3D Mechanical Earth Modeling and Risk Assessment For Optimum Selection Of Mud Weight In Zubair Oil Field.

Wellbore stability in Zubair oil field is the main problem during drilling
operations, particularly in highly deviated and horizontal wells. The instability of
the wellbore causes several problems included (poor hole cleaning, tight hole, stuck
pipe, lost circulation, bad cementing, and well kick or blowout). This is increasing
the Non Productive time (NPT), thus, the principal goal of drilling engineers is to
set up an appropriate drilling plan that mitigates these problems for further well
The aim of this study is to understanding the distribution of the pore pressure,
mechanical rock properties and far field stress and in-situ stresses throughout the the
Zubair oil field for an optimum selection of mud weights that compatible with the
planned wellbore trajectories for the intended highly deviated and horizontal wells..
Field data from six wells (logs, drilling and geological reports and as well as offset
well tests) were used to build one-dimensional mechanical earth modeling (1D
MEM) for the field of interest. The constructed, 1D MEM were then combined to
construct the three-dimensional mechanical earth modeling (3D MEM, which can
enables to distribute all estimated geomechanical parameter for each dome of the
Zubair oil field. Then, Monte Carlo simulations were then performed to quantify the
uncertainty of the rock mechanical properties as well as the effect of pore pressure
and in-situ stresses on the selected mud wiegths with less expected wellbore
problems. Finally, Tornado diagrams were employed to identify influence of the
major parameters on the forecasting of desired mud weight based on quantitative
risk assessment, thus more attention is required for the most influential parameters
for safe drilling operations.
A 1D MEM results have been calibrated with the triaxial and Brazilian laboratory
tests, Repeated Formation Test (RFT) and mini-frac test. The fault regime in the
Zubair oil field cab be categorized along two regions: strike slip and thrust fault
regime in carbonate rocks and normal fault in clastic rocks. Three failure criteria;
included; Mohr-Coulomb, Modified lade and Mogi-Coulomb were employed to
predict the rock failure. The results showed that Mogi-Coulomb failure criterion is
conservative in determining the minimum and maximum mud weights, and agree
with the determination of real failure from the image and calipers logs. The best
direction to drill the deviated and horizontal wells was towards the minimum
horizontal stress with 140o
azimuth from north. Based on the applied sensitivity
analysis, the vertical and the slightly deviated wells in orientations of (35-45
degrees) were more stable than other orientations. A quantitative risk assessment
was performed for three unstable formations (Tanuma, Ahmedi and Nahr-Umr
formtion). The results revealed that the optimum mud weights to drill these
formations were (1.2-1.22 gm-cm3
) with certainty (40-50%) for Tanuma formation,
(1.2-1.22 gm/cm3
) with certainty (60-70%) for Ahmedi formation , and (1.23-1.29
) with certainty (80-90%) for Nahr-Umr formation. The results of tornado
chart that diagnoses the parameters effect over the safe weight of the drilling mud
required to prevent shear failure and avoid tensile failure. The results appeared that
the uncertainty of the maximum horizontal stress has the greatest influence on mud
weight, and then followed by friction angle, cohesion, minimum horizontal stress
and pore pressure. The recommended range of mud weight along the sections of
12.25″ and 8.5″ of the highly deviated and horizontal wells were (between 12.2-13.2
ppg, 1.46-1.58 gm/cm3
) without any expected wellbore instability problems.
Based on the outcomes of 3D MEM, it is expected that the wellbore instability
problems is most likely to occur in the domes that have high Poisson’s ratio, low
Young’s modulus, low friction angle and low rock strength. These dome are
Shauiba and Hammar domes. In contrast, few problems are expected to expose in
Rafidya dome because of low Poisson’s ratio, high Young’s modulus as well as high
rock strength. Regarding formation, wellbore instability problems are expected to
expose in Tanuma, Ahmadi and Nahr-Umr due to high Poisson’s ratio, low Young’s
modulus as well as low rock strength.

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