پيش‌بيني فشار منفذي با استفاده از وارون‌سازي و تحليل سرعت لرزه‌اي

Porepressure prediction using seismic inversion and velocity analysis

فشار منفذي يا فشار شاره‌‌ درون حفره‌هاي‌‌ سنگ، از جمله كميت‌هاي مهم در بحث اكتشاف و حفاري منابع نفت و گاز است. وجود فشار منفذي زياد باعث ايجاد مشكلات عديده‌اي از جمله فوران چاه، ناپايدار بودن چاه، گيركردن لوله حفاري و يا از دست دادن چرخش گل حفاري مي‌شود. به‌همين‌‌علت به‌نظر مي‌رسد كه پيش‌بيني عددي فشار منفذي، در سازند‌هاي با فشار زياد، قبل از حفاري امري ضروري براي حفاري ايمن و اقتصادي است. همچنين در زمينه اكتشاف مي‌توان از مفهوم فشار منفذي در تحليل و بررسي مسيرهاي مهاجرت نفت و در بررسي كيفيت پوش‌سنگ استفاده كرد. در اين تحقيق به‌منظور پيش‌بيني كمي فشار منفذي از تبديل سرعت- فشار منفذي، كه با چاه‌هاي منطقه تنظيم شده، استفاده شده است لذا توليد مكعب سرعت لرزه‌اي مناسب در اين تحقيق اهميت ويژه‌اي دارد. بايد توجه داشت كه سرعت‌هاي به‌دست آمده از پردازش داده‌هاي لرزه‌اي معمولا به‌‌علت كم بودن وضوح و فرض‌هاي زمين لايه‌اي و برون‌راند هذلولي، داراي دقت كافي براي تحليل فشار نيست. در تحقيق پيش ‌رو سعي شده است تا با استفاده از داده‌هاي سرعت برانبارش و وارون‌سازي داده‌هاي لرزه‌اي، همچنين استفاده از داده‌هاي نگار سرعت صوتي در چاه‏هاي منطقه مدل سرعت مناسب بررسي فشار منفذي توليد شود. همچنين براي تبديل داده‌هاي سرعت توليد شده به فشار موثر، از رابطه باورز (1995) تنظيم شده با چاه‌هاي منطقه استفاده شده است. درنهايت نتايج حاصل از اعتبارسنجي حاكي از مناسب بودن مدل فشار منفذي حاصل از اين روش است.

Pore pressure could be estimated prior to drilling using seismic velocities. Applying this approach, the final calculated pore pressure is highly affected by accuracy of the obtained velocity data. In order to generate an accurate velocity cube with proper resolution, velocities from well logs and post-stack 3-D seismic inversion are used along with stacking velocities from velocity analysis of 3-D seismic data. Using Bowers’s effective pressure method (1995), including known velocity and density values at well locations, the coefficients of Bowers’s equation could be calculated. Also, through applying Gardener’s equation, one could relate velocity to density in each well location and finally, it is possible to determine overburden stress for the entire survey. Pore pressure prediction is one of the most important stages in drilling of the new wells. According to Huffman 2002, “pressure prediction at the basin scale can be very powerful in (1) determining where the source rock is actively maturing (2) determining where large scale fluid migration is occurring in a basin (3) predicting the behavior of large regional faults and structures (4) identify the presence of secondary pressure area (5) constraining the porosity model and (6) evaluating the integrity of vertical seal in the basin”. Moreover, pressure information prior to drilling can insure economic advantages for safe drilling. Overpressure is referred to state where the pore fluid pressure exceeds normal hydrostatic pressure at the particular depth. This phenomenon usually results from restriction of the fluid flow due to rapid sedimentation or fluid expansion, which in turn results in increase in pore pressure. Since the pioneer work by Pennebaker (1968), many papers has begun to deals with the pressure prediction prior to drilling. A paper by Dutta (2002) provides a good insight into using seismic based methods and possible pitfalls of using different velocities in estimation of pore fluid pressure. Bowers (1995) suggested an outstanding empirical equation, using data from the Gulf of Mexico which governs the compaction and fluid expansion mechanisms. Since these two mechanisms are the most frequent causes of overpressure, the Bowers’s equation is an appropriate approach in most cases. It is important to note that to obtain the desired accuracy, constants A and B (i.e. the virgin curve parameters) in the Bowers’s equation should be determined using the available data from offset wells in every survey. In the process of pressure estimation, overburden pressure needs to be estimated. Hence, Gardner equation (Gardner et al., 1974) is calibrated by calculating related parameters, using available density and velocity logs. Knowing the velocity and having the calibrated Gardner’s equation in hand, one can calculate the overburden pressure. Moreover, effective pressure can be acquired with acceptable accuracy using calibrated Bowers’s equation. Eventually, porepressure could be estimated using Terzaghi’s equation (Terzaghi, 1943). It should be noted again, generating velocities from processing of 3-D seismic data, rock densities and calibrated Bowers’s equation are crucial steps, through our workflow. The final goal of this paper is to present a proper method to generate and validate the 3-D pore pressure model using conventional data for a large survey at Mansouri oil field (one of the Iranian south west oil fields). It is shown that the accuracy and resolution of the velocity model could be greatly enhanced by using available well logs and a proper geostatistical method. This high resolution velocity model results high resolution pore pressure model which reveals much information concerning reservoir integrity and characterization. The effective pressure is calculated using Bowers’s method which is calibrated using measured pressure data at some well locations and at the end; the porepressure is calculated using Terzaghi’s equation. Studying the final pore pressure map reveals that there are no major pressure anomaly within the Asmari reservoir. In addition, a bizarre looking low pressure anomaly in the beginning of the second segment is not due to a real drop in pressure (base on measured pressure in the well). In fact, this error is introduced due to changes in formation petrology at the reservoir seal. It should be noted that the geological study prior to pressure interpretation is an imperative stage in pressure estimation. Comparison of the predicted pore pressure with the measured pressure at well location which was not included in calibration phase reveals that final pressure estimation is reliable and it is in good agreement with the measured pressure data. The high resolution pressure data is suitable for well planning and provides detailed information on pressure variation along the future well trajectories. The 3-D pore pressure cube, along with effective pressure, overburden pressure and fracture pressures are obtained and used for regional geomechanical and pressure assessments.