تعیین تنش‌های برجا و بررسی پتانسیل فعالیت مجدد شکستگی ها و گسل ها جهت تزریق co2 برای ازدیاد برداشت نفت در میدان نفتی گچساران

تزریق co2 در سازندهای زمین شناسی، مانند مخازن تخلیه شده نفت و گاز، علاوه بر مزیت های زیست محیطی یکی از روش های موثر ازدیاد برداشت نفت به‌عنوان ازدیاد برداشت ثالثیه است. وجود مخازن دارای افت فشار و نیازمند به تزریق گاز در جنوب غرب ایران از یک طرف و داشتن اثرات فنی و زیست محیطی از طرف دیگر سبب ایجاد پتانسیل مناسبی جهت تزریق گاز co2 برای ازدیاد برداشت در این منطقه شده است. برای انجام یک پروژه تزریق co2، در اولین گام، یک ارزیابی از وضعیت فشار منفذی مخزن، مقادیر و جهت‌گیری تنش‌های برجا و وضعیت شکستگی ها و گسل ها مورد نیاز است. در این پژوهش، ابتدا فشار منفذی بر اساس روش ایتون تغییر یافته[1]برای 47 چاه برآورد و با داده های آزمون مجدد سازند[2] و فشار گل اعتبارسنجی شد. مقادیر تنش‌های برجا برای 47 چاه در طول میدان گچساران، با استفاده از روابط پوروالاستیک، تخمین زده و با استفاده از داده های آزمون های نشت و نشت تمدید یافته[3] اعتبارسنجی شد. سپس جهت گیری تنش‌های برجای افقی و شکستگی ها بر استفاده نگاره های تصویری بررسی شد. رژیم تنش در میدان بررسی شده نرمال تعیین شد. در نهایت با استفاده تئوری گسل در حالت تنش بحرانی وضعیت شکستگی ها و گسل ها از نظر فعالیت هیدرولیکی و مکانیکی ارزیابی شد. در این بررسی گسل های شمارۀ 15، 6، 10 و 2 به‌ترتیب مستعدترین گسل ها برای فعالیت مجدد در طی تزریق co2 بودند.

Determination of in-Situ Stresses and Investigation of the Fractures and Faults Reactivation Potential for CO2 Injection to Enhanced oil Recovery in the Gachsaran Oilfield

IntroductionCO2 injection in deep geological formations, such as depleted oil and gas reservoirs, in addition to the environmental benefits, is one of the effective method for enhanced oil recovery (EOR) as tertiary EOR. Presence of reservoirs with a pressure drop which require injection of gas in the southwest of Iran and having the technical and environmental effects of CO2 injection have created a huge potential for CO2 injection to EOR in this region. In the first step, to perform CO2EOR, the geomechanical assessment is needed to find out pore pressure, insitu stress magnitudes and orientations and fractures and faults conditions. In this paper, the initial insitu pore pressure is predicted using modified Eaton method for 47 wells in the length of the study field and calibrated using repeat formation test and mud pressure data. Insitu stress was obtained by the poroelastic method for 47 wells in the length of the study field and calibrated using leak off test and extended leak off test. Then, the orientation of insitu stresses is obtained based on image logs. Hydraulical and mechanical activities of fractures and faults were performed by criticallystressedfault hypothesisMaterial and MethodsIn this paper, the initial pore pressure is calculated using modified Eaton method and other corrections that are proposed by Azadpour et al. (2015). The estimated initial pore pressure is validated using mud weight pressure (Pmw) and repeat formation tester (RFT) data. Insitu stresses are composed of three orthogonal principal stresses, vertical stress (SV), maximum horizontal stress (SH), and minimum horizontal stress (Sh) with specific magnitude and orientations. The magnitude of SV is calculated by integration of rock densities from the surface to the depth of interest. The poroelastic horizontal strain model is used to determine the magnitudes of the SH and Sh. Then, the estimated minimum horizontal stress from poroelastic horizontal strain model is validated against direct measurements of LOT and XLOT tests. The orientation of breakouts was determined based on compressively stressed zones observed in the UBI log and using Caliper and Bit Size (BS) logs. The hole elongates perpendicular to the SH and breakouts develop at the azimuth of Sh. Fractures and faults reactivation analyses are very important, they can potentially propagate upwards into the lower caprock and further through the upper caprock due to CO2 injection. Fractures and faults identification were performed based on image logs. Based on performed seismic interpretations by NISOC (National Iranian South Oil Company), 15 faults have been detected in the field. Fractures and faults conductivity and activity in the current stress filed affect on fluid flow and mechanical stability or instability of the CO2 injection site. Critically stressed fault hypothesis, introduced by Barton et al. (1995), states that in a formation with fractures and faults at different angles to the current stress field, the conductivity of fluids through their apertures are controlled by the interplay of principal stress orientations and fracture or fault directions. Hence, conductive and critically stressed fractures and faults in the current stress field were evaluated using critically stressed fault hypothesis. Fractures and faults are plotted in normalized 3D Mohr diagrams (normalized by the vertical stress), therefore conductive and critically stressed fractures and faults were determined.Results and discussionsThe maximum distribution of initial pore pressure was 2025 MPa in the field and the average of initial pore pressure was 25 MPa in the field. Unlike the World Stress Map, the stress regime is normal in the reservoir. Because the Kazeroon fault and Dezful Embayment act as a strikeslip tensional basin, resulting in the subsidence of Dezful compared with other regions. The frequency distribution of calculated insitu stress in 47 studied wells in the length of the field has been presented. The maximum frequency distribution of SV, SH and Sh were between 6070, 5060 and 3040 MPa, respectively. A large amount of fracturing is observed in 2025 m below the caprock. Based on the continuity of their low amplitude traces on the acoustic amplitude image of UBI, fractures are classified into 4 classes: discontinuousopen, continuousopen, possibleopen and closed fractures. OBMI and UBI image logs processing were performed in 7 wells. As can be seen from the image log, and caliper analysis the most dominant strike of SH around the well is 27◦ and Sh strike is 117◦. These have two dominant orientation, some faults are along the strike of the Zagros foldthrust belt (NWSE) and the others are perpendicular to the Zagros foldthrust belt strike (NESW).Based on the normalized 3D Mohr diagrams it is clear that the fractures and faults that are oriented to the SH will be the most permeable, because the faults and fractures experience the least amount of stresses in the direction of SH and they have minimum resistance to flow in this direction, therefore will have relatively high permeability. Also, results showed the faults number 15, 6, 10 and 2 will be the most dangerous faults during CO2 injection.