مدل‌سازي جانشيني سيال در سازند ايلام يكي از مخازن هيدروكربني جنوب غرب ايران

Fluid replacement modeling in Ilam Formation in one of the southwest Iranian oil reservoirs

با توجه به اينكه اكثر مخازن هيدروكربني ايران در كربنات‌ها واقع شده است، مطالعه جانشيني سيال و انتخاب يك مدل فيزيك سنگي مناسب جهت اين سنگ‌ها ضروري به نظر مي‌رسد. در اين مطالعه، به‌منظور مدل‌سازي جانشيني سيال جهت تشخيص نوع هيدروكربن، يك مخزن كربناته از ميادين نفتي جنوب ايران (سازند ايلام) انتخاب و مورد ارزيابي قرار گرفته است. در ادامه پس از بررسي‌هاي ميداني، اطلاعات زمين‌ شناسي و پتروفيزيكي لازم گردآوري شده و جهت مدل كردن پاسخ كشسان مخزن به‌كار رفته است. در اين راستا، ابتدا سنگ متخلخل با استفاده از روابط فيزيك سنگي كربناته مدل‌ شده و در مراحل بعد با استفاده از جانشيني سيال، سيال‌هاي مختلف در مخزن تزريق و پاسخ موج تراكمي و برشي آن به‌دست آمده است. نتايج مدل‌سازي نشان داد كه مخزن مورد مطالعه در صورت تغيير سيال داراي حساسيت مناسبي براي نشان دادن تغييرات پارامترهاي كشساني مي‌باشد. همچنين نتايج تحقيق نشان داد كه مخزن كربناته مورد مطالعه اگر اشباع از گاز، نفت و يا آب باشد به نحوه قابل‌ملاحظه با در نظر گرفتن يك مدل فيزيك سنگي مناسب از هم تفكيك مي‌شود. به‌علاوه در اين تحقيق يك الگوي مناسب جهت جدا‌سازي سيالات مختلف براي مطالعات ژئوفيزيك مخزن ارائه شده است.

Seismic technologies have been recently evolved into a central position in reservoir characterization and monitoring with the recent improvements and its cost efficiency. In this regards rock physics play an essential role by connecting seismic data to the presence of in-situ hydrocarbons. Modeling the effects of pore fluids on rock velocity and density is an essential part which normally is used to detect the influence of pore fluids on seismic signature. In recent years, one of the most important developments in rock physics has been the fast progress toward quantifying the relations between geologic processes and geophysical signatures. This quantification is normally done through application of different types of rock physics models: theoretical, empirical and hybrid models. However, fluid substitution methods make it possible to predict the elastic response of a rock saturated with one type of fluid from the elastic response of the same rock saturated with another fluid. This infers that seismic wave velocity could be predicted in geological formations for any possible hydrocarbon signature based on the measured velocities in the counterpart water-saturated formations. Therefore, fluid substitution is an important part of any seismic rock physics analysis (e.g., amplitude versus offset and time lapse studies), and can provides an efficient tool for fluid identification and quantification in a given reservoir. Fluid substitution commonly performed by using Gassmann’s equation which has already being discussed frequently. In general, Gassmann applicability is questionable in carbonates as it can under-predict, over- predict or even correctly predict seismic velocity changes by changing pore fluids. This is normally attributed to the violation of some of the Gassmann assumptions like their pore space connectivity in carbonates. The goal of this study is to perform fluid substitution and seismic modelling of one of the Iranian carbonate oil field to investigate validity of Xu and Payne (2009) for the carbonate field. This model generally emphasizes the behavior of rocks related to different pore types. Fluid substitution results are then compared and verified with the laboratory measurements of core sample taken from the same reservoir intervals. The final output of fluid substitution is saturated bulk modulus, shear modulus and density for either of the defined saturation scenarios. Our results show that Xu and Payne (2009) can be used on the studied reservoir. Also, the obtained results were confirmed using other source of information like ultrasonic measurements. Furthermore, this model was used to model frame bulk modulus as an input into the fluid substitution purposes. The results of the fluid substitution confirm the applicability of the introduced approach to discriminate different fluid responses in this field.