Construction of Mechanical Earth Modeling for Mitigating Wellbore Instability in Tanuma Shaly Formation -Southern Iraqi Oil Field

Abstract


Introduction
Wellbore instability is one of the main problems that engineers face during drilling.The causes of wellbore instability are often classified as either mechanical (for example, failure of the rock around the hole because of high stresses, and low rock strength) or inappropriate drilling practice (Bagheri et al., 2021).Treatment of wellbore instability issues is estimated to cost between 10 and 20 % of the overall cost of drilling.The petroleum industry experiences an annual economic loss of $1 to $6 billion globally due to wellbore instability issues (Albukhari et al., 2018).Wellbore instability is an important risk associated with drilling in shale formations, making it one of the most challenging operations.Shaly formations compose more than 85 % of instabilities and 75 % of all petroleum industry drilling formations (Abbas et al., 2018).Wellbore instability can be attributed to two primary categories tensile and shear failures (Shaban and Hadi, 2020).If the operating mud pressure falls below the breakout pressure, the shear failure will occur and formation fluid will flow into the wellbore (referred to as the "Kick" phenomenon).Conversely, if the mud pressure exceeds the breakdown pressure, tensile failure will occur, and drilling fluid losses to the formation, that influence the permeability of the rock (Rasouli et al., 2011).
Wellbore instability appears in various ways, including caving, which refers to the enlargement of the borehole; reduction of hole size, which results in a tied hole; loss of circulation; and stuck pipe (Zhang, 2013).To optimize well trajectories and ensure safe drilling operations, the analysis of wellbore instability plays a crucial role in future drilling strategies and field development.Any geomechanics application must address rock mechanical characteristics.Rock properties include Cohesion, Friction Angle, Tensile Strength, Young's modulus, Poisson's Ratio, and Unconfined Compressive Strength measured in laboratories (Aziz et al., 2021).These features are crucial to the mechanical earth model that utilizes different failure criteria to predict rock failure and set mud pressure limitations.These failure criteria serve as the fundamental element in assessing wellbore instability.By selecting the most appropriate failure criteria, engineers can accurately predict the conditions under which rock failure may occur and determine the mud pressure limits to ensure wellbore stability and prevent drilling-related issues.
As a result, it allows one to make critical decisions for planning and carrying out reservoir development (Tawfeeq and Aziz, 2023).The most common failure criteria are the Mohr-Coulomb and the modified Lade (Edan et al., 2023).Due to many problems that appear when employing Mohr-Coulomb criteria and a lower estimate of modified Lade criteria, Al-Ajmi and Zimmerman (2005) proposed a different criterion called the Mogi-Coulomb criterion.This failure criterion considers intermediate stress, which yields more realistic results.The shaly formation in the southern Iraqi oilfields comprises the Upper Zubair, Tanuma, Nahr-Umr, and Ahmadi Shales (Awadh et al., 2019;Awadh et al., 2021).The drilling operations have increased challenges when dealing with these formations (Alsultan et al., 2021;Awadh et al., 2021).Tanuma Shale Formation is a clastic sedimentary rock composed of fine-grained mud, consisting of clay mineral flakes and small shards of other minerals.Shales consist of both clay minerals and non-clay minerals.clay minerals may be categorized as smectite, vermiculite, vaolin, illite, and chlorite.non-clay minerals found in shales include quartz and feldspars (Mohammed et al., 2019).
Drilling in shale formations is regarded as one of the most difficult operations due to the significant risk of causing wellbore instability.Shale is a fine-grained sedimentary rock made up of small mineral particles combined with clay.Drilling in the Shale formation presents several challenges, including sloughing, swelling, caving, cementing, and casing landing problems, making it one of the most difficult formations to drill wells in.Stuck pipe occurrences are the principal drilling challenges in the petroleum industry.Due to the high cost of fishing, which makes over 250 million $ in losses in the North Sea and the Gulf of Mexico (Mohammed et al., 2019).One of the biggest drilling difficulties is stuck pipes in the Tanuma Formation, Nasiriyah oilfield, southern Iraq.Therefore, building a mechanical earth model that includes in-situ horizontal stresses (magnitude and direction), overburden stress (vertical stress), and rock mechanical characteristics is essential.The goal is to find the best failure criteria to develop a safe mud-weight window and anticipate rock failure.
The Nasiriyah oilfield extends approximately 34 Km in length and 13 Km in width.The field is located 38 Km north-northwest of Nasiriyah City within the unstable platform-Mesopotamian basin zone (Fig. 1) (Al-Sudani, 2019).All formations within this oilfield were formed during the Cretaceous, the stratigraphic column extends from Shiranish to Gotnia formations (Fig. 2).The lithology of Tanuma FORMATION is shale and marl, shale is considered an unstable rock and causes many problems during drilling operations (such as caving).It is considered one of the unstable formations of Nasiriyah Oilfield.This paper aims to construct a 1D mechanical earth model for a field in southern Iraq to understand and discuss the source and nature of problems that can occur during drilling operations and design the appropriate mud window to guide future wells drilling plans that are devoid of problems.

Methodology
The current study is based on data from two wells supplied by the Nasiriyah Oil Company and involves the available data of geophysical logs such as Spontaneous Potential, Gamma Ray, Density, Sonic, Neutron, and Resistivity logs of studied wells.The investigated interval consisted primarily of a thick Carbonate (limestone, dolomite) rock succession interbedded with shales, extending from Sadi to the end of the Shuaiba layers, and Clastic (Sandstone and Shale) rocks extending to the end of the Zubair Formation and the lower portion of the Nahr Umr Formation (Fig. 2).
Understanding the magnitude of stress and the mechanical properties of rocks is essential for any geomechanics study.The mechanical earth model was developed for wells X and B within the Nasiriyah oilfield.This was achieved through the application of three failure criteria; the Mogi-Coulomb failure criterion, the modified Lade and Mohr-Coulomb failure criterion to forecast the regions of failure within the wellbore.The mechanical earth model is a numerical representation of the state of stress and rock mechanical properties for a specific stratigraphic section in the Nasiriyah field.(Khudaier, 2019).

Fig. 2. Stratigraphy column of Nasiriyah oilfield
Initially, a comprehensive data audit was conducted to verify the accuracy of the collected data to create an accurate one-dimensional mechanical earth model and analyze the density log to estimate the overburden stress (Fig. 3).After that, the density log and acoustic compressional log data are used to compute the pore pressure profile.The mechanical properties of the rock, such as strength and elastic modulus, are vital considerations.Sonic logs, which detect shear and compressional effects, are compared to laboratory cores to ascertain these parameters.
The minimum and maximum horizontal stresses are calculated using static elastic characteristics, vertical stress, and pore pressure.Calibration is performed using data obtained from extended leak-off tests (LOT) and minifrac tests.The direction of the horizontal stresses can be determined by analyzing an aligned wellbore caliper log or formation micro-image log.To verify the mechanical earth model, the estimated wellbore failure, as indicated by various failure criteria, must be compared to the actual shape recorded by a caliper log.A sensitivity analysis is performed at a specific depth to determine the range of mud weight values to ensure stability and identify the optimal drilling direction (inclination and azimuth) for future wells in nearby locations.This analysis allows for the identification of the most favorable orientation.

Building the Mechanical Earth Model (MEM)
A mechanical earth model was developed to investigate the issue of wellbore instability in the Tanuma Formation, the data were supplied from the Nasiriyah Oil Company.This model was applied to wells X and Y that had significant instability events and high NPT values and the data of these wells were used, which included gamma-ray logs (GR), caliper logs (CALI), density logs (RHOZ), and sonic compression (DTCO), shear (DTSM) wave velocities for both wells, but core mechanical laboratory test, formation pressure test, and fracture pressure measurement was only available for the well X.
Vertical wellbore failure may be explained using this model, which takes into account the effects of stresses and rock mechanical characteristics.Understanding the mechanical effect on the wellbore enables mud weight optimization to stabilize the well (Azim et al., 2011).Beginning with the construction of the mechanical earth model relating to the target formation is the first step in the geomechanical analysis process, this requires the use of many data sets.These data contain rock mechanical parameters that were assessed in the laboratory on core samples, as well as logs and in situ stressors that were measured in the field.A mechanical earth model relating to the target formation may be characterized by four fundamental inputs, such as the pore pressure (Pp), the rock's dynamic properties, and the magnitude and direction of the in-situ stress.
There are three types of stresses in the area of interest: vertical stress (πv), minimum horizontal stress (πh), and maximum horizontal stress (πH).Some of the mechanical properties of the rock are its Young's modulus (E), Poisson's ratio (v), unconfined compressive strength (UCS), internal friction angle (φ), and tensile strength (To) (Al-Wardy and Portillo, 2010).Different methods were utilized for analyzing the wells, including lab data, gamma rays, density, porosity, acoustic (compression and shear wave speeds), resistivity, formation micro-imager (FMI) records, mud (master) measures, and caliper readings.

Rock Mechanical Properties
The rocks' mechanical properties are fundamental parts of the study of geomechanical analysis, which include Young's modulus (E), Poisson's ratio (v), internal friction angle (ϳ), and unconfined compressive strength of the rock (UCS).Continuous profiles for these properties show that the formation's ability and strength can vary naturally across several layers within the specific interval.Within this research, triaxial and multistage compressive tests were done in the lab on rock samples that were retrieved to check their mechanical properties (Abbas and Alsaba, 2018;Abbas et al., 2018).

• Elastic Rock Parameters
Rock elastic characteristics are the main things that are used to figure out in-situ stresses.For isotopically elastic materials, the static elastic parameters, like Poisson's ratio (v) and Young's modulus (E), show how the material deforms.Triaxial tests were done to find these static elastic values.To calibrate the elastic parameters of the rock, it must be transformed from dynamic to static elastic parameters, only Young's modulus (E) from equations 1 to 5, and dynamic Poisson's ratio (v) is the same as static (Abbas and Alsaba, 2018;Abbas et al., 2018). (1) (2) Where: Gdyn: shear modulus.Kdyn: bulk modulus.b: formation bulk density from density log.∆ and ∆ℎ: compressional and shear acoustic travel time.: dynamic Young's Modulus.: dynamic Poisson's Ratio.

• Strength Parameters of Rocks
Many rock strength variables denote the capability of a rock formation to resist the in-situ stress conditions that surround a wellbore.These parameters consist of the internal friction angle (φ), cohesive strength (C), and unconfined compressive strength (UCS).In the area of mechanical earth model of reservoirs, the rock strength parameters that are used the most commonly are the UCS and φ and their respective values.With the help of these characteristics, the wellbore failure that occurs during drilling and sanding as a consequence of formation pressure depletion is taken into consideration (Abbas and Alsaba, 2018;Abbas et al., 2018).Equation 6 Where: UCS: is an unconfined compressive strength.Φ: is the internal friction angle.: gammaray log.: is cohesion.

• Tensile Strength
The tensile strength (To) of the rock is an indication of its ability to resist tensile failure under certain conditions.Sudden and brittle failure of the rock materials occurs in the rock materials.The tested stress values ranged from 1/13 to 1/8 of their unconfined compressive strength (UCS).(Chang et al., 2013).
To find the tensile strength curve, the UCS was utilized (Rasouli et al., 2011).After testing plug samples, the expected tensile strength was compared to the data gathered in the lab.The mechanical properties of rocks were estimated from sonic wire logs for wells X and Y. Laboratory tests are in well X but not in well Y, so well Y will be calibrated based on well X (Fig. 4).While the predicted rock mechanical properties for well Y are shown in Fig. 5.

Pore Pressure
Deformation and instability of the wellbore are impacted by pore pressure.Consequently, it is one of the main factors of MEM.Pore pressure trends can be identified and calculated utilizing resistivity or sonic records.To calibrate the calculated pore pressure, pore pressure measurements are required.
To get more trust, the observed formation pressure points from the repeated formation test (RFT) were compared with the predicted formation pore pressure found using indirect methods (Abbas, 2019).The pore pressure curve and the individual RFT readings were very close to being accurate (Fig. 6).

Calculation of In-Situ Stresses
The main stress magnitudes in the regional stress field at any depth are; vertical stress, and horizontal principal stresses (maximum and minimum).
Vertical stress, which is also known as overburden stress, occurs at a specific location due to the gravitational force exerted by the overlying rock formations.Vertical stress is more efficient since it can be used to calculate pore pressure, horizontal stresses, and their relationship to classify the fault regime of formations, to estimate the vertical stress field component (σv) with depth using rock density data, as in equation ( 10).
Determination of minimum and maximum horizontal stresses can be achieved through the utilization of the poroelastic horizontal strain model (Thiercelin and Plumb, 1994).The formulations of this model, which are derived from the regional pore pressure, rock deformation, and Young's modulus of the rock masses, respectively, as in equations 11 and 12: (11) (12) While utilizing equations 13 and 14 to determine the magnitudes of the rock deformation εx and εy in the x and y planes, respectively, about the overburden stress (Kidambi and Kumar, 2016).
The minimum horizontal stress that could be calculated from the above methods was calibrated, using measurements directly from the mini-frac test.An estimation of the pore pressure, maximum horizontal stress, minimum horizontal stress, and vertical stress for two wells X and Y are indicated in Fig. 6.The tectonic stress regime observed in the Tanuma Formation appears to be a normal faulting regime ( σv > σH > σh).Fig. 6.Estimating the in-situ principal stress magnitudes for wells X and Y.

Direction of Horizontal Stresses
Stress direction analysis is a crucial aspect of geomechanical projects, especially in regions with tectonic activity.Understanding the magnitudes and orientations of stresses allows for the optimization of well trajectories to minimize wellbore instability (Alsahlawi et al., 2017).Wellbore breakout typically occurs where stress levels are highest and in alignment with the direction of the minimum principal stress.In a vertical well, the breakout direction coincides with the minimum horizontal stress direction, which aligns with the minimum principal stress.To determine the orientation of the horizontal stresses, an image log from a vertical well was utilized.
The log revealed that the minimum horizontal stress direction was at an azimuth of 115°, while the maximum horizontal stress occurred at an azimuth of 25°.This information was employed to establish the orientation of the horizontal stresses in the current geomechanical investigation (Fig. 7).These findings are consistent with stress measurements taken in the vicinity of Nasiriyah oilfield, southern Iraq.

Failure Criteria
Various failure criteria were employed to assess the stability of the wellbore and predict the likelihood of rock failure.The three criteria used in the current paper were the Modified Lade, Mogi-Coulomb, and Mohr-Coulomb criteria.Before applying these failure criteria, it was necessary to validate their accuracy by comparing them to actual wellbore failures that occurred under real mud weight conditions.This validation process involved matching the expected wellbore instability with the displacement observed in the real wellbore, using micro-imager and caliper logs.
Each failure criterion provides insights into how formations fail under shear stress.The Mohr-Coulomb model is considered over-conservative, meaning it predicts failure even in cases where the formations might still be stable.The Mogi-Coulomb criterion, on the other hand, is moderately conservative, providing a more balanced prediction of failure.Finally, the Modified Lade criterion is less conservative, indicating failure only when the formations are more likely to be truly unstable.

Mud Weight Window
The mud weight window for safe drilling operations can be estimated when the mechanical earth model parameters are available.The calculations were completed to determine the probability of borehole failure using the Mohr-Coulomb, Modified Lade, and Mogi-Coulomb failure criteria.The failure prediction along the Tanuma Formation was shown in Fig. (8a) using the Mohr-coulomb failure criteria, which was an overestimate in comparison to the actual failure (caliper log).
The reason for the results that were obtained is that the Mohr-coulomb failure criteria are not dependent on the intermediate principal stress.It appeared that the failure predicted by the Modified Lade was less accurate than the actual failure (Fig. 8b), this is because the Modified Lade failure criterion is considered to affect the intermediate stress when it comes to the prediction of failure.The fourth track, which depicts the caliper log shown in Fig. 8c, demonstrates a strong agreement between the projected failure by the Mogi coulomb and the actual wellbore failure for the majority of problem zones (Tanuma Formation).Hence, it is believed that the Mogi coulomb is the suitable failure criterion since the order of magnitude for stresses observed around a borehole in the case of breakouts is typically σθ ≥ σz ≥ σr and σr ≥ σz ≥ σθ concerning tensile failure.
Fig. 9 shows that well Y reflects the same results as well X.For drilling, the mud weight window is represented in the third track of these figures, while the first two represent depth, and formation name, respectively.The critical mud weight related to the kick is illustrated in grey on the left side of the third track.Minimum shear failure mode is indicated by the yellow pattern.Breakout failure is known if the mud weight falls below this yellow profile.The minimum mud weight necessary to ensure mechanical stability in a borehole is referred to as the failure limit.The model predicts re-opening natural fractures or fissures and allowing drilling fluid loss into the formation of the utilized mud weight that exceeds the blue profiles on the right.For drilling into the Tanuma Formation, the area between minimum shear failure (yellow) and tensile failure (blue) represents the safe operating mud weight window.

Model Validation
The validation of a geomechanical model is an essential requirement for its implementation.Once the mud weight window for an offset well has been determined, the actual mud weight utilized to drill the well can be utilized to forecast the predicted conditions of borehole failure (e.g., breakouts, tensileinduced fractures, and losses).the failure match can then be implemented by comparing the predicted wellbore instability to the actual rock failure presented in the image and/or caliper records.
The current study utilized the Mogi-Coulomb failure criterion to forecast the regions of failure within the wellbore.The caliper data indicate that significant breakouts were detected at intervals 1799.33-1850.6 m, 1976.68-1993.1 m for well X, and 1783.1-1847.431 m, and 1977.4-1988.6 m for well Y, as shown in Fig. 10.The anticipated events of the breakout regions exhibited an excellent match with the breakouts recorded in the caliper log.

Sensitivity Analysis at Tanuma Formation
According to the failure predicted by the Mogi-Coulomb criterion and the actual mud weight, a single-depth sensitivity analysis was implemented in this formation.Stereo net plots for depth 1839.47 m showed the breakouts with inclinations 0°-10° are the most safe and stable regarding shear failure, even when using low mud weight in directions of minimum horizontal stress 115°, but for inclinations 30°degree and above, shear failure occurs even with a high mud weight in both directions of minimum and maximum horizontal stress, and stereo net plots for breakdown show that tensile failure most likely occurs with inclination 60° and above towards of maximum horizontal stress.
While in the direction of minimum horizontal stress, breakdown may occur with an inclination of 80° and above (Figs.11a and 11b).Line plots displayed that the mud weight window was narrowing at inclinations above 10° (Fig. 11c), and there was not any effect of azimuth on the drilling mud weight window (Fig. 11d).Accordingly, failure appeared in the Tanuma formation, as it is one of the unstable formations within the Nasiriyah oilfield.

Wellbore Stability Forecast (Development Plan)
A wellbore stability prediction was performed to evaluate the drilling risk and analyze the mud weight window for vertical, deviated, and horizontal wells in the drilling planning phase.The objective was to address potential issues associated with borehole instability that occur in Tanuma formation.The analysis focused on evaluating the risks of wellbore deformation and the likelihood of breakout in weak shale sections along the planned vertical, deviated, and horizontal wells.The modified Mogi-Coulomb criteria were employed for this evaluation.The inclinations and outcomes of the wellbore stability prediction are presented in Table 1 and Figs. 12 and 13.Table 1 illustrates that as the wellbore inclination angle increases, the required mud weight to stabilize the wellbore also increases.The MIN_MW column represents the recommended mud weight for a specific inclination, while the MAX_MW column indicates the maximum mud weight to prevent breakdown.Both the vertical wellbore and the 10° deviated wellbore necessitated similar mud weights to prevent shear failure at the Tanuma Formation.

X Y
The chosen mud weight was designed to prevent tensile failure and minimize shear failure as much as possible.The results indicate that the wellbore inclination ranging from 0° to 10° is the most applicable well profile to drill a stable wellbore with the least shear failure along Tanuma Formation (Fig. 12).The mud weight window becomes narrower and higher mud weight is required for a planned deviated well at an inclination larger than 10° degrees with more complicated shear failure (Fig. 13).Generally, the wellbore stability forecasting results agreed with single depth sensitivity-analysis outcomes.Effective drilling procedures, such as routine borehole cleansing, vigilant tripping speed monitoring, appropriate mud conditioning, and ROP control during drilling through these zones, will improve instability management and prevent significant drilling problems.

Conclusions
The conclusions can be summarized as follows: The geomechanical analysis showed that laboratory core data a good agreement with both static mechanical rock properties and static mechanical rock parameters obtained from the selected model, which enhances our understanding of the geomechanical study.
The main risk is wellbore collapse due to shear failure of the Tanuma Formation and not tensile fracturing of the formation, which leads to losses.
The mechanical earth model developed for wells in the region showed good agreement with both caliper data and drilling reports.This was achieved through the application of three failure criteria; upon 30º 60º 89º

Fig. 4 .
Fig. 4. Predicted rock mechanical properties and laboratory measurements for well X.

Fig. 7 .
Fig.7.The image log illustrates the maximum horizontal stress and direction of the borehole breakouts(Allawi and Al-Jawad, 2023)

Fig. 11 .
Fig. 11.Sensitivity analysis using Mogi criterion at Tanuma Shale formations for well X, (a) Mud weight of breakout vs orientation, (b) Mud weight breakdown vs orientation, (c) Mud window vs deviation, and (d) Mud window vs Azimuth.

Table 1 .
Planned wellbore trajectory versus required stable mud weight of Tanuma and CR11 formations.