ISSN - ISRM

26 ISRM (India) Journaloccur when the axial stress falls below the rock tensilestrength.Nelson et al (2005), utilizing Anderson’s frictional faultingtheory and transverse drilling induced fractures,constrained insitu stresses in West Tuna area ofGippsland basin in Australia. With respect to the casestudy of this research the reservoir cap-rock in theMansouri-54 wellbore is located in 2335m (True VerticalDepth) and the final depth is about 3485m (TVD). Here,the wellbore breakout data was obtained from ultrasonicborehole imager (UBI). It can be realized that the azimuthof S his N135 ± 10S, which corresponds to the Zagrosthrust belt direction, approximately.basin. Mansouri oilfield is located in the northern Dezfulsubsidence at 60 Km south of Ahvaz. It is surrounded bythe Ahvaz field from the northwest, Abteimoor field fromthe west and Shadegan field from the northeast. At oilwatercontact surface, the Mansouri field is 39 Km longand 3.5 km wide.Fig. 1 : Breakout occurs along the minimum horizontalstress (S h) on the other hand it forms perpendicular todirection of the maximum horizontal stress (S H)(Tingay et al. 2008).2.0 GEOLOGY OF MANSOURI-54 WELLBOREThe Zagros fold-thrust belt stretches around 2000 kmfrom south-eastern Turkey through northern Syria andIraq to western and southern Iran (Alavi, 2004).Structurally, the Zagros basin is placed in the north ofthe Arabian plate. The case study is located at Dezfulembayment of the Zagros basin in Southwest of Iran(Figure 2). Sequential formations of the case study havebeen showed in Figure 2. As shown, studied formationsof the well include: Pabdeh, Gurpi, Ilam and Sarvak. TheZagros basin is part of the Tethys Ocean and is one ofthe most important petroleum reservoirs in the world(Alavi, 1994). The geological history of this basin includesa long time subsidence and deposition which isinterrupted by a short-term uplift. Its folding processbegan during Miocene and Pliocene anticlines andcontinued until now hence, forming a long anticline (Mottie1995). These anticlines constitute most of oil traps in thisFig. 2 : Mesozoic–Cenozoic stratigraphy correlation chart ofthe Zagros basin (Khoshbakht 2009).3.0 INSITU STRESS ORIENTATION IN MANSOURI-54WELLBORESo far as the depth of oil and gas exploration is concerned,insitu stress orientations manifest by a number ofphenomena. These include fault activity, boreholebreakout, core discing, center line fracturing in cores,drilling-induced fractures and natural and inducedearthquakes (Bell, 1996). Breakouts and drilling-inducedfractures are the most commonly encounteredphenomena (Bell, 2003). Here the wellbore breakoutsare illustrated using Ultrasonic Televiewer Imager (UBI)log which have been obtained from 3650 m to 3695 mdepth of the wellbore. Figure 3 highlights differentinstability phenomena in the wellbore. As shown, thewellbore breakouts appear as dark bands of lowreflectance on opposite sides of the wellbore at the depthinterval of XX1-XX3 and XX28-XX31 in Figure 3(a) andFigure 3(b), respectively. According to observedbreakouts at the Mansouri-54 wellbore, it can be inferredthat azimuth of S his about (N135 10S), with orientationof NE-SW direction. Since failure and collapse would leadto the high circumferential stress (σ θθ), the best directionVolume 1 v No. 1 v January 2012