Prediction of Stress Regime in an Oil Field Applying the Mechanical Earth Model; Case Study: Rag-e-Sefid Oil Field

Drilling boreholes in an oil field allocates considerable amount of time and energy to itself. This is while drilling is one of the first steps of a successful operation in an oil field. Hence, it is essential to gather the latest and the most precise information from the oil field in order to remove the obstacles in front of drilling operation firstly. Having comprehensive sight about the stress changes with depth in the earth is one of the most important parts of studies before drilling a borehole . Petrophysics logs always contain useful information about physical and mechanical properties of and oil field; however, the first information gained from drillings can authenticate or reject the information predicted by analytical methods. One of that information which could be assessed and compared with predictions is tectonic condition of the field. In spite of variety of available tectonic maps, stress distribution always should be studied in detail. This would lead to a better understanding of recent changes of tectonic in any specific area. Moreover, global tectonic maps cannot be singly sufficient in order to get through the details about stress distribution in a field. This would be more severe when the human change the stress distribution in the earth by underground activities in different scales such as drilling a borehole or depleting an oil field or etc. In this study, two of the most useful analytical methods for estimating the principal stress magnitudes are described in detail. The first one is poroelastic equations and the second called Zoback stress polygon. Then a drilling case study located in south of Iran is discussed. There are several borehole collapse in the field which there is no reasonable result for them. Analytical methods of stress magnitude prediction are applied for the mentioned case and a new graph of stress changes introduced for the area. Using poroelastic equations, stress magnitudes predicted for Rag-e-Sefid oil field. Then applying the Zoback stress polygon, allowed range of stress changes for horizontal stress estimated based on the average amount of vertical principal stress and the pore pressure. Results of the both analytical methods support each other logically. Finally, new stress distribution predicted for the mentioned oil field. Furthermore, it should be said that the main reason of borehole collapses in this field would be inadequate information about the stress distribution, while there is no considerable weakness in the formation layers.

کلیدواژه ها:

Oil field tectonics ، Mechanical Earth Model ، Zoback stress polygon