Wellbore instability management using geomechanical modeling and wellbore stability analysis for Zubair shale formation in Southern Iraq

AbstractWellbore instability problems cause nonproductive time, especially during drilling operations in the shale formations. These problems include stuck pipe, caving, lost circulation, and the tight hole, requiring more time to treat and therefore additional costs. The extensive hole collapse problem is considered one of the main challenges experienced when drilling in the Zubair shale formation. In turn, it is caused by nonproductive time and increasing well drilling expenditure. In this study, geomechanical modeling was used to determine a suitable mud weight window to overpass these problems and improve drilling performance for well development. Three failure criteria, including Mohr–Coulomb, modified Lade, and Mogi–Coulomb, were used to predict a safe mud weight window. The geomechanical model was constructed using offset well log data, including formation micro-imager (FMI) logs, acoustic compressional wave, shear wave, gamma ray, bulk density, sonic porosity, and drilling events. The model was calibrated using image data interpretation, modular formation dynamics tester (MDT), leak-off test (LOT), and formation integrity test (FIT). Furthermore, a comparison between the predicted wellbore instability and the actual wellbore failure was performed to examine the model's accuracy. The results showed that the Mogi–Coulomb failure and modified Lade criterion were the most suitable for the Zubair formation. These criteria were given a good match with field observations. In contrast, the Mohr–Coulomb criterion was improper because it does not match shear failure from the caliper log. In addition, the obtained results showed that the inappropriate mud weight (10.6 ppg) was the main cause behind wellbore instability problems in this formation. The optimum mud weight window should apply in Zubair shale formation ranges from 11.5 to 14 ppg. Moreover, the inclination angle should be less than 25 degrees, and azimuth ranges from 115 to 120 degrees northwest-southeast (NE–SW) can be presented a less risk. The well azimuth of NE–SW direction, parallel to minimum horizontal stress (Shmin), will provide the best stability for drilling the Zubair shale formation. This study's findings can help understand the root causes of wellbore instability in the Zubair shale formation. Thus, the results of this research can be applied as expenditure effectiveness tools when designing for future neighboring directional wells to get high drilling performance by reducing the nonproductive time and well expenses.

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Stability Analysis of Highly Deviated Boreholes to Minimize Drilling Risks and Nonproductive Time

Journal of Energy Resources Technology ◽

10.1115/1.4042733 ◽

2019 ◽

Vol 141 (8) ◽

Cited By ~ 6

Author(s):

Ahmed K. Abbas ◽

Ralph E. Flori ◽

Mortadha Alsaba

Keyword(s):

Stability Analysis ◽

Shear Failure ◽

Cost Effective ◽

Failure Criteria ◽

Horizontal Stress ◽

Rock Failure ◽

Wellbore Stability ◽

Tensile Failure ◽

Drilling Performance ◽

Deviated Wells

The Lower Cretaceous Zubair Formation is a regionally extended gas- and oil-producing sandstone sequence in Southern Iraq. Due to the weak nature of the Zubair Formation, the lack of wellbore stability is one of the most critical challenges that continuously appears during the drilling development operations. Problems associated with lack of wellbore stability, such as the tight hole, shale caving, stuck pipe, and sidetracking, are both time-consuming and expensive. This study aimed to construct a geotechnical model based on offset well data, including rock mechanical properties, in situ stresses, and formation pore pressure, coupled with suitable rock failure criteria. Mohr–Coulomb and Mogi–Coulomb failure criteria were used to predict the potential rock failure around the wellbore. The effect of the inclination and azimuth of the deviated wells on the shear failure and tensile failure mud weights was investigated to optimize the wellbore trajectory. The results show that the best orientation to drill highly deviated wells (i.e., inclinations higher than 60 deg) is along to the minimum horizontal stress (140 deg). The recommended mud weight for this selected well trajectory ranges from 1.45 to 1.5 g/cc. This study emphasizes that a wellbore stability analysis can be applied as a cost-effective tool to guide future highly deviated boreholes for better drilling performance by reducing the nonproductive time.

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New Interface for Assessing Wellbore Stability at Critical Mud Pressures and Various Failure Criteria: Including Stress Trajectories and Deviatoric Stress Distributions

Energies ◽

10.3390/en12204019 ◽

2019 ◽

Vol 12 (20) ◽

pp. 4019

Author(s):

Wang ◽

Weijermars

Keyword(s):

Shear Failure ◽

Axial Stress ◽

Failure Criteria ◽

Wellbore Stability ◽

Tensile Failure ◽

Wellbore Instability ◽

Wellbore Pressure ◽

Stress States ◽

Deviatoric Stresses ◽

Wellbore Failure

This study presents a new interface for wellbore stability analysis, which visualizes and quantifies the stress condition around a wellbore at shear and tensile failure. In the first part of this study, the Mohr–Coulomb, Mogi–Coulomb, modified Lade and Drucker–Prager shear failure criteria, and a tensile failure criterion, are applied to compare the differences in the critical wellbore pressure for three basin types with Andersonian stress states. Using traditional wellbore stability window plots, the Mohr–Coulomb criterion consistently gives the narrowest safe mud weight window, while the Drucker–Prager criterion yields the widest window. In the second part of this study, a new type of plot is introduced where the safe drilling window specifies the local magnitude and trajectories of the principal deviatoric stresses for the shear and tensile wellbore failure bounds, as determined by dimensionless variables, the Frac number (F) and the Bi-axial Stress scalar (χ), in combination with failure criteria. The influence of both stress and fracture cages increases with the magnitude of the F values, but reduces with depth. The extensional basin case is more prone to potential wellbore instability induced by circumferential fracture propagation, because fracture cages persists at greater depths than for the compressional and strike-slip basin cases.

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Geomechanical Analysis to Avoid Serious Drilling Hazards in Zubair Oilfield, Southern Iraq

Iraqi Journal of Science ◽

10.24996/ijs.2020.61.8.16 ◽

2020 ◽

pp. 1994-2003

Author(s):

Shaban Dharb Shaban ◽

Hassan Abdul Hadi

Keyword(s):

Gamma Ray ◽

Mechanical Test ◽

Horizontal Wells ◽

Failure Criteria ◽

Horizontal Stress ◽

Wellbore Instability ◽

Geomechanical Analysis ◽

Safe Mud Window ◽

Rock Mechanical Properties ◽

Minimum Horizontal Stress

Zubair oilfield is an efficient contributor to the total Iraqi produced hydrocarbon. Drilling vertical wells as well as deviated and horizontal wells have been experiencing intractable challenges. Investigation of well data showed that the wellbore instability issues were the major challenges to drill in Zubair oilfield. These experienced borehole instability problems are attributed to the increase in the nonproductive time (NPT). This study can assist in managing an investment-drilling plan with less nonproductive time and more efficient well designing. To achieve the study objectives, a one dimension geomechanical model (1D MEM) was constructed based on open hole log measurements, including Gamma-ray (GR), Caliper (CALI), Density (RHOZ), sonic compression (DTCO) and shear (DTSM) wave velocities , and Micro imager log (FMI). The determined 1D MEM components, i.e., pore pressure, rock mechanical properties, in-situ principal stress magnitudes and orientations, were calibrated using the data acquired from repeated formation test (RFT), hydraulic fracturing test (Mini-frac), and laboratory rock core mechanical test (triaxial test). Then, a validation model coupled with three failure criteria, i.e., Mohr-Coulomb, Mogi-Coulomb, and Modified lade, was conducted using the Caliper and Micro-imager logs. Finally, sensitivity and forecasting stability analyses were implemented to predict the most stable wellbore trajectory concerning the safe mud window for the planned wells. The implemented wellbore instability analysis utilizing Mogi-Coulomb criterion demonstrated that the azimuth of 140o paralleling to the minimum horizontal stress is preferable to orient deviated and horizontal wells. The vertical and slightly deviated boreholes (1ess than 30o) are the most stable wellbores, and they are recommended to be drilled with 11.6 -12 ppg mud weight. The highly deviated and horizontal wells are recommended to be drilled with a mud weight of 12-12.6 ppg.

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Redefining Technical Limit – Managed Pressure Directional Drilling Solution in Mexico Homol Field

10.2523/iptc-21398-ms ◽

2021 ◽

Author(s):

J. L. Lopez Mata ◽

S.. Perez ◽

H. H. Vizcarra ◽

Alex Ngan ◽

E. A. Garcia Gil ◽

...

Keyword(s):

Communication Protocol ◽

Wellbore Stability ◽

Plastic Behavior ◽

Directional Drilling ◽

Wellbore Instability ◽

Well Control ◽

Drilling Performance ◽

Innovative Solution ◽

Drilling Technology ◽

History Of

Abstract This paper will discuss the Managed Pressure Directional Drilling fit-for-purpose solution deployed to meet drilling challenges in Mexico's offshore Homol field. This innovative solution integrates a new state-of-the-art Rotary Steerable System (RSS) with Managed Pressure Drilling (MPD) technology. Drilling hazards such as the ballooning effect due to drilling plastic formations, losses, wellbore instability, and stuck pipe were effectively mitigated, and improved drilling performance with reduced NPT was delivered compared to other directional drilling systems. The solution requires the integration of two highly technical disciplines, MPD and Directional Drilling. Hence, a Joint Operating & Reporting Procedure (JORP) and a defined communication protocol proved crucial for effective execution. The solution is based on a rigorous Drilling Engineering process, including detailed offset well analysis to deliver a comprehensive risk assessment and mitigation plan jointly with the Operator to tackle drilling hazards such as ballooning without compromising the directional drilling requirements. In addition, flow processes and procedures were developed for contingency events, including but not limited to losses, stuck pipe, wellbore instability, and well control. After successfully deploying the new RSS tool in Mexico offshore, the Operator came across a challenging directional well with a history of ballooning effect, losses, stuck pipe, and wellbore instability. Combining the RSS tool with MPD Constant Bottom Hole Pressure (CBHP) technique to mitigating the ballooning effect while maintaining constant surface back pressure (SBP), the well was drilled while minimizing the downhole pressure fluctuation to mitigate against wellbore instability until reaching the lower paleocene formation, taking care to maintain an equivalent circulating density (ECD) of 2.04g/cc while drilling, and 1.99g/cc during connections, in order to reduce the ballooning effect observed in offset wells. As a result of careful planning, the RSS and downhole-surface communication continued to work well, while the MPD CBHP variant successfully mitigated against ballooning and well control hazards. The paper will also discuss the effective communication protocol between directional drilling, MPD services, and rig contractors to ensure safe operational alignment. Rotary steerable systems (RSS) for directional drilling must drill in increasingly hostile environments and with different challenges inherent to formations; examples of this are formations with plastic behavior that cause ballooning effect. This phenomenon can confuse drilling crew cause its behavior is very similar to kicks from wells. Homol is an oilfield with marked ballooning characteristics, causing significant Non-Productive Time (NPT). Drilling challenges in the Homol field require the utilization of both Directional Drilling technology and MPD techniques to improve drilling performance and reduce NPT at the same time. However, the technologies need to be optimized for one another. Also, directional services had to ensure reliability and accurately position wells, while the MPD technology to discern ballooning from actual influx and managing wellbore stability. This article describes the teamwork carried out by the directional team and MPD to avoid/minimize the ballooning effect while drilling directional jobs, improving operational time. The paper also includes a planning and operational blueprint to reduce NPT related to, while increasing drilling performance in terms of rate of penetration (ROP) and wellbore quality to allow the liner to be run to section TD in the Lower Paleocene formation.

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New insight on studying the effect of both chemical sensitivity and rock mechanical properties in shale formation to minimize wellbore instability problems

E3S Web of Conferences ◽

10.1051/e3sconf/202020503012 ◽

2020 ◽

Vol 205 ◽

pp. 03012

Author(s):

Mohammad H. Alqam ◽

Hazim H. Abass ◽

Abdullah M. Shebatalhmad

Keyword(s):

Drilling Fluid ◽

Wellbore Stability ◽

High Porosity ◽

Shale Formations ◽

Wellbore Instability ◽

Testing Procedures ◽

Shale Formation ◽

Rock Mechanical Properties ◽

Core Volume ◽

Shale Permeability

Historically, many of the wells drilled in in shale formations have experienced a significant rig downtime due to wellbore instabilities. Most of the instability problems originated from the encountered shale formations. The objectives of this study include (1) to measure the properties governing shale strength and drilling fluid/shale interaction, and (2) to establish a reliable and efficient rock mechanical testing procedures related to wellbore stability. Preserved shale core has been recovered from shale formation and special core handling procedure was implemented. Mineral oil was used for plugging and core preservation. Rock mechanical characterization was conducted on core samples using both XRD/SEM techniques to study the core mineralogy. In addition, shale permeability was determined by two methods: flow testing and pressure transition methods. The results indicated that shale has high percentage of quartz (30-40%) which causes the shale to have high porosity and high permeability. The unconfined compressive strength of shale is very low which any drilling fluid that contains water phase further reduces. The Young’s modulus is very low which makes near wellbore deformation high. Based on the shale swelling testing, the all-oil fluid show no volume change occurred to the shale. When the same shale was exposed to the 7% KCl, about 16% increase in core volume occurred in 48 hours. This means that all samples allowed the water to flow into the shale formation.

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Analysis of Wellbore Instability in an Offshore Field: A Case Study

Volume 8: Polar and Arctic Sciences and Technology; Petroleum Technology ◽

10.1115/omae2018-77928 ◽

2018 ◽

Author(s):

Nubia Aurora González Molano ◽

José Alvarellos Iglesias ◽

Pablo Enrique Vargas Mendoza ◽

M. R. Lakshmikantha

Keyword(s):

Field Effect ◽

Point Of View ◽

Wellbore Stability ◽

Geomechanical Model ◽

Wellbore Instability ◽

3D Geomechanical Model ◽

Shale Formation ◽

Major Factors ◽

Offshore Field

Several wellbore instability problems have been encountered during drilling a shale formation in an offshore field, leading to the collapse of the main borehole and resulting in several sidetracks. In this study, an integrated 1D & 3D Geomechanical model was built for the field in order to investigate the major factors that control the instability problems from a Geomechanical point of view and to design an optimum mud window for planned wells in the field. Effect of bedding on wellbore stability was the most important factor to explain the observed drilling events. Optimized well paths for planned wells were proposed based on results of a sensitivity analysis of the effect of bedding orientation on wellbore stability. It has been observed that bedding exposed depends not only on well inclination but also on dip of the formation, attack angle, and azimuth.

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INTEGRATED OF GEOMECHANICS WELLBORE STABILITY & SWEET SPOT ZONE ANALYSIS TO UNCONVENTIONAL WELL DRILLING OPTIMIZATION

PETRO ◽

10.25105/petro.v9i1.6115 ◽

2020 ◽

Vol 9 (1) ◽

pp. 21

Author(s):

Allen Haryanto Lukmana

Keyword(s):

Organic Carbon ◽

Total Organic Carbon ◽

Shear Failure ◽

Horizontal Stress ◽

Wellbore Stability ◽

Brittleness Index ◽

Sweet Spot ◽

Collapse Pressure ◽

Laboratory Test Results ◽

Shale Formation

<strong><em>Abstract </em></strong>Central Sumatra Basin is one of the largest hydrocarbon producer basins in Indonesia. The largest hydrocarbons accumulation in this basin does not rule out the possibility of hydrocarbons also trapped in shale source rock. The potential for hydrocarbon shale is in the Brown Shale Pematang Group layer. The obstacle to development is the depth of the Brown Shale layer so deep that further case studies are needed. This study aims to analyze the geomechanical wellbore stability modelling for drilling and determination sweet spot zone supported by x-ray diffraction (XRD), brittleness index (BI), total organic carbon (TOC) analysis. The geomechanical wellbore stability modelling based on pore pressure, shear failure gradient/collapse pressure, fracture gradient, normal compaction trend, minimum horizontal stress, maximum horizontal stress and overburden gradient analysis. Brittleness index considers each parameter from XRD data which dominantly contains clay, quartz, and calcium. Based on XRD analysis of shale samples from Limapuluh Koto Area, it showed that the samples included the brittle shale group because of the dominant quartz, while the samples from Kiliran Jao were shale brittle because of dominant carbonate (carbonate-rich). From laboratory test results of 8 rock samples from Brown Shale Formation outcrop in Limapuluh Koto Area, it was obtained total organic carbon (TOC) value is 4-17% (average 8%). The shale thickness estimated > 30 m, the brittleness index shale estimated 0.71, and the gradient of over-pressure on Brown Sahle Pematang Group estimated 0.57 psi/ft & 0.53 psi/ft from log data analysis. So the output of this results the study is expected to get stable borehole, minimum of non-productive time (NPT), the problem when drilling such as caving and sloughing. Based on (Mt, 2013), the prospect criteria results can be concluded that the Brown Shale Formation has good unconventional hydrocarbon shale potential. It can be carried out with further research.

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The Innovative Integration of Wellbore Strengthening and Managed-Pressure Drilling Redraw the Line Between Undrillable and Drillable - Case Study from Offshore Mediterranean Deepwater

10.2118/206230-ms ◽

2021 ◽

Author(s):

Mahmoud El-Husseiny ◽

Taher El-Fakharany ◽

Samir Khaled

Keyword(s):

Total Concentration ◽

Pressure Control ◽

Control Mode ◽

Integrity Test ◽

Gas Field ◽

Drilling Performance ◽

Managed Pressure Drilling ◽

Weight Window ◽

Minor Losses ◽

Wellbore Strengthening

Abstract Managed pressure drilling (MPD) has a reputation for enhancing drilling performance. However, in this study, we use it as a technology for making undrillable wells drillable. In the deepwater of the Mediterranean of Egypt, a gas field has been producing for few years. Water broke through in one well, thus, we must drill a new well to compensate for the reduction in production. Years of production led to pressure depletion, which makes it difficult to drill this well conventionally. In this study, we will discuss the combination of MPD and wellbore strengthening (WS). In addition, we will discuss the challenges we met while drilling and how we tackled them, and the best practices and recommendations for similar applications. The 12¼" × 13½" hole section passed depleted sands, followed by a pressure ramp. First, we drilled the depleted sands and confirmed the pressure ramp top. To strengthen the sand, we spotted a stress-cage pill of 645 bbls with a total concentration of 29 ppb. In addition, we conducted a formation integrity test (FIT), but its value was lower than the required value to drill to the section target depth (TD). Then, we switched to MPD and increased the mud weight. MPD in annular pressure control mode (AP) enabled us to walk the edge as near as possible to the impossible. Drilling this section was challenging due to the narrow mud weight window (MWW). We faced kick-loss cycles, where we had high-gas levels (from 20% to 55%) while drilling with a loss rate from 60 to 255 bph, at the same time. The 8½″ × 9½″ hole section will cover a depleted reservoir. Therefore, we decided to use the MPD to drill this section. To widen the MWW, we decided to stress-caging the hole, as we drill. We loaded the active-mud system with stress-cage materials totaling 39 ppb. We drilled the hole section while keeping the bottom hole pressure (BHP) at 14.6 ppg. We drilled using MPD by maintaining 525-psi surface back pressure (SBP). We used the SBP mode (semi-auto mode) to add connections, resulting in minor background gases and minor losses. This study discusses the application of a novel combination of MPD and WS. It emphasizes how MPD can integrate with other technologies to offer a practical solution to future drilling challenges in deepwater-drilling environments.

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Influence of the Weakening Effect of Drilling Fluid on Wellbore Stability in Anisotropic Shale Formation

Frontiers in Physics ◽

10.3389/fphy.2021.745075 ◽

2021 ◽

Vol 9 ◽

Author(s):

Fan Zhang ◽

Hou-Bin Liu ◽

Shuai Cui ◽

Ying-Feng Meng ◽

Jia-Jun Wang

Keyword(s):

Shear Failure ◽

Drilling Fluid ◽

Great Influence ◽

Pressure Rise ◽

Wellbore Stability ◽

Distribution Model ◽

Longmaxi Formation ◽

Collapse Pressure ◽

Long Time ◽

Shale Formation

For horizontal wells in Longmaxi Formation, oil-based drilling fluid that soaks for a long time is more likely to cause a wellbore collapse. Therefore, in this paper, the downhole core method of shale formation, in Longmaxi Formation, was adopted. First, rock samples were selected from different sampling angles and soaked with field drilling fluid. Second, a triaxial mechanics experiment was carried out. Based on the anisotropic wellbore stress distribution model, the stability of shale wellbore was calculated and analyzed. The results show that the compressive strength and cohesion of the shale are reduced after soaking in the drilling fluid. Hence, the reduction range of various sampling angles obviously differs as well. Shear failure occurs in vertical stratification; shear slip failure occurs along the weak plane, showing strong anisotropy. Combined with the experimental results, the collapse pressure is calculated, and it is found that the weakening effect of drilling fluid makes the overall collapse pressure rise by about 0.2 g/cm3. Finally, the shale bedding dip and dip direction have a great influence on the collapse pressure. The lower critical mud weight always takes the minimum value when the borehole axis is perpendicular to the bedding.

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Optimize Internal Phase Salinity to Improve Wellbore Stability and Mitigate Lost Circulation

10.2118/204347-ms ◽

2021 ◽

Author(s):

Jianguo Zhang ◽

Alan Rodgerson ◽

Stephen Edwards

Keyword(s):

Water Activity ◽

Wellbore Stability ◽

Chemical Effect ◽

Wellbore Instability ◽

Lost Circulation ◽

Main Challenge ◽

Internal Phase ◽

Naturally Fractured ◽

Drilling Operations ◽

Shale Formation

Abstract Wellbore instability and lost circulation are two major sources of non-productive time (NPT) in drilling operations worldwide. Non-aqueous fluid (NAF) is often chosen to mitigate this and minimize the chemical effect on wellbore instability in reactive shales. However, it may inadvertently increase the risk of losses. A simple method to optimize internal phase salinity (IPS) of NAF is presented to improve wellbore stability and mitigate the increased possibility of losses. Field cases are used to demonstrate the effects of salinity on wellbore instability and losses, and the application of the proposed method. IPS is optimized by managing bidirectional water movement between the NAF and shale formation via semi-permeable membrane. Typically, higher shale dehydration is designed for shallow reactive shale formation with high water content. Whereas, low or no dehydration is desired for deep naturally fractured or faulted formation by balancing osmotic pressure with hydrostatic pressure difference between mud pressure and pore pressure. The simple approach to managing this is as follows: The water activity profile for the shale formation (aw,shale) is developed based on geomechanical and geothermal information The water activity of drilling fluid (aw,mud) is defined through considering IPS and thermal effects The IPS of NAF is manipulated to manage whether shale dehydration is a requirement or should be avoided If the main challenge is wellbore instability in a chemically reactive shale, then the IPS should be higher than the equivalent salinity of shale formation (or aw,shale > aw, mud) If the main challenge is losses into non-reactive, competent but naturally fractured or faulted shale, then IPS should be at near balance with the formation equivalent salinity (or aw, shale ≈ aw, mud) It is important that salt (e.g. calcium chloride – CaCl2) addition during drilling operations is done judiciously. The real time monitoring of salinity variations, CaCl2 addition, water evaporation, electric stability (ES), cuttings/cavings etc. will help determine if extra salt is required. The myth of the negative effects of IPS on wellbore instability and lost circulation is dispelled by analyzing the field data. The traditional Chinese philosophy: "following Nature is the only criteria to judge if something is right" can be applied in this instance of IPS optimization. A simple and intuitive method to manage IPS is proposed to improve drilling performance.

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