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Mechanical earth model to mitigate wellbore instability of Nahr Umr formation in Southern Iraq oilfield

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Abstract

Borehole instability problems have caused non-productive time while drilling operations in the Nahr Umr shale formation. The drilling difficulties, including stuck pipe, caving, and tight holes, have been identified as significant problems in this formation. This study aims to comprehend the causes of wellbore failure and choose the most suitable drilling strategy. In this regard, a mechanical earth model (MEM) was performed using open-hole wireline logging measurements from the offset wells data. Those data included gamma ray, density log, compressional wave velocity, shear wave velocity, neutrons porosity, and image log. Moreover, there were also measured data such as leak-off tests and modular formation dynamics testers that were applied to validate the model’s accuracy.

The analysis exhibited that the leading cause of wellbore instability problems was improper mud weight (10.6 ppg). In addition, inappropriate drilling practices and the heterogeneity of the Nahr Umr formation have magnified the risk of drilling problems. The MEM outcome proved that the mud weight should be 11.6–13.5 ppg to safely drill a vertical well. Thus, this work can be applied as a cost-effective tool when designing future neighboring deviated wells.

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Abbreviations

ρ b :

bulk density of the formation (g/cm3)

Z :

depth

Δt 𝑛 :

shale slowness at a normal trend line

Δt 0 :

shale slowness derived from sonic log

x :

exponent values

∆t COMP :

compressional slowness of the bulk formation us/ft

∆t shear :

shear slowness of the bulk formation us/ft

K dyn :

dynamic bulk modulus (Mpsi)

G dyn :

dynamic shear modulus (Mpsi)

v dyn :

unitless Poisson’s ratio

Vp:

compressive sonic wave (ft/ms)

Vs:

shear sonic wave (ft/ms)

GR:

value of gamma ray log

GRmax:

maximum values of gamma ray log

GRmin:

minimum values of gamma ray log

Ф:

porosity from neutron porosity log

Vshale:

shale volume

(E) :

Young’s modulus measured in (Mpsi)

Esta:

static Young’s modulus (Mpsi),

υsta:

static Poisson’s ratio (dimensionless)

Gsta:

static bulk modulus (Mpsi)

Ksta:

static shear modulus (Mpsi)

α :

Biot’s constant

ε handε H :

tectonic strain factors in minimum and maximum horizontal direction, respectively

E horz :

horizontal static Young’s modulus, ex YME_H_VTI_STA Mandatory curve input

E vert :

vertical static Young’s modulus, ex YME_V_VTI_STA Mandatory curve input

v horz :

horizontal compression-horizontal expansion static Poisson ratio, ex PR_HH_VTI_STA mandatory curve input

v vert :

vertical compression-horizontal expansion static Poisson’s ratio, ex PR_VH_VTI_STA mandatory curve input

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Authors and Affiliations

  1. Thi-Qar Oil Company, Dhi-Qar, Nasiriyah, Iraq

    Raed H. Allawi

  2. Petroleum Engineering College, AL-Ayen University, Dhi-Qar, Nasiriyah, Iraq

    Raed H. Allawi

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  1. Raed H. Allawi
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Correspondence to Raed H. Allawi.

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Allawi, R.H. Mechanical earth model to mitigate wellbore instability of Nahr Umr formation in Southern Iraq oilfield. Arab J Geosci 17, 218 (2024). https://doi.org/10.1007/s12517-024-12023-1

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