Wellbore stability analysis and application to optimize high-angle wells design in Rumaila oil field, Iraq

Abstract

Mechanical wellbore instability is one of the main issues during the drilling operations, this problem causes around 14% of drilling nonproductive time (NPT) for new wells in Rumaila oil field; these breakouts were commonly observed in the shale and weak shaly sandstones along Rumaila geological column. In this study, a quantitative wellbore failure analysis was carried out through applying modified Lade criteria on five wells in Rumaila oil field (A, B, C, E and F). The results gave a prediction of the appropriate mud density that can control the wellbore shear failure for any well type in order to eliminate or minimize wellbore instability and indicated that the preferred direction to drill deviated and horizontal wells should be parallel to maximum horizontal stress (NE–SW). The study also introduced the recommended drilling mud weight for vertical and deviated wells in Rumaila oil field for the sake of gaining wellbore stability which requires raising the drilling mud weight to more than 1.27 gm/cm³ starting from bottom of Sadi Formation to the well total depth. The results also showed that wells with low inclination (less than 30°) are less stable than highly inclination wells (more than 30°).

Introduction

Maintaining wellbore stability is one of the main costly problems during drilling new wells; it is estimated that issue causes 14% of the total nonproductive time (NPT) in Rumaila oil field. Wellbore stability aims to prevent the surrounding wellbore rocks from plastic distortion or brittle collapsing because of mechanical stress (Osisanya 2012).

Once drilling a well, the rocks surrounding borehole will support the weight that was earlier endured by the removed rock. Consequently, the stresses are noticeably changed which would escalate the stresses around the well, more exactly, a concentration of stress, that concentration may cause the failure of the borehole wall (Al-Ajmi and Zimmerman 2009). Drilling engineers can modify the concentration of stress in order to avoid or decrease breakdown of the borehole throughout optimizing mud pressure and choosing the suitable azimuth (Aslannejad et al. 2013).

Recently, the interest in wellbore stability had increased dramatically due to raised numbers of horizontal and highly deviated wells, where this type of wells offers a great economic profit by lowering development costs, higher production rates and higher recovery factors (Zhou et al. 1994).

Materials and methods

Wellbore stability analysis

The comparison between different failure criterions revealed that the modified Lade criterion had predicted the suitable mud weight that can be used for successful drilling (Rahimi 2014). The wellbore stability prediction using modified Lade criterion is a direct criterion that takes into consideration the intermediate principal stress impact on the shear strength in a representative approach. Therefore, it can be considered a suitable criterion for practical computations (Fjaer et al. 2008).

Based on experimental examinations, Lade (1977) concluded that the increase in the average normal stress in cohesionless soil correspond with the decrease in friction angle. The earliest equation that Lade established with regard to first and third invariants of stress [I₁, I₃] involves parameters of atmosphere pressure and material constant (m); Lade criterion for frictional matters failure can be represented as follows (Ewy 1999):

$$\left( {\frac{{I_{1}^{3} }}{{I_{3} }} - 27} \right)*\left( {\frac{{I_{1} }}{{P_{\text{a}} }}} \right)^{m} = \eta 1$$

(1)

I₁ and I₃: Stress invariants predicted from the three principal stresses:

$$I_{1} = \sigma_{1} + \sigma_{2} + \sigma_{3}$$

(2)

$$I_{3} = \sigma_{1} *\sigma_{2} *\sigma_{3}$$

(3)

Ewy (1999) developed and modified the Lade criterion, in Ewy’s version; values of material constant were considered as zero so that old criterion can be altered in a way where the linear shear strength rises with rising invariant of first stress, or average normal stress invariant (Zoback 2007). Additionally, as original Lade criterion was defined for cohesionless material and the stress-invariant parameters, I₁ and I₃, were not valued based on the effective stress conception, Ewy (1999) proposed a new approach with effective stress influence and presented the parameter ($\eta$) that depend on cohesion, leading to reforming the old Lade criterion into:

$$\frac{{(I_{1}^{\prime \prime } )^{3} }}{{I_{3}^{\prime \prime } }} = 27 + \eta$$

(4)

where $I_{1}^{\prime \prime }$, $I_{3}^{\prime \prime }$ are first and third modified stress invariants and expressed as:

$$I_{1}^{\prime \prime } = \left( {\sigma_{1} + S - P_{0} } \right) + \left( {\sigma_{2} + S - P_{0} } \right) + \left( {\sigma_{3} + S - P_{0} } \right)$$

(5)

$$I_{3}^{\prime \prime } = \left( {\sigma_{1} + S - P_{0} } \right)*\left( {\sigma_{2} + S - P_{0} } \right)*\left( {\sigma_{3} + S - P_{0} } \right)$$

(6)

$I_{1}^{\prime \prime }$ and $I_{3}^{\prime \prime }$ invariants can also be calculated using the following equations instead of Eqs. (5), (6):

$$I_{1}^{\prime \prime } = \left( {\sigma_{X} + S - P_{0} } \right) + \left( {\sigma_{Y} + S - P_{0} } \right) + \left( {\sigma_{Z} + S - P_{0} } \right)$$

(7)

$$\begin{aligned} I_{3}^{\prime \prime } & = \left( {\sigma_{X} + S - P_{0} } \right)*\left( {\sigma_{Y} + S - P_{0} } \right)*\left( {\sigma_{Z} + S - P_{0} } \right) \\ & \quad + 2\tau_{XY} \tau_{YZ} \tau_{ZX} - \left( {\sigma_{X} + S - P_{0} } \right)\tau_{YZ}^{2} \\ & \quad - \left( {\sigma_{Y} + S - P_{0} } \right)*\tau_{ZX}^{2} - \left( {\sigma_{Z} + S - P_{0} } \right)*\tau_{XY}^{2} \\ \end{aligned}$$

(8)

The parameter (S) is related to the rock cohesion, whereas (η) describes the internal friction angle; these parameters can be directly determined from friction angle (ϕ) and cohesion (S_o) as follows (Ewy 1999):

$$S = \frac{{S_{0} }}{\tan \phi }$$

(9)

$$\eta = \frac{{4\tan^{2} \phi *(9 - 7\sin \phi )}}{(1 - \sin \phi )}$$

(10)

By presenting the parameter (S) in the stress-invariant components, the modified Lade criterion can be applied in the wellbore instability evaluation, where the failure function (F) is stated as follows (Yi et al. 2006):

$$F = 27 + \eta - \left( {\frac{{I_{1}^{\prime \prime 3} }}{{I_{3}^{\prime \prime } }}} \right)$$

(11)

Failure occurs when F ≤ 0 as the strength of rock is lower than the stress; in this situation, an unstable wellbore will be observed (Ewy 1999).

Optimization of drilling mud weight

Knowledge of suitable mud weight is one of the imperative aspects in well design; it can be predicted using the next calculations which utilize the borehole wall stresses that have been anticipated through lined elasticity. These calculations correspondingly assume the absence of connection between the pore pressure of formation and borehole pressure. The suitable borehole pressure to restrict instability (P_w) calculated by the modified Lade criterion Eq. (4) can be stated as follows (Ewy 1999):

$$P_{\text{w}} = (B - C^{{\text1} /{\text2}})/(2A)$$

(12)

where

$$A = \sigma_{z} + S - P_{0}$$

(13)

$$B = A*\sigma_{{\theta^{n} }} - \tau_{{ \theta_{Z} }}^{2}$$

(14)

$$C = B^{2} - 4A\left\{ {D - \left( {S - P_{0} } \right)\left[ {A\left( {\sigma_{{\theta^{n} }} + S - P_{0} } \right) - \tau_{{\theta_{Z} }}^{2} } \right]} \right\}$$

(15)

$$D = \left( {\sigma_{{\theta_{n} }} + \sigma_{z} + 3S - 3P_{0} } \right)^{3} /(27 + \eta )$$

(16)

$$\sigma_{{\theta_{n} }} = \sigma_{x} + \sigma_{y} - 2\left( {\sigma_{x} - \sigma_{y} } \right)\cos 2\theta - 4\tau_{xy} \sin 2\theta$$

(17)

$$\sigma_{z} = \sigma_{zz} - v\left[ {2\left( {\sigma_{x} - \sigma_{y} } \right)\cos 2\theta + 4\tau_{xy} \sin 2\theta } \right]$$

(18)

$$\tau_{{\theta^{z} }} = 2(\tau_{yz} \cos \theta - \tau_{zx} \sin \theta )$$

(19)

The suitable wellbore pressure can be later converted to drilling fluid density or mud weight, measured in gram per cubic centimeters (Ewy 1999).

Results and discussion

Instability zonation and orientation

Wellbore stability analysis for Rumaila oil field requires identifying the borehole breakout intervals along the geological column; these breakouts were mainly associated with shale layers in Tanuma, Ahmadi, Nahr Umr and Zubair Formations. Shale is a fine-grained sedimentary rock that has a low permeability. Therefore, the time it takes for the stress to redistribute after a new hole is being drilled is very long, leading to possible failure in the borehole even after a few days of drilling; this is because the pore pressure in low-permeable formations is very high compared to high-permeable formations (Aadnoy 2010).

A correlation between six vertical wells in Rumaila oil field (A, B, C, D, E and F) using caliper log that measures well diameter indicated that there are eight breakout zones, as illustrated in Fig. 1; these zones (for the purpose of this study) were termed: Tanuma, Ahmadi Shale1, Ahmadi Shale2, Nahr Umr Shale1, Nahr Umr Shale2, Nahr Umr sand2, Upper Shale and Middle Shale.

Fig. 1

Breakout zonation using caliper log in five wells in Rumaila oil field

The breakouts’ orientation identified from oriented caliper and image logs in Rumaila oil field was 130°–150° (NW–SE) (S_hmin orientation). Therefore, the S_Hmax orientation, which is perpendicular to minimum horizontal stress, was 40°^–60° (NE–SW).

Wellbore stability analysis

The wellbore stability analysis resulted from applying modified Lade criterion on five vertical wells in Rumaila oil field (A, B, C, E and F) revealed that the shear failure pressure is greater than the used mud pressure, especially in the shale layers of Tanuma, Ahmadi, Nahr Umr and Zubair Formations, that low mud pressure had led to wellbore instability and formed breakouts and washouts; these predictions were corresponding to the actual borehole breakouts indicated by caliper log. Figures 2, 3, 4, 5 and 6 show the calculated pore pressure, shear failure pressure and minimum horizontal stress (S_hmin) plotted with used mud pressure and actual breakouts.

Fig. 2

Predicted shear failure pressure with used mud weight pressure values and comparison with actual breakouts observed from Caliper log in well A

Fig. 3

Predicted shear failure pressure with used mud weight pressure values and comparison with actual breakouts observed from Caliper log in well B

Fig. 4

Predicted shear failure pressure with used mud weight pressure values and comparison with actual breakouts observed from Caliper log in well C

Fig. 5

Predicted shear failure pressure with used mud weight pressure values and comparison with actual breakouts observed from Caliper log in well E

Fig. 6

Predicted shear failure pressure with used mud weight pressure values and comparison with actual breakouts observed from Caliper log in well F

Stable wellbore design involves raising mud weight (and/or change well trajectory) appropriately to limit the initial breakout of the wellbore to an acceptable amount. The optimal mud pressure shall be higher than shear failure pressure but in the same time lower than minimum horizontal stress to prevent formation fracture which may cause mud losses (Zoback 2007).

Well design optimization

To avoid borehole failure and increase wellbore stability, the concentration of stress can be adjusted suitably through modifying the pressure of drilling mud and choosing optimum azimuth of the borehole in relation to in situ stresses (Al-Ajmi and Zimmerman 2009).

Generally, there are two primary types of borehole trajectory (Rabia 2002):

• 1. Straight or vertical.

• 2. Directional, which include horizontal and deviated borehole trajectory.

Vertical wells optimization

In accordance with the mud properties results that used in successful drilling, it is recommended to sustain the lowest possible mud density in order to reduce contamination of the reservoir. Nevertheless, if the mud density was too low, then the drilling mud pressure will not be possibly sufficient enough to maintain borehole stability (Al-Bazali et al. 2007).

Average shear failure pressure and its equivalent mud weight (EMW) predicted from five wells’ analysis in Rumaila oil field (A, B, C, E and F) are addressed in Table 1. The observed high EMW that exceeds 1.20 gm/cm³ (the common mud weight is used to drill that section) was corresponding to shale. Values of shear failure pressure that are measured in pound per square inch (psi) can be converted to equivalent mud weight (EMW) in gram per square centimeter (gm/cm³).

Table 1 Average values of shear failure pressure with their equivalent mud weight

Optimization of drilling mud weight for vertical wells in Rumaila oil field requires raising the drilling mud weight to more than 1.27 gm/cm³ starting from bottom of Sadi Formation to the bottom of Zubair Formation. This mud weight can help to decrease and minimize most of the breakout intervals except Nahr Umr Sand2 and Middle Shale where these zones require higher mud weight (1.31 gm/cm³). However, very high mud weight may create induced fractures in the weak zones leading to severe lost circulation; also, it may possibly cause reservoir damage as the mud solids penetrate deeply into the reservoir (Mitchell and Miska 2011).

Directional and high-angle wells optimization

Directional drilling is a broad term that concerns all required activities for designing and drilling a wellbore to reach the reservoir target, or number of targets, located at some horizontal distance from top of the hole. In other words, the purpose of directional drilling is to connect the surface location with oil or gas reservoirs that are not located right below it; also, directional drilling can be the solution in the event of the drill pipe becoming stuck in the hole by simply drill around it, or plug back the well to drill to a replacement target (Burgess 1991).

Horizontal wells include the wells where inclination is more than 85° (Rabia 2002). The advantages of directional and horizontal wells include (Manshad et al. 2014):

• 1. Increase the production rates because of longer wellbore length exposed to the pay zone.

• 2. Reduce the water coning by means of decrease drawdown in the reservoir for a certain production rate, leading to reduction in the needed remedial work in the future.

• 3. Decrease the pressure drop with time around the wellbore.

The results of shear failure minimum mud weight estimated using modified Lade criteria in Rumaila oil field are illustrated in polar plots as shown in Fig. 7 for each instable zone; these polar plots display the values of wellbore shear failure breakout as a function of borehole orientation (deviation and azimuth), and the color shading in the plots specifies wellbore damage equivalent mud weight in gm/cm³. The azimuth indicates the borehole azimuth and distance from the center point to the borehole deviation in degrees. All zones indicated that the maximum horizontal stress on the NE–SW direction influences the wellbore and causes breakout in the weak zones of shale; that direction is in accordance with the regional horizontal stress regime caused by the movement of Arabian plate and collision with the Eurasian plate.

Fig. 7

Polar plot for shear failure equivalent mud weight

Outcomes also indicated that the required mud weight to avoid excessive wellbore shear failure (breakout) as a function of position is defined in darker red color representing orientations with higher mud weight, whereas light blue and blue colors characterize orientations with lower mud weight (parallel to maximum horizontal stress (S_Hmax) direction).

It is recognizable from the figures above that the best direction to drill high deviated and horizontal wells to avoid wellbore breakout and failure should be parallel to the S_Hmax orientation (NE-SW). The S_hmin azimuth (NW–SE) had a low sensitivity. Nonetheless, breakout pressure is highly sensitive to inclination in the S_Hmax azimuth (Lade 1977).

Conclusion

• 1. Eight breakout zones were identified in Rumaila lithological column based on borehole enlargement.

• 2. Breakouts occur when the shear failure pressure is higher than the mud pressure, especially in the shale layers of Tanuma, Ahmadi, Nahr Umr and Zubair Formations.

• 3. Drilling mud weight for vertical wells in Rumaila oil field needs to be raised to 1.27 gm/cm³ starting from Sadi Formation to the bottom of Zubair Formation to gain wellbore stability. This mud weight can help to decrease and minimize most of the breakout intervals except Nahr Umr Sand2 and Middle Shale where these zones require higher mud weight (1.31 gm/cm³). However, reaching increased mud weight may create induced fractures in the weak zones, which could lead to severe lost circulation.

• 4. Optimal direction to drill deviated wells is parallel to the maximum horizontal stress direction that is 50° or 230° (NE–SW).

• 5. Deviated wells (more than 30° inclination) in the direction of maximum horizontal stress are more stable than other well types in Rumaila oil field.

Recommendations

• 1. Assess the current study results with future deviated and horizontal wells in Rumaila oil field and possibility to implement the same practice on other fields in southern Iraq such as Zubair, West Qurna, Tuba and Nahr Umr as the deviated and horizontal wells drilling can add significant increase in production in these fields.

• 2. Implement the extended leakoff test (XLOT) or mini-fracture test (DFIT) on different formations in Rumaila oil field to confirm the values of minimum and maximum horizontal stress.

• 3. Carry out a study about the chemical effect of the drilling mud on the shale layers and possible interaction between them, which may be possible reason for bore hole instability.