S/m = electric permittivity ~ F/m H/m

48 3. Frequency is too low for wave propagation in rocks, but does constrain length scale through skin depth : z s At a single frequency 2 E = µ E becomes t 2 E = i µ E =2 f ( ) which for uniform fields has solutions of the form where we have defined a skin depth E(z) =E o e z/z s+i( t z/z s ) z s = 2/ µ For every skin depth the fields decay by 1/e (~37%) and phase lags by one radian (~57 ). In 1 Ωm sediments: frequency skin depth 0.1Hz 1,600 m 0.25 Hz 1,000 m 1.0 Hz 500 m 5.0 Hz 225 m

49 Although you need a low enough frequency to reach your target, sensitivity can increase with frequency (Eradial, 3,000 m, phase): degrees 3000m, E rad 0.25Hz degrees m, E rad 0.75Hz 150 Northing, km Hz Hz N = 623 rms = 4 90 N = 642 rms = m, E rad 1.75Hz degrees 280 degrees 3000m, E rad 3.25Hz Northing, km Hz Hz N = 602 rms = Easting, km 330 N = 348 rms = Easting, km 460 (But your signal size gets smaller.) These are data over the Scarborough gas field offshore NW Australia, collected by Scripps with support from BHP Billiton.

50 Multiple frequencies Single frequency Hz Single frequency Hz Single frequency Hz 1 y transmitter Constable, Orange, and Key, 2015 Depth (km) Good inversion technology is key to getting the most out of EM data, and inversions of multiple frequencies are much better than single frequency inversions Truth 1 Hz 0.1, 1Hz 0.1, 0.3, 1, 3, 10 Hz Resistivity (ohm m) Key, 2009

51 With a single frequency and no phase data, a shallow resistor (say, hydrate) has an almost identical response to a deep hydrocarbon reservoir. This led to actual drilling errors. Reservoir Ey Shallow slab Receiver Reservoir 0.33 ohm-m Shallow slab 0.33 ohm-m 1 ohm-m 1 ohm-m Courtesy Arnold Orange

52 Northing, km Northing, km o 0 Azimuth Field purely radial (in-line geometry) Azimuth 4000m, E rad 0.75Hz, log 10 (V/Am 2 ) N = 690 rms = 10% m, E az 0.75Hz N = 297 rms = 9% Easting, km log 10 (V/Am 2 ) Receiver Transmitter fields have a dipole geometry. The radial (inline) and azimuthal 210 (broadside) fields behave differently. The greater vertical component 220 of the radial fields creates greater sensitivity to thin, 230 sub-horizontal resistors (reservoirs). You can use 12.8 this to tell thick from thin 240 resistors. The modes also behave differently to anisotropy m, E rad 0.75Hz N = 690 rms = 4 Radial field Er Eø Azimuthal field N = 297 rms = Easting, km 7805 Northing, km degrees 250 At intermediate degrees azimuths the m, E az 0.75Hz data are a trigonometric mixture of the 110 radial and azimuthal fields N = 690 rms = 10% Hz amplitude, 4km Tx-Rx offset 4000m, E az 0.75Hz 13 log 10 (V/Am 2 ) N = Transmitter o 90 Azimuth Field purely azimuthal (broadside geometry) N = 297 rms = 9% Easting, km N =

53 Inverting either inline Bx or Ey provides best resolution to reservoir targets. Note Ez: don t confuse sensitivity with resolution. x Crossline Tx x Inline Tx x Vertical Tx E B Bx B B E E Ez Ey y y y x transmitter y transmitter z transmitter Depth (km) Truth Ex By Bz Ex,By,Bz Depth (km) Truth Bx Ey Ez Bx,Ey,Ez Depth (km) Truth Bx Ey Ez Bx,Ey,Ez Resistivity (ohm m) Resistivity (ohm m) Resistivity (ohm m) 1D Inversions of synthetic 0.1 and 1.0 Hz data with 1% noise added: Key, 2009

54 CRIPPS INSTITUTION OF OCEANOGRAPHY UCSD 6. CSEM works best in deep water, where the air wave is small. But, it works OK in shallow water because the air wave and target response are coupled. Don t try to remove the air wave. 'Air wave' Seawater propagation host rock propagation target signal Flood From: Orange, Key and Constable, Geophysics, 2009 Orange, Key, and Constable, 2009

55 7. Depth sensitivity in CSEM is largely determined by geometry. Sensitivity is always greatest right below the Tx and Rx. Depth sensitivity is about half Rx-Tx spacing, and so is sensitivity to off-line structure. Plan 0.25 Hz and perturbation of 1 Ωm halfspace Section inline (m) Constable, 2010

56 Targets have to be bigger than their depth of burial to be detected (note that this has nothing to do with transmitter power!). -8 Radial mode fields (1 Hz) (3D calculation courtesy C. Weiss) -9 log10(e field magnitude, V/m/(Am)) km 2 km 3 km 1000 m, 1 Ωm 4 km 5 km diameter 100 m, 100 Ωm 1D layer D halfspace Range, m Constable and Weiss, 2006

57 8. Anisotropy is important. Aligned crystals and inter-bedding can produce anisotropy on microscopic to macroscopic scales. CSEM cannot tell micro- from macro-anisotropy until the layers get very thick. NR O Brien, J. Sed. Res

58 To a good approximation (10% or so), the anisotropic response follows the vertical resistivity for the radial mode, and the horizontal resistivity for the azimuthal mode. Inverting the modes separately using isotropic models may give reasonable results, but joint inversion of both modes will fail without anisotropy. 1 Ωm 0.5 Ωm Red - magnetic fields Blue - electric fields Constable, 2010 Anisotropic Horizontal Vertical -8-8 Amplitude (log 10 V/m/(Am), T/(Am)) Radial Bx: Anisotropy Ey: Anisotropy Bx: HS_half Ey: HS_half Bx: HS_one Ey: HS_one Amplitude (log 10 V/m/(Am), T/(Am)) Azimuthal By: Anisotropy Ex: Anisotropy By: HS_half Ex: HS_half By: HS_one Ex: HS_one -17 RADIAL: Frequency: 0.25 Hz Horizontal Range (km) -17 AZIMUTHAL: Frequency: 0.25 Hz Horizontal Range (km)

59 9. Seawater conductivity matters. Getting it wrong can introduce spurious structure into CSEM models (red = correct, black = 1-layer approximation). Expensive mistakes have been made Truth Free sea, 1 layer, rms 3.6 Fixed sea, 20 layers 1.5 Depth (km) Resistivit y (ohm m) Key, 2009

60 10. Like DC resistivity, CSEM is mainly sensitive to the resistivity-thickness product of thin resistive layers. This is called T-equivalence, where T = transverse resistance (i.e. resistivity times thickness) I V 100 m, 1 Ωm 10 m, 100 Ωm I V 100 m, 1 Ωm 1 m, 1000 Ωm Apparent Resistivity Electrode Spacing If resistivity is proportional to net pay, this may not be much of a problem.

61 The Future

62 Marine EM was almost certainly over-sold between 2002 and 2007 or so. WesternGeco pulled out of the marine EM business in 2010 and OHM was absorbed into EMGS in EM contractor share price The marine EM market was recently worth around $200m/year, or about 5% of the marine seismic market. Marine EM is a capital-intensive business and there is a persistent fear of patent lawsuits, so it is difficult for smaller, innovative companies to enter the market. On the other hand, there probably isn t enough profit for the big companies to care. But there is some progress.

63 PGS surface towed streamer system is providing good data in water depths up to 400 m or so, and allows data collection co-incident with 2D seismics. PGS website

64 Petromarker has a novel vertical-vertical system which can operate in deep water, but has limited lateral capabilities. Petromarker website

65 On the other hand, there is only one contractor offering the standard approach that works so well in deep water. EMGS website However, the custom vessel, large (3D) survey model has made CSEM a relatively expensive commodity.

66 Does it work? Jonny Hesthammer thinks so. False positives have been drilled (mostly lithological resistors), but false negatives are very rare. That is, without a CSEM signature the likelihood of a commercial discovery is very low - but industry just keeps drilling and providing more data for Jonny s plot - without more confidence in the method it is difficult to realize its value. Strong EM response (NAR>15%) Weak EM response (NAR<15%) Increasing EM signature % 300% 4 250% % 3 150% % 2 50% % 0.5 Ivory 32 mmboe Goliat 238 mmboe Havis 250 mmboe Wisting 132 mmboe Skrugard 286 mmboe Norvarg 189 mmboe Skavl 35 mmboe Hanssen mmboe Storbarden - Dry Salina 38 mmboe Eik - Dry Mercury 6-12 mmboe Atlantis Minor gas Apollo - Dry* Phoenix Dry* Breiflabb Heilo - Dry Mostly discoveries (76%) Mostly dry wells (68%) (2.9% commercial discovery rate) Discovery - Exploration Discovery - Calibration Discovery Unknown commerciality Discovery Non-commercial Dry well - Exploration Dry well - Calibration Wisting Alt. - Dry Darwin - Dry Byrkje - Dry Bønna - Dry Recent CSEM wells NAR=15% Hesthammer et al., First Break, 2012; Hesthammer pers. comm. 2015

67 Much of the CSEM data is collected by the majors (ExxonMobil, Shell, Chevron, etc.) and the large NOCs (Petrobras, Pemex, etc.). These companies keep their success rate statistics fairly confidential. But they are still collecting data I think the future lies in smaller, portable, ship-of-opportunity systems (like the ones academics must use), expanding the market to smaller client companies. This may take the passing of a few years and a rise in oil prices, but I think we can count on both of these. Thank you!

68 Connecting the world of applied geophysics