Oil & Gas Journal - September 7, 2015

FIG. 3 MECD CHANGE: STRIKE-SLIP FAULTING STRESS REGIME

0.84

0.80

0.76

0.72

0.68

0.63

0.59

Inclination angle, degrees Relative azimuthal angle, degrees
Ma
x
im
u
mE
CD,
g/
c
c

90 75 60 45 30 15

0 90 75 60 45 30 15 0

MECD increases with the increase of inclination angles and weakens with the increase of relative azimuthal angles for both the new model and the conventional model. The variation of MECD is sensitive to inclination angles.

When the inclination angle is fixed, the impact from the change of relative azimuthal angles on MECD is small. When drilling in the horizontal section of a well (i=90º), MECD is always large at a random relative azimuthal angle, showing that the wellbore is easier to collapse.

Fig. 2 illustrates the changing trend of MECD with inclination angles and relative azimuthal angles in the NF-SS regime. The horizontal section of the wellbore is most stable when it is drilled in the direction of minimum horizontal stress (α=90º) where the highest production rates are obtained. 10

The wellbore is most unstable when the inclination angle is 0-45º. To keep the wellbore stable in the kick-off section, UBD should only be applied in the horizontal section.

Fig. 3 illustrates how MECD decreases with the increase of inclination angles in the SS stress regime when the relative azimuthal angle is random. MECD reaches the minimum value and the mud-weight window is the widest in the horizontal section.

Fig. 4 shows that MECD obtains maximum value when i=0º (vertical section) or α=90º in the SS-RF stress regime. The wellbore is most unstable at this time.

Fig. 5 indicates that MECD in the RF stress regime is much smaller at a higher inclination angle (i=60-90º) when it is drilled in the direction of maximum horizontal stress (α=0º).

effective minimum principal stress, MPa, and φ and C represent the internal friction angle of rock mass and the cohesion strength of rock, respectively. The conventional model can also be leveraged by using the Mohr-Coulomb model when maximum principal stress, intermediate principal stress, and minimum principal stress are σθmax, σθmin, and σr respectively, in Equation 4.

Model testing

Data collected from a gas field in China where UBD of horizontal wells is ongoing tested the models. Table 1 lists rock mechanics data, well data, and in-situ stress data for the five stress regimes listed in Table 2. Both tables present the five stress regimes’ (NF, NF-SS, SS, SS-RF, and RF) effects.

Figs. 1-5 show the maximum equivalent collapse density’s (MECD) change with both inclination angles and relative azimuthal angles. MECD occurs at a certain inclination angle and relative azimuthal angle when θ changes from 0 to 90°.

The new model calculated the figures’ smooth area, with the mesh area calculated by the conventional model.

The figures show that the changing trends of both models are the same, but MECD when fluid seepage is taken into account is greater than when it is ignored. The mud-weight window is also narrower and the wellbore is more unstable.