Experience of GPR application in oil-and-gas industry

Transcription

1 Experience of GPR application in oil-and-gas industry V. V. Kopeikin, P. A. Morozov, F. D. Edemskiy and D. E. Edemskiy Department of radio wave propagation Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN) Troitsk, Moscow region, Russia B. R. Pavlovskii and Yu. A. Sungurov Institute of Physical Diagnostics and Modeling (IPDM) Moscow, Russia Abstract Technical characteristics of enhanced power ground penetrating radar (GPR) developed at IZMIRAN are discussed. Examples of successful application of LOZA GPR at the stages of projecting, pipe-laying and pipeline maintenance are given. The geological structure of a pipeline path has been explored in a joint IPDM-IZMIRAN expedition. Keywords - ground penetrating radar, GPR, pipeline diagnostics, pipe, GPR case study, horizontal directional drilling, HDD. I. INTRODUCTION Nowadays, in Russia and abroad, successful development is obvious of non-invasive GPR technologies, in contrast with traditional direct methods of ground probing such as boring and pit sampling. GPR sounding supplements common survey methods in the intermediate depth range which is scarcely covered by traditional geophysics. Our experience shows that subsurface radar step by step becomes an indispensable tool of a practical geophysicist. However, up to date the use of popular commercial GPRs in engineering geology still cannot be considered as widespread. The reason is the limited power and penetration depth of the equipment using standard 50 V transistor transmitters. Therefore, the classical GPRs can work properly only in low-absorption soils, such as dry sand or permafrost. In those conditions penetration depth usually reaches just the first few meters, whereas on wet clay or loam soils operation becomes practically useless. As the gas and oil pipeline routes often pass areas with highly wet soils and complicated geological conditions, successful solving of the arising problems requires improved subsurface probing equipment. So the use of a traditional GPR often comes merely to detecting buried pipes in relatively dry soils. II. EQUIPMENT In our work we use LOZA GPR developed at IZMIRAN and produced by JSC VNIISMI (Khimki, Moscow region). Its distinctive feature, compared with popular foreign and domestic analogs, is enhanced radar potential that allows operation in highly conductive absorbing media. In order to achieve high energy values, we completely changed the classical GPR schematic diagram [1]. So, the transmitter pulse power has been increased by a factor of 10000, and stroboscopic transformation in the receiver has been substituted with the direct signal registration without reduction to low frequencies [2]. As the main emitting antenna element, a resistively-loaded dipole is used in LOZA GPR series [3]. This radically reduces so called ringing usually arising in the widespread bow-tie GPR antennas. Such ringing noise may utterly corrupt the GPR performance by masking weak return signals with the tails of preceding stronger ones. Mid-frequency LOZA-V GPR has the radar potential of 120 db, which makes possible to reach penetration depth of about 10 m in average conditions and m in light soils. Such a GPR already deserves the name of a GEOphyisical research tool. It can be applied in a wide area of geological tasks requiring probing up to m depth. However, not being completely satisfied with m penetration depth, geologists want to look even deeper. At m depth a lot of geophysical problems appear where GPR could be applied. To reach such a depth, there are two ways: Tending to the theoretical limit of transmitter power. For the low-frequency version LOZA-N GPR, an extra-power 20 MW transmitter (21 kv peak voltage) has been designed [4]. It exceeds the classical transistor transmitter power by a factor of Further increase could be achieved, however such a device would be constrained in operation conditions. A transmitter with 50 kv peak voltage can be used only in dry or frosty weather. On the other side, our experience proves that a 20 kv transmitter is practically all-weather one. Some users cope to work with such a transmitter even from water surface, with a special antenna design. Reducing effective frequency of the probing pulse. The attenuation of the probing signal is frequencydependent: the lower is frequency, the less is attenuation. At MHz frequency, a 120 db GPR dynamic range can provide the return signal amplitude sufficient for object detection at m depths. In

2 order to efficiently emit a probing pulse at such a frequency, quite large antennas 6, 10 and 15 m long are used in LOZA-N GPR. Some inconvenience of managing such a long antenna is entirely repaid by new opportunities and the quality of the received information. GPR, equipped with long dipole antennas, can be moved along the earth surface with strolling speed, giving in real time, on the portable display, a geological cross section up to 100 m depths. Summarizing the aforementioned, the main characteristics of LOZA GPRs are: LOZA-V. Frequency band: MHz; transmitter peak power: 1 MW; antennas: 100, 150, 500 MHz; depth range: 0-20 m. LOZA-N. Frequency band: 1-55 MHz; transmitter peak power: 20 MW; antennas: 10, 15, 25 MHz; depth range: m. Examples of LOZA-V and LOZA-N practical use are illustrated by Fig. 1. III. OIL-AND-GAS INDUSTRY SPECIFICS Experience of LOZA GPR usage for the benefit of oil-andgas industry showed excellent results on all the stages of pipelaying process. A. Pipeline route project While planning a pipeline route, one should take into account geological features of the ground, relief and soil peculiarities. Otherwise the line may encounter different obstacles (caverns, already existing underground utility lines, etc.) and other hazardous zones. As an example, consider the horizontal directional drilling (HDD) technology which is commonly used at recent time. This trenchless pipe laying approach is very effective and causes minimal harm to nature, as well as to the comfort of the people living in the area. Also it makes pipe repairing procedures possible under rivers, woods, in protective zones and specific ground. Despite doubtless advantages of HDD, there are some problems here to deal with. So, different obstacles on the pipeline route (large boulders, rock outcrop, etc.) may occur that can damage the drilling tool or make the route impassable. Cavities, drift-sand and landslides can cause emergency situations too. In a number of cases GPR may help to reveal these anomalies and prevent the accidents. B. Pipe laying process Using GPR one can carry out the backfilling control, solve different operational tasks that may arise if some problems with drilling occur, and so on. Such problems can emerge when the drilling is conducted for the purpose of geological survey as well. For instance, if one starts to drill above a cock pit, the boring tool can fall inside and be lost. C. Pipeline Maintenance Technical conditions of pipelines require a regular monitoring for preventing natural or man-caused accidents. Fig. 1. Surveying with LOZA-V and LOZA-N GPRs

3 GPR allows one to survey hazardous areas: zones that are in danger of floods, swelling, uncontrolled ground dynamics, etc. Unauthorized pipeline junctions can also be revealed by means of GPR. So GPR is a powerful useful instrument not only for HDD, but also for trench and above-ground pipeline routing. The advantages of surface penetrating radar are well known [5]: Non invasive measurements Relatively high resolution up to a few cm. Wide variety of detected objects (metal, liquids, voids, etc.) These features provide a dramatic increase of efficiency and rate of the pipe-laying works, compared with traditional techniques based on drilling and DC vertical electric sounding. Having such a high power potential as LOZA-V and LOZA-N, subsurface radars demonstrate their main advantage with respect to other geophysical exploration techniques and drilling high spatial resolution. Whatever the density of bore sampling be (in practice, not better than m, and even m along a pipeline route), it is difficult if not impossible to reconstruct a vertically developing geological structure (fault, karst, etc). On the other hand, the GPR typical sampling step (20-30 cm) guarantees detecting and detailed characterizing of such structures. IV. SURVEY SAMPLES Below we illustrate with selected examples some areas of application where LOZA GPR is being actively used for the benefit of the oil-and-gas industry. A. GPR profiling along a pipeline route In special technical literature publication appear on the relation between paleorelief and the statistics of detected corrosion defects of gas or oil pipes [6]. Collaborative experimental surveys performed by IZMIRAN and IPDM in Noyabrsk district, West Siberia, have confirmed those observations. The measurements were carried out with LOZA- V GPR (150 MHz antennas, probing depth m). The highest defect occurrence sites coincide with the buried banks of paleochannels [7] (Fig. 2.) Regular maintenance GPR surveys would allow one to preventively detect the corrosion hazardous zones of the pipeline route. B. GPR characterization of a karst danger area Karst zones present a danger not only to a built pipeline but even at the stage of pre-pipe-laying exploration (cases of losing the boring tool occurred to our engineer colleagues. GPR helps to reveal them before the active operation stage and to avoid material losses). An example of detection of cock pits is displayed in Fig. 3. In the first example a blind attempt of HDD pipe-laying has been interrupted by the jamming of the boring tool at the 44 m mark at the 20 m depth. A posterior GPR survey (Fig. 4) has shown that the stop had been caused by entering a fault between two landslide rock blocks. Basing on GPR data, the following successful HDD had been conducted along a deeper path, below the zone subjected to landslide activity. The GPR profiling was carried out with LOZA-N (25 MHz antennas, 10 MW transmitter). Our second example refers to HDD pipe-laying under the bottom of a wide river. Preliminary GPR survey performed with LOZA-N (same parameters) revealed boulders in the subbottom layer of sediments at the depth of about m (see Fig. 5). Following our recommendation, the HDD path has been drawn below the dangerous zone and the drilling was carried out without obstacles. V. CONCLUSION As the above examples show, GPR can provide priceless information during subsurface exploration and engineering works in hard geological conditions. Very often pipe-laying routes pass swamps, scarp slopes, etc. Traditional method of geophysical exploration encounter difficulties and drilling is often unavailable in such environment. More examples could be presented where GPR serves as a good supplement to common geophysical techniques by providing a higher-level information of the geological structure. Ultra-wide band EM pulse sounding reveals detailed small-scale subsurface features and yields practically continuous data by B-scan profiling. Many previously inaccessible sites become possible to explore. The prospects of GPR application in gas-and-oil industry look very optimistic. REFERENCES [1] J. C. Cook, "Proposed monocycle-pulse VHF radar for airbone ice and snow measurement", Trans. Amer. IEE, 1960, Vol. 79, No. 51, pp [2] V. V.Kopeikin, D. E. Edemsky, V. A. Garbatsevich, A. V. Popov, A. E. Reznikov, A. Yu. Schekotov, "Enhanced power ground penetrating radars", 6th International Conference on Ground Penetrating Radar, 1996, pp , Sendai, Japan. [3] T. T. Wu, R. W. P. King, "The cylindrical antenna with nonreflecting resistive loading", IEEE Transactions on Antennas and Propagation, 1965, Vol. 13, No. 3, pp [4] V. V. Kopeikin, I. V. Krasheninnikov, P. A. Morozov, F. Guangyou, L. Xiaojun, Z. Bin, "Experimental verification of Loza-V GPR penetration depth and signal quality", 4th International Workshop on Advanced Ground Penetrating Radar, 2007, pp , Naples, Italy. [5] D. J. Daniels, "Ground Penetrating Radar", 2004, London, IEE, 731 p. [6] A. N. Rasputin, V. A. Zhelobetskiy, S. N. Kuimov, K. V. Postautov, "Applying geoninformational systems to assess the influence of natural factors on pipeline technical condition", Gas industry, 2009, No. 11, p [In Russian] [7] C. S. Bristow, H. M. Jol. "Ground Penetrating Radar in Sediments", The Geologocal Society, 2003, UK, London, 330 p. C. GPR survey of the planned pipe-laying paths for horizontal directional drilling Consider two examples of successful GPR application to horizontal directional drilling (HDD).

4 paleochannel terrace Fig. 1. GPR profile showing character of subsurface structure along the pipeline route Fig. 3. GPR profile showing character of near-bottom soil structure. The survey was carried out from an icy surface of the river Red line - initially planned HDD route that encounters several huge boulders Green line - corrected HDD route beyond the hazardous boulders zone

5 Fig. 2. GPR profile acquired on a building site in a karst dangerous zone. Ufa, Bashkiria Fig. 4. GPR profile showing character of subsurface structure along the planed HDD route 1 - Outlined sliding rocky blocks 2 - Fault zones and zones of increased rock fracturing 3 - Undisturbed bedrock