بررسي فرو نشست زمين در اثر استخراج مواد نفتي با استفاده از روش تداخل سنجي رادار

(Study of land subsidence due to the oil extraction using Radar Interferometry (InSAR

توليد از مخازن هيدروكربوري، سبب افت فشار منفذي در اين مخازن مي‌شود. اين افت فشار، تنش ناشي از رسوبات روباره‌ي سنگ مخزن را كه پيش از عمليات برداشت، توسط فشار سيال داخل مخزن و سنگ‌هاي پوششي كنترل مي‌شد افزايش داده و موجب تراكم محيط متخلخل اطراف مي‌شود. در صورتي كه ميزان تراكم مخزن از حدي فراتر رود، سنگ‌هاي روباره در اثر وزن خود شروع به فرونشست خواهند كرد كه اين امر مي‌تواند تأثيرات مخربي از جمله شكستگي چاه‌ها، مچاله شدگي لوله‌هاي جداري و خسارات سرچاهي را به دنبال داشته و در نتيجه فرآيند توليد از اين مخازن را با مشكل جدي مواجه كند. بنابراين مطالعه پديده‌ي فرونشست ناشي از بهره‌برداري منابع هيدروكربوري، حائز اهميت بوده و نياز به توجه و بررسي دقيق دارد. براي اين منظور روش‌هاي متعددي مي‌تواند مورد‌ استفاده قرار گيرد؛ لذا روشي كه داراي سرعت و دقت بالا و هزينه‌ي پايين باشد همواره در اولويت خواهد بود. بدليل هزينه‌بر بودن روش‌هاي ترازيابي دقيق و نقشه‌برداري زميني و نيز عدم دسترسي به مشاهدات آنها در برخي شرايط خاص، بكارگيري روشي سريع‌تر و ارزان‌تر پيشنهاد مي‌شود. خوشبختانه پيشرفت در زمينه‌ي ماهواره و تكنولوژي رادار باعث شده است كه قادر به اندازه‌گيري جابجايي‌هايي بسيار كوچك سطح زمين در نواحي مستعد جابجايي از جمله ميدان‌هاي تحت برداشت سيال‌هاي زير‌سطحي باشيم. روش تداخل‌سنجي تفاضلي رادار (InSAR) فناوري نويني است كه از تصاوير ماهواره‌اي جهت آشكارسازي دگرريختي شكل سطح زمين استفاده مي‌كند. در اين راستا دو ميدان بزرگ نفتي يكي واقع در منطقه‌ي جنوب غربي ايران و ديگري در كاليفرنياي مركزي توسط تكنيك تداخل‌سنجي راداري مورد بررسي قرار گرفت. نتايج بدست آمده بيانگر كارايي مناسب اين روش به منظور بررسي جابجايي ناشي از فرونشست در ميادين مذكور مي‌باشد.

Land subsidence is a vertical movement of the earth surface relative to a stable reference level. It occurs as a result of plate tectonic and human activities. The common causes of subsidence from human activities are pumping under-ground water, oil and gas from overlying reservoirs. Withdrawal of fluids from hydrocarbon reservoirs causes their pressure to decrease. This pressure reduction rises the stress of reservoir’s overburden sediments which was previously controlled by the pressure of inside fluids before exploitation, and consequently increases the density of their porous surroundings. If the reservoir’s density exceeds a specific threshold, overburden rocks start to subside because of their weight. Therefore pressure drawdown leads to reservoir compaction, movement of the overburden and subsidence over the reservoir. This subsidence can prove costly for production and surface facilities. So study of the subsidence caused by hydrocarbon exploitation is an important task which needs precise considerations. Several methods are available to monitor land subsidence. Classical surveying such as Leveling and global positioning system (GPS) can produce some related data whereas they are expensive and cannot also produce the needed map at a particular period of time. Recent advances in satellite and Radar technology have made it possible to measure very small movements of the earth surface. Interferometric Synthetic Aperture Radar (InSAR) is a novel technology for measuring the surface deformation. Using the InSAR technique at relatively large subsidence areas can be monitored. The pros of InSAR are that it is not necessary to physically access the deformation areas and also the high spatial and temporal resolution of its data. Sub-centimeter accuracy has been reported for InSAR derived surface deformations. Interferometric Synthetic Aperture Radar relies on repeated imaging of a given geographic location by space-borne radar platforms. Synthetic Aperture Radar sensors measure both magnitude and phase of the transmitted electromagnetic signal that is backscattered from the earth surface. The phase measurement is used to derive information on heights and deformations of the terrain. This phase represents a combination of the distance scattering effect. If a second SAR data set is collected then from comparing the phase of the second image with the phase of the first, an interferogram can be formed. The basic principle of interferometric SAR is that if the surface characteristics are identical for both images, the phase differences are sensitive to topography and any intrinsic change in position of a given ground reflector. The interferogram can be corrected for topographic information using an external digital elevation model (DEM). The change in distance is along the line of sight to the satellite, preventing it from directly distinguishing vertical and horizontal movement. As geometrical and temporal baseline de-correlations and atmospheric noise are limitation factors to assess slow movements in subsidence areas, recent developments in multi temporal InSAR (MTI) algorithms have enabled the detection and monitoring of the slow deformation with millimetric precision. In this paper, Marun oil field; the second-largest oil field which is located in the south west of Iran has been studied. The Small Base Line Subset (SBAS) approach that is an (InSAR) algorithm has been performed for generating mean deformation velocity map and displacement time series from a data set of subsequently acquired SAR images. SBAS technique identifies coherent pixels with phase stability over a specific observation period which has been implemented in StaMPS software. This method which is based on multiple master interferograms, works with interferograms with small spatial baselines and short temporal intervals to overcome de-correlations by increasing spatial and temporal sampling and coherent areas. For this study, we have used 10 ASAR images acquired by the ENVISAT satellite from European Space Agency (ESA) during 2003 to 2006 and have generated 22 interferograms by the SBAS method. All interferometric processing were implemented using DORIS software. A SRTM Digital Elevation Model (DEM) with 3-arcsecond geographical resolution has been used to remove the topographic phase. SBAS processing was then implemented using the Stanford Method for Persistent Scatterers (StaMPS) software. As a result, the mean velocity map obtained through InSAR time series analysis which is in the Line-Of-Sight (LOS) direction of satellite to the ground. The time series analysis results of InSAR have been then compared with field production data. This sampled data allows us to evaluate potential of non-tectonic effects such as petroleum extraction on surface displacements and the relationship between both deformation and oil production rate. The results of InSAR analysis reveal the maximum subsidence on order of 13/5 mm per year over this field due to the extraction and geological characteristics in the time period of 2003-2006.