A Three-Dimensional Finite-Element Model in ABAQUS to Analyze Wellbore Instability and Determine Mud Weight Window

Round 1

Reviewer 1 Report

This contribution aims at simulating a three-dimensional (3D) finite-element model in ABAQUS to investigate in-situ stresses and calculate the MWW during the drilling operation of wellbore-D in the Azar oilfield. The research design, methodology, and outcomes are all good. This work, on the other hand, has to focus more on its novelty. Some suggestions for the authors to improve this work are as follows:

(1) The abstract, as well as the introductory section of this contribution, should emphasize the study's novelty in a better way.

(2) Line 80 – 84, the authors may go into further detail about the chemical factors, as wellbore instability is mostly driven by chemical and mechanical causes.

(3) Line 150 – 160, the author may go into considerable detail regarding the modeling description and analysis of the 3D finite-element numerical model.

(4) Line 444 – 459, please enhance the implications of the findings (results) by adding more applicable conclusions from this study.

Author Response

Response to Reviewer 1 Comments

 

 

 

This contribution aims at simulating a three-dimensional (3D) finite-element model in ABAQUS to investigate in-situ stresses and calculate the MWW during the drilling operation of wellbore-D in the Azar oilfield. The research design, methodology, and outcomes are all good. This work, on the other hand, has to focus more on its novelty. Some suggestions for the authors to improve this work are as follows:

 

Point 1: The abstract, as well as the introductory section of this contribution, should emphasize the study's novelty in a better way.

 

 

Response to Point 1:

The authors are grateful to Reviewer #1 for this helpful comment. We tried to highlight the novelty of this study further, as pointed at the last paragraph of Section 2. According to Reviewer #1 comment, a clarified novelty statement is added to the Abstract. We emphasized the implication of this study in the Conclusion too.

 

The literature on the wellbore stability misses the importance of the wellbore's deviation angle and azimuth. Concerning the availability of geo-mechanical data, this research investigates the optimization of the drilling route, i.e., the wellbore's deviation angle and azimuth, in wellbore-D of the Azar oilfield, Iran. To this aim, a three-dimensional (3D) finite-element model in ABAQUS is simulated to analyze in-situ stresses and determine the MWW in the drilling operation of wellbore-D in the Azar oilfield. Drawing upon this strand of research into the appropriate plan, this research attempts to evaluate the mechanical properties of rock, elasticity modulus, properties of rock failure, pore pressure, and in-situ stress of wellbore in the various deviation angles. This particular research finding also provides insights for obtaining the lower limit of the mud weight window and determining the optimal path of the wellbore when using directional drilling technology.

 

 

 

Point 2: Line 80 – 84, the authors may go into further detail about the chemical factors, as wellbore instability is mostly driven by chemical and mechanical causes.

 

Response to Point 2:

We would like to appreciate Reviewer #1 for this helpful comment. Following statements are added to the revised manuscript regarding to cover the chemical factors:

 

There are various chemicals in the drilling fluid which physically and chemically interact with formations, result in the production of swelling stress, and alleviate the mechanical strength of the wellbore wall [Yan et al., 2013]. Chemical treatments can focus on changing the chemical composition of formations and forming chemical sealants in fractures [Aston et al., 2007; Feng and Gray, 2017]. Also, wettability-alteration treatments attempts to change filter cake from oil-wet to water-water in terms of enhancing fracture healing while using oil-based muds [Quintero et al., 2012].

 

Feng, Y.; Gray, K.E. Review of fundamental studies on lost circulation and wellbore strengthening. J Petrol Sci. Eng. 2017, 152, 511–522. https://doi.org/10.1016/j.petrol.2017.01.052.

 

Yan, C.; Deng, J.; Yu, B. Wellbore stability in oil and gas drilling with chemical-mechanical coupling. The Scientific World J. 2013, 720271. https://doi.org/10.1155/2013/720271.

 

Aston, M.S.; Alberty, M.W.; Duncum, S.D.; Bruton, J.R.; Friedheim, J.E.; Sanders, M.W. A new treatment for wellbore strengthening in shale. Soc. Pet. Eng. 2007, http://dx.doi.org/10.2118/110713-MS.  

 

Quintero, L.; Brege, J.J.; Christian, C.F.; Clark, D. Reducing fracture propagation during the drilling process by altering wettability. Soc. Pet. Eng. 2012. http://dx.doi.org/10.2118/134032-MS.  

 

 

Point 3: Line 150 – 160, the author may go into considerable detail regarding the modelling description and analysis of the 3D finite-element numerical model.

 

Response to Point 3:

Thank you for this constructive comment. We added some more statements in relation to 3D finite-element model application.

Gomar et al. [22] stated that analytical solutions can be used for the distribution of principal stresses around a smooth circular wellbore wall. To better simulate actual bottom wellbore conditions of lost circulation, the 3D finite-element model allows simulation of wellbore circulation and fluid loss at the same time [Feng and Gray, 2016]. In a dynamic state, the 3D finite-element model was used to predict lost circulation with drilling mud in the Azar Oilfield’s wellbore-D and compute the principal stresses around the deviated wellbore.

 

Feng, Y.; Gray, K.E. A parametric study for wellbore strengthening. J Natural Gas Sci. Eng. 2016, 30, 350-363. https://doi.org/10.1016/j.jngse.2016.02.045.

 

Point 4: Line 444 – 459, please enhance the implications of the findings (results) by adding more applicable conclusions from this study.

 

Response to Point 4:

The authors are grateful to Reviewer #1 for this comment. The following statements are added to the revised version that elaborates further on the implications of the findings.

 

The wellbore instability is one of the main issues in oilfields while drilling an angular oil well. Determining the mud weight window is a practical solution for controlling the wellbore instability. This study aimed to calculate the lower limit of the mud weight window in the Azar oilfield’s wellbore-D. The first set of analyses referred to constructing a 3D model with the finite element method. In this method, characteristics of a porous media, including porosity, permeability, pore pressure, percentage of saturation, and specific weight, were represented. The 3D finite-element model in ABAQUS was designed to determine the modeling and analysis solution for the wellbore's deviation angle and azimuth in the Azar oilfield. The optimization of the drilling route was investigated to analyze in-situ stresses and determine the MWW in drilling operations. Kirsch's equation was used to validate numerical modeling. Analytical relations of Kirsch's equation showed an acceptable error to validate the numerical modeling process. The results showed that the drilling wellbore in the direction of the maximum horizontal stress needs lower mud weight compared to the direction of the minimum horizontal stress. In short, it can be stated that the active stress regime is a pressure system in the Azar oilfield that has created a combination of reverse and strike-slip faults throughout the structure. It is a prominent finding from this analysis that lower mud weight would reduce the cost of the drilling operation. Besides, these achievements on the wellbore instability would help us establish greater accuracy on this matter and provide insights into the drilling of wellbores in petroleum engineering.

Author Response File: Author Response.docx

Reviewer 2 Report

    In this study, a three-dimensional (3D) finite-element model was simulated in ABAQUS to analyze in-situ stresses and determine the MWW in the drilling operation of wellbore-D in the Azar oilfield. The results from the 3D finite model revealed that the Azar oilfield is structurally under the impact of a complex tectonic system dominated by two reverse faults with a configuration of ?H>?h>?v across the Sarvak Formation. My impression of the manuscript is positive, but following comments have to be considered by the authors:

    (1) The Introduction section is week. For example, the anisotropy and heterogeneity of geomaterials have a great influence on wellbore instability. However, the corresponding reviews are missing. Besides, the authors only mentioned that in-situ stresses will affect well instability, which actually needs to be explained in depth. Especially, the varying principal stress direction will influence the detailed failure characteristics of wellbores. In additiona, when the depth of wellbores is deep, some characteristic fracturing phenomena that are different from shallow wellbores may occur due to the high in-situ stresses. But the corresponding reviews are also missing. Please refer to the following literature:

[1] Research on zonal disintegration characteristics and failure mechanisms of deep tunnel in jointed rock mass with strength reduction method. Mathematics. 2022; 10(6): 922.

    (2) The legends of Figs. 8, 9 and 10 are missing. What do the different colors mean? Meanwhile, in Fig. 9, the format of “?Hmax” and “?hmax” should be uniform.

    (3) Please check through the paper and change the degrees such as “30 and 210 degrees”, “120 and 300 degrees”, etc, into “30° and 210°” and “120° and 300°”. The Latin abbreviations like “i.e.” should be italic.

    (4) What are the unites of the stresses displayed in Figs. 11 and 12?

    (5) In the Conclusions, the authors mentioned that the purpose of this study is the numerical modeling of the instability issue of the angular well in the Azar oil field, to calculate the lower limit of the mud weight window. Actually, this paper is more like a project report instead of a research paper. As a study, what are your core innovation and contributions to the literature? Please revise your abstract and conclusion sections to emphasize these.

    Hence, the above comments must be considered, and the minor revision is needed to further improve the manuscript.

Author Response

Response to Reviewer 2 Comments

 

 

 

In this study, a three-dimensional (3D) finite-element model was simulated in ABAQUS to analyze in-situ stresses and determine the MWW in the drilling operation of wellbore-Din the Azar oilfield. The results from the 3D finite model revealed that the Azar oilfield is structurally under the impact of a complex tectonic system dominated by two reverse faults with a configuration of ?H>?h>?v across the Sarvak Formation. My impression of the manuscript is positive, but following comments have to be considered by the authors:

 

Point 1:

The Introduction section is week. For example, the anisotropy and heterogeneity of geo materials have a great influence on wellbore instability. However, the corresponding reviews are missing. Besides, the authors only mentioned that in-situ stresses will affect well instability, which actually needs to be explained in depth. Especially, the varying principal stress direction will influence the detailed failure characteristics of wellbores. In addition, when the depth of wellbores is deep, some characteristic fracturing phenomena that are different from shallow wellbores may occur due to the high in-situ stresses. But the corresponding reviews are also missing. Please refer to the following literature:

[1] Research on zonal disintegration characteristics and failure mechanisms of deep tunnel in jointed rock mass with strength reduction method. Mathematics. 2022; 10(6):922.

 

Response to Point 1:

The authors are grateful to the Reviewer #1 for this overall appreciation. We do try the best to cover the highlighted points appropriately.

 

Chen et al. [2022] examined a series of 3D heterogeneous tunnel models considering varying joint dip angles with the aim of understanding the zonal disintegration phenomenon. They found that the zonal disintegration is induced by the stress redistribution of surrounding rock masses. Chen et al. [2022] also realized that the model with larger inclination angle is extremely damaged further before the final collapse of the wellbore.

The literature on the wellbore stability misses the importance of the wellbore's deviation angle and azimuth. Concerning the availability of geo-mechanical data, this research investigates the optimization of the drilling route, i.e., the wellbore's deviation angle and azimuth, in wellbore-D of the Azar oilfield, Iran. To this aim, a three-dimensional (3D) finite-element model in ABAQUS is simulated to analyze in-situ stresses and determine the MWW in the drilling operation of wellbore-D in the Azar oilfield. Drawing upon this strand of research into the appropriate plan, this research attempts to evaluate the mechanical properties of rock, elasticity modulus, the properties of rock failure, the pore pressure, in-situ stress of wellbore in the various deviation angles.

 

 

 

 

 

 

 

Point 2:

The legends of Figs. 8, 9 and 10 are missing. What do the different colors mean? Meanwhile, in Fig. 9, the format of “?Hmax” and “?hmax” should be uniform.

 

Response to the Point 2:

The authors are grateful for your constructive comment. Figures 9 and 10 have been re-sketched, presenting them as Figs. 8 and 9, respectively, in the revised manuscript.  Fig. 8 from the initial version is removed because of its similarity with the Fig. 10. The format of horizontal stresses is unified in the revised manuscript. 

 

Figure 8. The direction of maximum and minimum horizontal stresses, according to the reverse fault's configuration in the Azar oilfield. Depths are in meter.

 

 

Figure 9. The tectonic structure resulted from reverse faults performance in the Azar oilfield.

 

 

Point 3:

Please check through the paper and change the degrees such as “30 and 210degrees”, “120 and 300 degrees”, etc., into “30° and 210°” and “120° and 300°”. The Latin abbreviations like “i.e.” should be italic.

Response to the Point 3:

Thank you so much for your helpful comment. The proper symbol ‘°’ is replaced in the revised manuscript. Suggested Latin abbreviation is marked italic in the revised manuscript.

 

Point 4:

What are the units of the stresses displayed in Figs. 11 and 12?

Response to the Point 4:

These figures present the direction of maximum and minimum horizontal stresses shown by a rose diagram. We have emphasized on the geographical direction rather than the units of principal stresses.

 

Point 5:

In the Conclusions, the authors mentioned that the purpose of this study is the numerical modeling of the instability issue of the angular well in the Azar oil field, to calculate the lower limit of the mud weight window. Actually, this paper is more like a project report instead of a research paper. As a study, what are your core innovation and contributions to the literature? Please revise your abstract and conclusion sections to emphasize these.

 

Response to the Point 5:

The authors would like to appreciate Reviewer #2 for this suggestion. We have tried to revise both Abstract and Conclusions accordingly.

 

Abstract:

Wellbore instability is one of the main problems of the oil industry that causes high costs in the drilling operation. Knowing about the mechanical properties of involved formations and in-situ stresses is a privilege in determining an appropriate mud weight window (MWW). To this aim, a three-dimensional (3D) finite-element model was simulated in ABAQUS to analyze in-situ stresses and determine the MWW in the drilling operation of wellbore-D in the Azar oilfield. The results from the 3D finite model revealed that the Azar oilfield is structurally under the impact of a complex tectonic system dominated by two reverse faults with a configuration of ?_H>?_h>?_v across the Sarvak Formation. The amount of vertical, minimum, and maximum horizontal stresses was 90.15, 90.15, and 94.66 MPa, respectively, at a depth of 4 km. Besides, the amount of pore pressure and its gradient was 46 MPa and 11.5 MPa/km, respectively. From drilling wellbore-D in the direction of the maximum horizontal stress, the lower limit of the MWW was obtained 89 pcf. In this case, the results showed that the wellbore with a deviation angle of 10° is critical with a mud weight lower than 89 pcf. It caused the fall of the wellbore wall within the plastic zone sooner than other deviation angles. Also, in the case of drilling the wellbore in the direction of minimum horizontal stress, the lower limit of the MWW was 90.3 pcf. Moreover, in the deviation angle of approximately 90°, the wellbore wall remained critical while the mud weight was below 90.3 pcf. A comparison of the numerical and analytical modeling results showed that the modeling error falls within an acceptable value of <4%. As a result, the wellbore with the azimuth of the maximum horizontal stress needed less mud weight and decreased the drilling costs. This particular research finding also provides insights for obtaining the lower limit of the mud weight window and determining the optimal path of the wellbore when using directional drilling technology.

 

Conclusions:

The wellbore instability is one of the main issues in oilfields while drilling an angular oil well. Determining the mud weight window is a practical solution for controlling the wellbore instability. This study aimed to calculate the lower limit of the mud weight window in the Azar oilfield’s wellbore-D. The first set of analyses referred to constructing a 3D model with the finite element method. In this method, characteristics of a porous media, including porosity, permeability, pore pressure, percentage of saturation, and specific weight, were represented. The 3D finite-element model in ABAQUS was designed to determine the modeling and analysis solution for the wellbore's deviation angle and azimuth in the Azar oilfield. The optimization of the drilling route was investigated to analyze in-situ stresses and determine the MWW in drilling operations. Kirsch's equation was used to validate numerical modeling. Analytical relations of Kirsch's equation showed an acceptable error to validate the numerical modeling process. The results showed that the drilling wellbore in the direction of the maximum horizontal stress needs lower mud weight compared to the direction of the minimum horizontal stress. In short, it can be stated that the active stress regime is a pressure system in the Azar oilfield that has created a combination of reverse and strike-slip faults throughout the structure. It is a prominent finding from this analysis that lower mud weight would reduce the cost of the drilling operation. Besides, these achievements on the wellbore instability would help us establish greater accuracy on this matter and provide insights into the drilling of wellbores in petroleum engineering.

Author Response File: Author Response.docx