Abstract
Zubair oilfield is an efficient contributor to the total Iraqi produced hydrocarbon. Drilling vertical wells as well as deviated and horizontal wells have been experiencing intractable challenges. Investigation of well data showed that the wellbore instability issues were the major challenges to drill in Zubair oilfield. These experienced borehole instability problems are attributed to the increase in the nonproductive time (NPT). This study can assist in managing an investment-drilling plan with less nonproductive time and more efficient well designing.
 To achieve the study objectives, a one dimension geomechanical model (1D MEM) was constructed based on open hole log measurements, including Gamma-ray (GR), Caliper (CALI), Density (RHOZ), sonic compression (DTCO) and shear (DTSM) wave velocities , and Micro imager log (FMI). The determined 1D MEM components, i.e., pore pressure, rock mechanical properties, in-situ principal stress magnitudes and orientations, were calibrated using the data acquired from repeated formation test (RFT), hydraulic fracturing test (Mini-frac), and laboratory rock core mechanical test (triaxial test). Then, a validation model coupled with three failure criteria, i.e., Mohr-Coulomb, Mogi-Coulomb, and Modified lade, was conducted using the Caliper and Micro-imager logs. Finally, sensitivity and forecasting stability analyses were implemented to predict the most stable wellbore trajectory concerning the safe mud window for the planned wells.
 The implemented wellbore instability analysis utilizing Mogi-Coulomb criterion demonstrated that the azimuth of 140o paralleling to the minimum horizontal stress is preferable to orient deviated and horizontal wells. The vertical and slightly deviated boreholes (1ess than 30o) are the most stable wellbores, and they are recommended to be drilled with 11.6 -12 ppg mud weight. The highly deviated and horizontal wells are recommended to be drilled with a mud weight of 12-12.6 ppg.
Highlights
Zubair is one of the hugest and most challenging oilfields in the world
The most complex sort of the encountered problems is categorized as the wellbore instability dilemmas that contribute to the majority of the nonproductive time (NPT) consumed to drill in Zubair oilfield
Minimizing the relatively high cost attributed to the instability issues can be managed by conducting an integrated geomechanical analysis
Summary
The vertical stress is one of the most essential input parameters in any mechanical earth modeling. The vertical stress is fundamentally attributed to the weight of both overlaying formations and the fluid they confined [6]. The overburden stress was computed by integrating the densities extracted from the Bulk Density log (RHOZ) that covered rocks from Sadi to the end of Zubair formation, using equation (1) [7]. Where ρ is the bulk density of the overlaying rock integrated with respect to the depth of interest (z), and g equates the gravity acceleration (m/s2). Where ρ mud is the density at ground level (soil density 1.65 gm/cm3), Air Gap is Rig floor height from the ground level (m), TVD is the true vertical depth (m), and A0 and α are Fitting parameters. The computed overburden stress ranges from 0.95 to 1.00 psi/ft, as illustrated in the seventh track from Figure-3
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