Determination of a safe mud window and analysis of wellbore stability to minimize drilling challenges and non-productive time
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The instability of a wellbore is still one of the common problems during drilling. The cause of such a borehole failure can often be mitigated by suitably determining the critical mud pressure as well as the best well trajectory. Therefore, we could save the time and the cost of drilling and production significantly by precluding some drilling problems. The main objective of this paper is to apply a geomechanical model based on well data including the in situ stresses, pore pressure, and rock mechanical properties coupled with suitable rock failure criteria in order to obtain a safe mud window and a safe drilling direction. The Mogi–Coulomb failure criterion was used for deriving the failure equations for tensile and compressive failure modes. For comparison, the analysis was also carried out using the traditional Mohr–Coulomb failure criterion. Variations of wellbore inclination and azimuth were also used to recommend upper and lower mud pressure bounds and the most stable borehole orientations. The best trajectory selection for the inclined borehole was also investigated. Furthermore, the effect of drilling mud pressure and wellbore orientation (θ = 0o) and (θ = 90o) on wellbore stability and stress distribution around the wellbore was assessed. The stability model has been applied to a well located in an oilfield in Iran and showed that the new model is consistent with field experience.
KeywordsWellbore stability Rock failure criterion Safe mud weight window Drilling
Wellbore instability is one of the main problems that engineers encounter during drilling. Borehole stability requires the knowledge of interaction between rock strength and in situ stress. The drilling of an in-gauge hole is an interplay of two factors: uncontrollable and controllable. Uncontrollable factors are the earth stresses (horizontal and vertical), pore pressure, rock strength and rock chemistry. Controllable factors include mud weight, wellbore azimuth and inclination (Mohiuddin and Khan 2007). Therefore, the way to prevent wellbore instability during drilling is to adjust engineering practices by choosing optimal wellbore trajectories and mud weights. From the mechanical perspective, a wellbore can fail by induced stresses (usually two types): shear failure and tensile failure, which can lead to a stuck pipe, wellbore breakout, induced fracture, poor cementation, side track, and loss of drilling mud (Mclean and Addis 1994). Therefore, critical mud pressure should be considered in order to mitigate wellbore instability-related problems. Although, the selection of a suitable failure criterion for wellbore stability analysis is difficult and controversial (Al-Ajmi and Zimmerman 2009), numerous drilling engineers tried to predict wellbore stability prior to drilling, via different rock failure criteria, and they also investigated stress concentration around the wellbore in order to propose an optimum mud weight window for a successful drilling operation.
Awal et al. (2001) indicated that the optimal well path can be vertical, deviated, or horizontal, depending on stress regimes. Al-Ajmi and Zimmerman (2009) used the Mogi–Coulomb criterion to develop a model for wellbore stability analysis and indicated that the Mogi–Coulomb criterion shows field conditions more realistically. Zhang et al. (2010) evaluated five rock failure criteria, namely, the Mohr–Coulomb, Drucker–Prager, modified Lade, Mogi–Coulomb and three-dimensional (3D) Hoek–Brown criteria and found that 3D Hoek–Brown and Mogi–Coulomb criteria are suitable for wellbore stability analysis.
In the present study we first review and define the Mohr–Coulomb and Mogi–Coulomb criteria. We then use Mogi–Coulomb failure criterion for predicting wellbore stability and determining of mud weight window and stress distribution around the wellbore wall. For comparison, the analysis is also carried out using the traditional Mohr–Coulomb failure criterion. Then both criteria were applied to a well located in an oilfield in Iran.
A series of sensitivity analyses that demonstrate the influences of drilling mud pressure and borehole orientation on wellbore stability are then presented and discussed.
Rock strength criteria
Considering the Mohr–Coulomb criterion, shear failure occurs if F ≤ 0, and accordingly, the required mud weight to prevent failure in each mode of failure can be calculated.
Considering the Mogi–Coulomb criterion, shear failure occurs if F ≤ 0.
Determination of stress orientation in deviated wells
When the maximum principal stress exceeds the effective strength, failure takes place at that location. Eventually, the calculated principal stresses can be used in rock failure criteria in order to assess wellbore stability.
Wellbore failure mechanisms
Drilling a well in a formation changes the initial stress state and causes stress redistribution in the vicinity of the wellbore. The redistributed stress state may exceed the rock strength and hence, failure can occur. Generally, a wellbore fails either by exceeding the tensile strength of the formation or by exceeding the shear strength of the formation (Chen 1996). These two types of failures are explained in detail below.
Compressive shear failure
Shear failure usually results in borehole collapse or breakout. Borehole breakouts are collapsed regions located on the least horizontal principal stress for vertical wells and are generally formed by compressive shear failure. Therefore, compressional failure will occur in the direction of the minimum horizontal stress because the tangential stress will reach a maximum here.
In general, the borehole tensile failure is defined by the minimum principal stress. Therefore, this failure becomes the upper limit of the mud weight window in safe drilling operation. Fracture initiates when the minimum effective stress (i.e., the total stresses minus the formation pore pressure) at the wellbore wall reaches or exceeds formation rock tensile strength, T (Fjaer et al. 2008). Thus, failure occurs when σ 3 − P o ≥ − T.
The proposed methodology can be applied by following algorithm and the results obtained from this solution are shear failure and tensile failure determination in order to calculate the optimum mud window.
Data of carbonate formation for wellbore stability analysis
Depth of investigation
Maximum horizontal stress
Minimum horizontal stress
Internal friction angle
Mud weight window versus depth
Mud weight window versus wellbore inclination and azimuth
Effective stress distribution around the wellbore
It should be pointed out that the reason of applying inclination of 30° and azimuth of 67° for this study is understanding of effective stress distribution around the wellbore drilled in this study.
From Figs. 6 and 7 can be concluded that the effect of mud pressure on stress distribution in the radial direction is affected by the orientation of the wellbore stresses. At θ = 90o the tangential stress reaches its highest value for a greater section of the radial distances from the wellbore. At θ = 0o the tangential stress reaches its lowest value and the axial or radial stresses may become maximum at the radial distances from the wellbore as mud weight increases.
As illustrated in Fig. 8a and b, at mud pressure of 46 and 50 MPa the maximum tangential stress is at 80° 260°. Since the mud pressure is low the radial stress is the minimum principal stress and shear failure may occur. In Fig. 8c axial stress is the maximum principal stress and radial stress is minimum principal stress that at orientations of 0°, 170°, 350° it is nearly abutted with tangential stress indicating fracture initiation at theses orientations. The maximum magnitude of the tangential stress at which compressive failure will occur is 39.31 MPa.
Figure 8d shows that at mud pressure of 60 MPa the radial stress is greater than the tangential stress for the ranges 0°–20°, 130°–200°, and 310°–360° indicating that tensile failure may occur in those ranges. The tangential stress is greater than the radial stress between 20° and 130°, and also between 200° and 310° thus indicating that compressive failure may occur. The axial stress is the maximum principal stress at mud pressures of 57 and 60 MPa. Therefore, increase in the drilling mud pressure causes an increase in radial stress and a decrease in the tangential stress around the wellbore wall.
We observed that the agreement between both Mohr–Coulomb and Mogi–Coulomb criteria is excellent. If maximum and minimum horizontal stresses are too close, the Mogi–Coulomb failure criterion results would be very close to the results from the Mohr–Coulomb failure criterion.
At a wellbore inclination of 30°, drilling at azimuths 90° and 270° are the most stable states and the highest safe mud weight window is found in these two azimuths. The least mud weight windows, which represent the least stable state, are found at azimuth 0 and 180°.
Since the resultant stress difference between the minimum and maximum horizontal stresses is smaller than that between the overburden and horizontal stresses, vertical direction is the most stable well trajectory.
At wellbore inclination of 30°and orientation of 80° and 260°, the tangential stress is maximum principle stress, however at orientation of 170° and 350°, the tangential stress is minimum and the radial and axial stresses are maximum depending on mud pressure.
An increase in the hydrostatic mud pressure indicates an increase in the radial stress and a decrease in the tangential stress; they are also influenced by the wellbore orientation.
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