OTC 17639 How to Plan for a Successful C.S.E.M. Survey D.G. Peace, AOA Geomarine Operations LLC A.G.O. Copyright 2005, Offshore Technology Conference This paper was prepared for presentation at the 2005 Offshore Technology Conference held in Houston, TX, U.S.A., 2–5 May 2005. This paper was selected for presentation by an OTC Program Committee following review of information contained in a proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Papers presented at OTC are subject to publication review by Sponsor Society Committees of the Offshore Technology Conference. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to a proposal of not more than 300 words; illustrations may not be copied. The proposal must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract Electro-Magnetic or EM Techniques & Surveys have been around for decades now and most explorationist’s will have limited exposure to one or more of them. Such EM techniques include land magneto-tellurics MT, audio MT, marine MT and now Controlled Source EM - CSEM. The first three have been successful commercial exploration tools for some time now and are very good for answering regional geological questions in areas where seismic fails or is ambiguous...areas such as sub-salt, sub-volcanics sub-carbonates...ie poor seismic regions. The EM methods help the explorationist by measuring the passive electrical energy flow in the earth which allows for identification of regional conductor and resistor rocks. However the new CSEM technique that is getting the recent strong attention is rather different. Although it uses similar receiver technology to the passive method it then couples it with an active dipole EM source. Thus CSEM offers an entirely new set of applications for EM by its ability to image much thinner resistors down to 10's of meters rather than the 100's of meters of passive EM. CSEM has been effectively proven over the last 3 years as an exceptional new geophysical tool that can and already is changing the way companies work in exploration, appraisal and development. Of CSEM the CEO of a major US Oil Corporation recently stated something like : “Research in non-seismic electro-magnetic hydrocarbon detection has led to new technology for remotely detecting and imaging hydrocarbons from the earth’s surface. This new technology “CSEM” holds great promise for accurately assessing potential hydrocarbon resources and reducing exploration risk” ….and others have described the technology as the most significant new E & P development since the transition from analog to digital…cdp stacking, 3D….and more recently DHI’s and AVO. This talk describes how to plan and carry out a successful CSEM survey. “How to Plan for a Successful CSEM Survey” Contents : A. Introduction – EM can be a Life Changing Experience ! B. So You Really Want to do a CSEM Survey ? C. Some Critical Things To Consider About Your Prospect D. Survey Layout and Design E. Processing and Interpretation of Results F. Summary…and Good Luck A, Introduction To all of you geologists and geophysicists used to working with seismic and now about to read this paper, please be aware that getting involved with EM surveys may be a life changing experience…you may never be the same again, so consider yourself warned. I just don’t want you blaming me later on for not telling you upfront. You see I used to be a “normal” seismic guy happily doing my usual seismic geophysics and geology and exploring until 1990… then someone asked me to do an EM survey…and life has somehow never ever been quite the same since. This is not a hard complicated paper…in fact just the opposite, what we have tried to do in our 20 minutes is to condense some 30 years of hard won EM experience into a few basic guidelines to help you with your first CSEM survey. We hope we are able to help even in a modest way to your future success with CSEM. 2 OTC 17639 it was so exciting and in doing so unfortunately missed most of the talk, a fact you have never missed in the last 30 years of your career …that is until now. For most of us seismic based folks, EM at first seems alarmingly simple… then almost trivial and easy to do, especially when someone else is talking about it…….but on closer inspection, usually around the week before you mobilise the crew, you suddenly start to understand why you should have stayed awake in the school those 30 years back….the devil as they say is in the details…and boy are there a few details ! Figure 1 The PolarBjorn - A Superb CSEM Vessel So, here you are sitting at the OTC, you are in the special CSEM session although you might not really know why. You have seen the recent EM adverts, read the press hype, maybe been visited by a couple of EM companies selling their wares……..you have listened to the previous 5 papers this morning… and managed to convince yourself that you really ought to get your management and technical team on board and give this CSEM thing a whirl as it seems to have some interesting points that could maybe help you in your day to day exploration business. This perception of yours is correct, figure 2 might help shows why as it shows a high resistivity response over a potential prospect. What I want to do here in the next 20 minutes is to try and make your life easier…to start to guide you to a successful first CSEM survey over one of your prospects, to show you some of the important things you need to know, to guide you as to what type of prospect you might consider… then outline a few of the practicalities of how you should go about it all. Figure 1 Example CSEM data across a potential drilling prospect The previous papers were good, they explained it all and you now think you understand pretty well how it works. However, if you are honest you know the second you get out of the room and try to explain it to someone else, you find you don’t really understand it too well and maybe it’s actually more complicated than it first seemed. Please don’t worry…this is normal, and you are not alone as around 95% of the exploration community would have a hard time explaining what CSEM really means. It is a truth that for most seismic based geophysicists…their total exposure to EM stopped that damp, wet grey afternoon in the first year at grad school when the professor was going through “alternate” geophysical methods….you fell asleep as Figure 3 Nothing Will be the Same after the Leap into the EM world ? But first….are you really sure why you want to do this ?? Once you have taken this first step into the EM world…nothing will be the same after…don’t say I didn’t OTC 17639 3 warn you. You are a paid up member of the exploration community, you love your job and you are a consenting adult with an inquiring mind…… and most importantly no one has actually coerced you to do this….you only have yourself to blame. From this highly typical but promising start…let’s see what we can do to help you on your way. B, So…You Really Want to Do a CSEM Survey ?? Ok lets keep this simple…you are right, there are in reality many very good reasons why this is a very good thing to do. Several large oil companies have spent the last 2-3 years carrying out pretty secret trial surveys of which you have just seen the first few results. For these companies now it is all about how far they can push the method and what level of detail is possible (see figure 4). Number Of CSEM Surveys information can predict the fluid fill of a target hydrocarbon reservoir ahead of the drilling bit. 4. CSEM provides a unique non seismic look at your reservoir in a direct measurement (resistivity) that you as a geologist are very familiar with. The data is worked together with available seismic data to provide relevant seismic and structural constraints on the Electro-Magnetic data for modelling and inversion purposes. CSEM is also great to integrate with other well data such as downhole, well to well and well to surface resistivity data. 5. CSEM can be used for appraisal, exploration, development and in the future probably reservoir monitoring purposes. One of the great things about EM as well, is that prior to doing a survey you can actually model whether it will work or not. Modelling target prospects ahead of CSEM field surveys is compulsory standard procedure for EM surveys and will not only tell you if you prospect is a valid CSEM target, but will also guide you to the best survey layout and choice of equipment. 2004 Widespread Commercial Surveys More Complex Test Targets Push the limits New Companies taking an Interest 2003 Longer Term Trials First Commercial Surveys Wider Geological Areas 2002 Follow up CSEM Trials New Areas 2001 First CSEM Trials 1975 1980 1985 1990 1995 2001 2002 2003 2004 Early R & D by Scripps & Southampton Figure 4 2005 Time Increasing Use of CSEM Technology The Good Reasons to use CSEM include: 1. The method actually works! MMT works by identifying regional resistivity variations due to large scale structures at a basin scale level…whereas CSEM works by being targeted by its geometry at specific target reservoirs where it can establish the resistivity of the target. 2. CSEM has been trialled extensively over the last 3-4 years in many different geological basins and over many different types of geological prospect in differing structural styles. Research is now pushing the boundaries to see how far it can be developed. 3. CSEM can truly identify the shape of thin resistive bodies and establish their resistivity value, and with offset Figure 5 A Suite of CSEM data showing the presence of a shallow resistor Unfortunately…. as in all the best things in life there are a few catches along the way, some aspects you might have to be wary of as you stick your toe tentatively into the EM water. These Catches Include : 1. CSEM works best in 500mtrs of water or more…more is good, beware shallow water first time around…..it can be done but its not as simple to understand and interpret. Recent experiments indicate that while reasonable data can be obtained in shallow water, the results are not easy to process and interpret. Hence the jury is partly out on this one at present. These water depth considerations are caused by the 4 OTC 17639 presence of the energy air path (often called the air wave) which comes in at around 1 to 1.5 times the water depth from seabed. This energy can be likened in simple terms to a very strong seismic multiple in as much as it is a very strong signal many times larger than the CSEM signal we are seeking to record from the target reservoir. Once the air wave arrives it completely masks the target signal. See figures 6 and 8 This of course means that in deep water where the original surveys were done the method can see deeply down to many of today’s commercial target depths, however in shallow water, the depth of investigation is presently severely limited. 2. There are target-water depth-depth to target considerations that may make your target more difficult to evaluate. At present. 1-1.5 times water depth is a good target depth below seabed…deeper is possible but needs more care and attention. Energy Propagation Modes 4. Simple big target structures are great, more complex ones are tougher, closely stacked targets are hard to separate while wider spaced stacked targets are easier. EM data interpretation gets more interesting in complex geology…large bumps are straightforward, but complex and stacked or inter-twining channels are more difficult. Targets that are close over or under salt or basement type rocks are more challenging and will surely sort the men from the boys. 5. Target aerial size is important. A rule of thumb suggests that targets bigger than their depth of burial (from WB) are easily imaged, targets around the same size as the depth of burial should be ok but need to be modelled to be sure, however targets smaller than their depth of burial may not be possible to see and detailed modelling will be required to establish what is possible. Seawater transmitter Think diffusion not wave fronts -10 Direct Wave -11 Air Path Sediment 1 Ohm-m Target Wave -12 -13 -14 Towed Dipole Source Direct Path Log10(E-Field) V/Am2 -15 Air Wave -16 200 0 Recent sediments 1-3 ohm-m Target Path 200 400 800 12000 Source-Receiver Separation (m) oil – gas sand 10 – 50 - 80 ohm-m Figure 6 Showing CSEM energy propagation pathways 3. CSEM identifies changes in resistivity. Thus if there is no resistivity contrast, or only a very small resistivity contrast between the target and the surrounding matrix rock then there is nothing to observe, so usually don’t do an EM Survey in such areas. In this case of little or no resistivity contrast then it is unlikely you are looking at a commercial size target, most of which have good resistivity contrasts!! CSEM CSEM Survey Ship Towed Dipole Source 50 meters from Seabed Seawate transmitte -10 Sediments 1 Ohm-m -11 Oil Sat 50 Ohm-m Sediments 1 Ohm-m Log10(EField) Direct Wave Target Wave -12 -13 -14 200 -15 Air Wave -16 200 0 Sea Water 2 Km 400 800 12000 Source-Receiver Separation (m) Figure 8 showing plots of the different “waves” with and without a target Sea Bottom Sands/shales 0.5-2 ohm-meters Sands/shales 2 Km Oil Sand ~40+ ohm-meters Description - Resistor Mapping Figure 7 Showing good Target Parameters 6. Unfortunately, these are early days in the use of CSEM and there are actually are not many people who really really understand how the whole CSEM thing works, and of those who think they do, I am afraid many are wrong or misguided or even swayed by heavy commercial pressures (heaven forbid). …thus there is a real EM brains shortage at present. OTC 17639 C, Some Critical Things To Consider About Your Prospect More than anything else we want your first CSEM survey to be a success. We want that because it will reinforce the CSEM method and its future usage while at the same time increase your standing in your company and at the same time will probably give you access to more funds to do more CSEM surveys…..a real winwin-win situation. 5 3. Most important of all you will have a discovery well or two that will confirm that the hydrocarbon reservoir section has a higher resistivity value than the surrounding matrix rocks…this is very important as it is both the value that CSEM can measure…i.e. the difference in resistivity between the matrix and the reservoir…..AND it will help confirm that a high resistivity thin feature actually is an economic hydrocarbon bearing reservoir…… RATHER than some other thin high resistivity feature like a sill, or tight sand, or salt etc. See figures 9 and 10. There are several target considerations for your first survey such as: 1. Water depth over target and nature of water bottom…flat or channelised, steeply sloping, does the target sit above the water bottom as you go off a shelf edge ?...or combinations of these 2. Target depth from water bottom and the aerial size of the target compared to its depth of burial from water bottom. Targets smaller in aerial extent than their depth of burial are tougher to find than targets aerially bigger than their depth of burial. 3. Geologic type of target…big bump/fault closure/channels/ salt play? 4. Wide and fat or slim and thin ?, single reservoir or stacked ? 5. Is there a resistivity contrast between the target and host rock? If not then please don’t do an EM survey. 6. Are there any other resistors near your target that could be mistaken for your target, such as authigenic carbonates, gas hydrates, tight sands, salt or volcanics 7. What questions do you want the CSEM to answer…simple YES-NO or more complex YES-NO, details of reservoir edge and thicknesses etc? 8. All the usual is it a single or multiple prospect area?, The impact of CSEM increases when it is used as a risk-reduction tool, and used to evaluate and rank a number of possible leads in a block. Are there other companies who can share the survey costs type of questions? So with all this in mind we would suggest that your first CSEM survey takes place over an appraisal prospect situation. This was where the first ever CSEM surveys were carried out and for some very good reasons: 1. In an appraisal situation you usually have very good seismic, 3D, often good AVO on which you drilled your first discovery well. 2. You will have a pretty good idea of the structural shape and the reservoir shape which may or may not be the same. Offset Discovery Well Structure defined on 3D seismic with similar AVO amplitude effects 1st Discovery Well Figure 9 An appraisal plan based on seismic & AVO doesn’t know the reservoir resistivity and where it changes 4. Planning for a high cost high visibility development project, you will have very good financial reasons to avoid drilling dry or fizz gas wells, and the more £$£$ you can save your boss the happier he or they will be. 5. As you have just had a discovery, your boss will most likely not moan too much at the thought of forking out say $1.0 to 2.0 million $$$ to ensure you don’t drill dry wells in the appraisal….a fact that somehow escaped his notice before drilling the discovery. Spending say $1.5m on the survey to save the cost of one or more $40-50m wells will get most peoples favourable response. (Small aside - Its amazing how many companies wont spend a small amount of $$$ pre-spud to ensure an exploration well is best located but once the initial discovery is made money is less of a problem. Excuse me but a well is a well is a well.) 6 OTC 17639 Prospect Modelling - Try Before You Buy - MODEL MODEL MODEL So you have your appraisal project in-hand, you have checked the water depth target depth relationship works, the boss say’s OK why don’t you look at it more closely….what do you do next ? More accurate picture of reservoir characteristics pre-appraisal drilling. CSEM will outline the high resistivity reservoir Figure 10 Whereas including the resistivity data highlights where the reservoir is changing and shows where not to drill NB - Technical note to very successful companies…IF you are lucky enough to be drilling 100% successful wells from your current methods… i.e. your seismic and AVO driven wells are 100% ok…then you don’t need CSEM. BUT please don’t throw away this paper as your run of luck will for sure come to an end sooner or later. After Appraisal CSEM Of course once you have established in a geological area that a CSEM anomaly equates to a hydrocarbon bearing reservoir and no other geological feature, then the method can be used for near field and further away exploration purposes. A positive CSEM anomaly in open acreage adjacent to a CSEM discovery is going to have much more attractive risking than one without. Having the CSEM anomaly also allows more accurate economics to be considered and a better value placed on any subsequent bid for the Acreage. CSEM before 3D ? is a question being asked more and more. Once you have proven that CSEM works in your area and you have a 2D prospect with a related positive CSEM anomaly…then why not drill and prove the prospect before going to the additional expense of a 3D seismic survey? Take this argument a bit further, then you will arrive at the scenario where you have maybe 2 or 3 CSEM successes, some on 3D some on 2D and you might ask well why don’t I cover my block with CSEM and then if there are any other CSEM anomalies present…just drill those first…even if you have only limited regional 2D seismic or maybe only a line or two. At this point you may think I have gone too far…but I can assure you others in Oil companies are thinking this way already…its all a matter of proving the method to suit the level of confidence you require for any given geological area. Well one of the very nice aspects of using EM surveys is that you can model your target pretty accurately before committing to acquire any MMT or CSEM survey data. From simple geological cartoons of your prospect it is possible to model the CSEM response and thus tell if it is both a suitable target for CSEM and if so what survey parameters and equipment will be required to conduct a successful survey.. So the next step is to meet with a CSEM contractor, explain the project and problems you are trying to solve. They should firmly point you in the direction of modelling the prospect in detail. They will probably charge a little to do this properly. We cannot over emphasise the importance of good modelling pre-survey. There are several simple 1D modelling packages around that can be had for little or nothing; in fact there is a simple free 1D model generating programme on Prof Steve Constables website (Scripps Institution of Oceanography www. mahi.ucsd.edu/steve). Most companies have got and use 1D and 2.5D modelling packages. 1D is fine for quick and simple robust simple geology models and will give you a quick feel for whether your prospect is potentially do-able. The next level of complexity is to use a 2.5D modelling programme, which is a halfway house to full 3D capability and in fairly common use nowadays. A full complex 3D modelling programme should give the highest level of detail and flexibility but they require more time, care and input data and many presently are rather slow and sometimes not as accurate computationally as we would like. However 2.5D or 3D modelling should be mandatory for successful modelling of anything approaching a complex geological target..i.e. for any real life exploration target most of us look at nowadays !! OTC 17639 7 D, Survey Layout and Design One of the outputs from the modelling process will be that you get some necessary information to help you plan the survey design (See figure 11). This information includes things such as: 1. The range of source to receiver offsets where the signal change due to the presence of the resistor can be seen. 2. The offset which has the maximum difference in recorded signal between the matrix host rock half-space and the ontarget response. Figure 11 showing results from a modelling test Good CSEM companies will work with you on the modelling and be transparent about the implications of your models. See fig 11. Good 3D modelling packages will not only give you a YES (it’s a target) or NO (It’s too difficult at present), but they will also allow you to experiment with new or novel acquisition techniques that can make the difference between the target being seen or not. As an example we recently were asked to look at a deep stratigraphic lead located partially under a deep salt dome. The salt was very accurately imaged from good 3D seismic; the water depth was 1500 mtrs and the target depth approaching 3000 mtrs from water bottom. By our normal CSEM target criteria we could see this was at best a difficult project maybe an impossible target and certainly requiring detailed modelling. From this unpromising start we were able to : 1. first see that the target alone (without the salt) could be imaged by CSEM using simple 1D models 2. See that even with the salt included the target could be seen, but unfortunately apparently requiring higher specification recording system than was available at the time. 3. Later, by creatively experimenting with survey layouts, we were able to make the salt our friend rather than the enemy at which point we also found we could image the target with lower specification equipment than was originally thought to be required…ie today’s level of equipment. Full 3D model So with detailed modelling we were able to take an un-doable prospect and show how it could be done with today’s equipment and survey techniques. This level of analysis wouldn’t be possible with 1D or 2.5D models. So Model Model Model closely with your EM contractor, pick their brains and pay close attention to the detail and don’t skimp on it. 3. The level of recording accuracy & sensitivity required to “see” your target is limited by the overall signal to noise levels of your system. These are usually measured in electrical field strength normalised to the source electrical dipole moment. EField in V/Am2 typically are in the range of -13 to – 16 V/Am2. Early CSEM surveys were capable of measuring signals as small as -13 to -14 V/Am2, but the latest cutting edge surveys nowadays are a couple of orders of magnitude better, and can measure signals as low as 10 -16 on a good day. Theoretical minimums are around 10 -17 V/Am2 so we are now pretty close to the limits of recording; but maybe some blue sky R & D thinking will move these goalposts soon? 4. Preferred positions for source and receiver line layouts. Which also depends on the type of survey Question you want to answer…ie simple Yes-No or to gain more detailed reservoir information. NB, as the method measures the difference in the signal response between the on and of target section, it is by default necessary to measure both the on and off target responses, which becomes more interesting to achieve when separate prospects are close or intertwined n some way (such as channel complexes) Survey Layout To date there have been four main types of CSEM survey layout : a. Simple or detailed grids b. Star pattern centred on the target centre (if you know where that is ? and if you think you do is it really correct ? c. Simple two line cross again centred d. Single line…dip or strike ? 8 OTC 17639 Early surveys tended towards the grid format, later surveys used the Star pattern where the line directions can help define the reservoir edges, and later as clients wanted to save money ….cross line and then single line surveys have also become more popular although there are a few things to consider when doing a single line survey. and the CSEM method up for failure….which is not our recommended approach. As an example, let’s take six identical prospect blobs that we have interpreted pretty rigorously from our seismic. They all look pretty similar and the shape is more or less the same. The AVO response looks similar on each and we are having a hard time homing in on one or the other as the best target. We know that our well commitment say’s that we should drill 3 wells, but we also know that our company will for sure pull the plug after the first dry well and head for the hills….what are we to do?. Figure 13. Hey lets run a single CSEM line over each of the blobs and that will tell us…great idea ? unfortunately no it’s a bad idea and you would be fooling yourself if you think it will work…you might get lucky…but how lucky do you feel ??? Some survey design styles All cases need on and off target data Figure 12 showing some survey design types Brief Technical Aside A brief word about the last 3-4 years – The 2 or 3 main clients who drove CSEM on in the early days were astute enough to know that the method needed rigorous proving over a variety of different geological cases. They carried out extensive trial surveys in different global, geological and structural situations and in situations varying from appraisal to surveys where the rig was steaming over the horizon to the next location. Over a period of time and analysis they came to better understand what type of survey is appropriate for what sort of target, where and when etc. These type of R & D surveys are still going on and will do for many years as companies push the boundaries of CSEM and establish what it can really achieve. New dry wells are already being drilled as these edges are found. The key to success in any single line survey is knowing for sure that when you cross each blob you are measuring the maximum response possible from a hydrocarbon filled reservoir that might or might not be there. Now in the case of our six blobs (See figure 13) we decided that as they were all the same we would run the line straight down the middle. Keeps it simple, no deviations, if the AVO had varied maybe we would have used that, but it didn’t so we had to make a judgement call. For the first 3 prospects with the blue line you are lucky, the blue single line works well, the reservoir is there, its full of oil, the response is good and hey its in the middle of each blob…great. The first is the best, the second smaller and third far too small and the CSEM response was actually pretty ratty, but hey you hit them good, especially the first one. Unfortunately for you the next three prospects turn out to look bad, not because they really are bad, but because you missed the reservoir in each one with your single red line. That’s too bad as the seismic anomaly is ok, its just a shame that the reservoir wasn’t centred on the seismic blob. Now the word on CSEM’s capability is truly out. Other smaller companies with less EM experience are realising that CSEM can help them and so the last year we have been getting a new type of enquiry that goes something like …..”wow I wanted to talk with you because I have heard CSEM is a real oil finder and that if I ran a single CSEM line over my 10 prospects identified in my permit in Wogga Wogga land it would tell me where the oil is and which is the good one to drill P.S.….the rig will be ready in 3 weeks time and I have budget approval for $200k to do the survey… please can you help??” signed “desperate from Wogga Wogga HQ”. Now I can empathise sincerely with this class of inquirer, in fact I am sure we have all been in similar situations…but their expectations are unrealistic and they are setting themselves Figure 13 One Line is Rarely Enough ! OTC 17639 9 So you missed all of the second three, and feel bad, but hey its ok because you got a really good one on the first prospect so you have a great drilling target. Its only a couple years later after you have relinquished the other 5 prospects and the next company comes in and drills 5 straight successes in a row that you feel maybe you cut corners a bit too much? The key point to this little tale is that even on the best of seismic and AVO datasets we all know that the actual reservoir outline is not always easy to see, and frequently changes shape as new more detailed information becomes available. By just running a single line over a series of similar prospects you run a very high risk of missing the best part of the target and in doing so missing the very thing you are seeking. Please don’t get me wrong here, these Yes-No prospect ranking surveys are an excellent use of the CSEM method once you know that high resistivity = hydrocarbon reservoir in an area. But you do need to make the CSEM survey big enough to give a representative response from over most of the target otherwise you are setting the scene for failure. If the blobs scenario didn’t convince you then consider how more difficult it becomes in a more complex area where several channel systems run side by side over a shelf margin…what are the odds of getting a single line to sample the sweet spot in each of those ??...not very good we think. SO please design in enough lines to adequately sample your targets. Another reason for acquiring more data earlier is that it allows more information to be gained about the reservoir that can be very useful once a specific target has been drilled and proven hydrocarbon bearing (See Processing and Interpretation 10 ). Understanding Regional Resistivity Variations - USE MMT !! Knowing the background regional resistivity of your target area is essential to be able to both place the CSEM results into a local context and to be able to remove or adjust for any significant local variations in resistivity across the target area. It may be that differential sedimentary and compaction factors across the area have resulted in a resistivity slope across your target that need’s to be taken into consideration. One of the earliest CSEM surveys done had a very large salt feature located next to the survey area that wasn’t pointed out to the CSEM survey crew before the survey. When some rather spectacularly large anomalies started coming in it was clear that there was a huge resistor in the area that wasn’t the target. In similar terms, we don’t always know as much about the local or regional geology and its resistivity as we would like. In a project a couple of years back in NW Europe it was explained to us that recent Tertiary section was 1-5 ohm mtrs, and Mesozoic was 10-20 ohm mtrs. The survey found a much thinner Tertiary than expected, while the Mesozoic was much thicker than expected and not of the same structural style…ie it was flat and sedimentary rather than having a large fault block appearance. The EM data didn’t fit at all well with the seismic data in the area. At the time of the survey we were given a very hard time about this, but couldn’t explain it other than saying that there was another unknown higher resistivity layer present where the lower Tertiary should be. It was only a year or so later when this deeper section was drilled for the first time that it was discovered that the deeper part of the Tertiary had a different sand provenance and structure and was indeed much higher resistivity at 10-20 ohm mtrs. The EM data were fully correct and vindicated as an excellent predictive tool. Also, the most cost effective and time effective period to acquire a larger data set is when you have the boat there the first time. It is much more expensive and takes longer to remobilise a new CSEM crew for a prospect infill survey after it has departed the area. Example MMT Results Block one Block 2 Sub Salt ? Salt ? Salt Rank Exploration Applications – More difficult Geology Figure 14 More Complex Geology is More Fun to Unravel ? Example of regional MMT data results with an accuracy of around 5-10% of depth which in the Faeroe Islands is about 250-300 mtrs Figure 15 Showing MMT Regional Resistivity Results CSEM surveys are different to MMT. In CSEM surveys we are from the start specifically targeting the geometry of the method at one or two selected reservoirs. The method is in EM not MT mode and so we gain none of the required regional 10 OTC 17639 background resistivity data. For this reason we always record about 10-50% of our CSEM receiver locations as MMT (Passive MT) sites. This ensures we have adequate knowledge of the regional resistivity gradients and variations. This is a VERY important part of the survey and should not be omitted. Regional 3D MMT volume draped over 3D seismic volume Figure 17 Showing CSEM receivers ready to deploy 4. An “A” frame on the stern to facilitate the deployment of the source and source cables, plus sufficient deck space for the source power equipment. Image courtesy of Figure 16 Regional MMT Resistivity Data can be draped over 3D seismic Survey Boats & Operational Considerations This is not a section on how to use survey boats, as I am sure you all know how to do that. But rather a few points on the specific needs and requirements of current day CSEM operations The survey boat should ideally have the following capabilities: 5. A navigation spar hole in the ship’s bottom 6. Sufficient dry scientific laboratory space for the instrument loading-downloading and initial data processing. 7. Sufficient deck or scientific space for the system navigation equipment. This is often carried in a separate container system to facilitate moving from ship to ship. 8. Scientific crew space for around 15 people. To date several different vessels have been used for CSEM surveys from the early surveys on the NERC vessel the “RRS Charles Darwin” through to long term projects on the “Polarbjorn”, a superb vessel. 1. A large open back deck that will allow the assembly and deployment of the EM receivers. Ideally this would be partly covered so the receivers can be assembled and tested in the dry. 2. A crane large enough to lift and deploy the receivers far enough over the ships side so that the receiver legs do not get fouled on the ships side. 3. Storage space for some 40-100 EM receivers and their related backup kit. Together with rough storage space for the concrete anchors which weigh around 200 kilos each. Each receiver recording point will require a separate anchor so that if you are recording 100 points, then you will need 100 anchors plus a few spares. Anchors can be held in a suitable transport container to prevent movement in heavy weather. Figure 18 RRS Charles Darwin OTC 17639 E, Processing and Interpretation of Results Again this section is not intended to be a full description of all EM data processing and interpretation methods as that would take many papers in their own right. The following are few of the more important points to be aware of when you start processing and interpreting your dataset. 1. When compared with present day seismic processing, EM processing is relatively simple and quick to do. The raw CSEM data are downloaded immediately when each receiver is recovered to check that the data is ok before abandoning any receiver site. This data is quickly run through an early processing sequence to also confirm that the data are correct and meaningful. 2. This early on-board processing allows first hand quality control and also means that the data can be quickly evaluated for CSEM response content. It helps allow real time changes to the survey design and plans which can surveys right up to the wire in tight drilling situations, in some past cases we have literally had the rig steaming over the horizon to spud as we were doing surveys. 3. Most of this early processing will be on laptop based computers, although in the future we expect more comprehensive computing capability will be available on board as well. 4. Another processing aspect to be aware of is the fact that everything is so new. Most processing programmes have their origins in academia somewhere and as such are not necessarily the best or most flexible for working with complex present day structural shapes. While much progress is being made, don’t expect the same level of choice or capability as with present day seismic processing. 11 8. That said there are also literally only a handful of EM experts who are capable of making sensible CSEM interpretations. Unfortunately it also has to be said at present of these, even fewer have any experience of working in an Oil Company environment or with the pressures that are present in the run up to most drilling operations. Thus you need to ensure they work very closely with experienced oil company geologists and geophysicists to ensure their work is properly guided and validated with all other forms of geological and geophysical data available in the prospect area. 9. Iteration and integration are keys words for the best overall risk reduction using CSEM. Integration….with all the other G & G data to ensure the best possible constraints can be placed on the CSEM inversions and models, and iteration….around the improving CSEM model…using that to improve the seismic model…using that improved seismic model to improve the CSEM model again….and so on. Use all data-sets, reiterate and reinterpret….re-invert and reprocess until the data has converged as far as seems sensible to a “Best Earth Model”. 10. Often for simple YES-NO type answers this rigorous approach is not required as one or more prospect responses will truly stand out clearly from the rest even on early processed data. In such cases, that might be sufficient for a first cut for a drilling location. However after the well has been drilled and proven, then depending on the survey design, the CSEM data can later provide more information about the reservoir shape and size and edge locations. This is another reason for acquiring more CSEM data early. Having more data coverage over the entire prospect can lead to detailed reservoir edge shapes, reservoir sizes, locations and relative volumetric sizes. If you only record one line then this capability is greatly restricted. 5. Of the three main contractors offering EM processing capability only one has more than 2-4 years experience of processing modern commercial EM data sets. Also, current CSEM data processing is really in its early days despite the fact that the technique has been known for around 25 years. 6. Thus the 3 present contractors are rather paving the way and rather making it up as they go along in true pioneering spirit. Each has their own way of showing and displaying the data with pro’s and con’s for most of the processes. Care has to be taken with some of the displays in these early days as the limits of present day modelling and inversion techniques are being pushed all the time especially when talking about true 3D capability versus various alternative imaging techniques. 7. It has to be said that at present there are NO really true expert CSEM data interpreters around. The method as used at present has only been around for 4 years and of that the first 2 years were very experimental. While many surveys have now been done and results interpreted the actual number of proven results…I.E. drilled and proven correct is numerically very small. Figure 19 above shows an example of regional contour data draped over a seismic time slice. If you had this type of data set showing the high resistivity variations as red would it make you more confident about drilling several of them ? F, Summary…and Good Luck To sum up, if you have gone through this simple set of thoughts and applied them to your targets then you will have started down the right path to a successful first CSEM survey. 12 OTC 17639 It for sure will make you think hard about some old geophysics we have until recently largely ignored. This to me is a wonderful new example of what we know as the explorers serendipity biting us again when a fringe black box geophysical technique, ridiculed and abused by most of us can suddenly with a little progress blossom into a new technique that maybe we cant do without in the future. As you do your survey, you are probably treading new ground in your area of interest as although now some 100 or more surveys have been acquired, not that many geological provinces have been sampled and even less wells drilled on CSEM results. Acknowledgements I would very much like to acknowledge the help, advice, comments and experience of all my colleagues at AGO and Sclumberger including : Arnold Orange Ransom Reddig Lionel Fray Kambiz Safinya. From wider CSEM viewpoint, I also feel that we should all acknowledge the main people who have over the last 3-4 years and in some cases the last 10-20 years contributed and brought the CSEM method to where it is today. These individuals include : Chip Cox of Scripps Institution of Ocenaography Steve Constable also of Scripps Len Srnka and others unknown of ExxonMobil Terje Eidesmo of Statoil Svein Ellingsrud of Statoil Lucy McGregor of Southampton University Martin Sinha of Southampton University Thank you also to the OTC for both this special CSEM session and for accepting this paper As we sail off to our next CSEM survey…….. Maybe it will be for you………??
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