|Theme||Visible||Selectable||Appearance||Zoom Range (now: 0)|
Arlanoglu, C.. (The University of Texas at Austin) | Feng, Y.. (The University of Texas at Austin) | Podnos, E.. (The University of Texas at Austin) | Becker, E.. (The University of Texas at Austin) | Gray, K. E. (The University of Texas at Austin)
Abstract In drilling deep horizons, the mud weight window between pore pressure gradient and fracture gradient often narrows due to rock properties and underground stress state. Non-productive time (NPT) events such as lost circulation, wellbore instability, kicks, underground cross-flow, and pipe sticking are more likely. Such problems greatly increase drilling costs. Plugging preexisting natural fractures or drilling-induced fractures with lost circulation materials (LCM) is often used to increase fracture gradient and widen the mud weight window. This technique, called ‘wellbore strengthening’, includes several strengthening methods, but there are several factors affecting these procedures are not thoroughly understood. To reduce the risk of loss circulation while drilling in formations with narrow mud weight windows such as pressure depleted reservoirs and deep-water formations, a good understanding of the mechanisms of wellbore strengthening in different down-hole scenarios helps engineers to optimize the design of drilling fluids and operation procedures. This paper discusses the mechanism of wellbore strengthening in elastic and pore-elastic models, utilizing the finite element method to evaluate wellbore and near-wellbore stresses during fracture initiation and propagation, and after plugging fractures with LCM. Factors affecting fracture behavior such as formation stress anisotropy, LCM bridging location, initial pore pressure, and fluid leak-off are investigated. A better understanding of the several interacting events local to the wellbore and near-wellbore regions can result in improved operational practices related to lost circulation events.
Abstract Wellbore pressure containment, the maximum pressure that a wellbore can withstand before whole drilling mud begins to leak off into formations, defines the upper bound of the mud weight window. Drilling costs are heavily associated with this mud weight window in terms of expensive casing programs or nonproductive time because of narrow mud weight windows. Although various methods have been developed using different materials for curing mud losses, the performance has been poor as a result of the lack of engineering design for the materials and placement techniques. The commonly used "trial and error" approach reveals inadequate understanding of the challenge, although a number of advances have been accomplished in recent years. In this paper, results are reported from a fundamental study aimed at revealing the basic physics that control the success of treatments strengthening wellbores or widening the mud weight window. It first focuses on reviewing and identifying factors that can reduce the upper bound of this mud weight window, followed by analysis of wellbore strengthening that can increase this upper bound of mud weight window, especially by sealing hydraulically conductive cracks at the wellbore. Introduction With the move to deeper reservoirs that requires penetrating depleted formations or those located in deeper water, the safe drilling mud weight window is becoming smaller. This has been translated into much higher costs because of either expensive casing programs or nonproductive time (NPT). More than 12% of NPT has been reported1 for Gulf of Mexico (GOM) area shelf drilling as a result of lost circulation alone. It is a common practice to use lost circulation materials to cure mud losses to drill ahead2. However, because of a lack of a clear understanding of mechanisms and loss conditions, a "trial and error" approach is commonly used by following a predetermined flowchart. This is obviously not the most efficient approach. In recent years, work3–16 has been performed to address this problem and the concept of "wellbore strengthening" has been gradually emerging, replacing the old concept of "plugging a hole." This important change reveals that the industry is now embracing the understanding of the technology and focusing on the physics behind wellbore problems and potential solutions. Among these solutions, it is important to discuss wellbore strengthening with particulate lost circulation material (LCM) treated drilling fluids. An example of this technique is the "stress cage" method12,13. This approach shows great potential for solving lost circulation, especially when drilling depleted formations. Although some report success, the stress cage method has been challenged by others17. Some issues, such as fracture stability and the maximum wellbore pressure containment, have been reviewed in a previous publication18. A detailed study of this method is probably needed to better understand its effect on the physics of wellbore strengthening. This paper begins with the analysis of factors that weaken wellbore pressure containment at or near the wellbore. It appears obvious that reversing of some of those factors could strengthen the wellbore. However, this is not so apparent for other factors, such as fractured wellbores. Therefore, this paper uses a boundary element analysis method, a numerical approach, to demonstrate how a wellbore can be strengthened by sealing the fractures. A similar analysis that involves the far field stress affected by pore pressure depletion would reveal the mechanism for lost circulation in depleted formations. This will be discussed in a separate paper with an analysis of a different strengthening approach. Wellbore Pressure Containment and Wellbore Weakening Wellbore pressure containment (WPC) is defined as the maximum pressure that a wellbore can withstand before the wellbore starts to leak its mud into the formation16. When large pores, vugs, or open natural fractures exist, WPC is defined primarily by the formation pressure. The loss of mud into large pores, vugs, or open natural fractures is beyond the scope of this paper. Because hydraulic fracturing is a tensile failure process, the near wellbore stress concentration pertinent to WPC is the tangential stress around the wellbore, sometimes called hoop stress. Sedimentary rock has a very small tensile strength, usually not exceeding 200 psi. It is often neglected to simplify stress analyses.
Abstract Reservoir depletion alters formation pressure, far-field in-situ stresses, and stresses around the wellbore. All of these factors influence collapse and fracture gradients, and therefore the drilling mud weight window. For deviated wells in depleted reservoirs, this situation is even more complicated due to the effect of well inclination and azimuth on the mud weight window. Analytical model results are presented in this paper for mud weight window calculations, considering formation pressure depletion, initial in-situ stress ratio (SHmax/Shmin), well inclination and azimuth. In-situ stresses change due to reservoir depletion are first determined from poroelastic theory, assuming the reservoir experiences vertical deformation, but no deformation in the lateral directions. Next, far-field in-situ stresses after depletion are transformed to a deviated wellbore with arbitrary well inclination and azimuth. Then, utilizing the Mohr-Coulomb shear failure criterion and tensile strength failure criterion, collapse and fracture gradients are calculated for various ratios of initial horizontal stress, degree of depletion, wellbore inclination, and azimuth. Finally, the sensitivities of collapse and fracture gradients relative to these factors are compared and discussed. The most problematic situation is to drill highly deviated or horizontal wells in reservoirs with large depletion along the maximum horizontal stress direction. In this case, fracture gradient can reach extremely low levels, resulting in very narrow drilling window and very small margin for mud weight adjustment. Drilling along or near the direction of minimum horizontal stress is preferred. An interesting finding in this study is that fracture gradient does not always decrease for deviated wells, which is contrary to common understanding that fracture gradient always decreases as the pore pressure gradient declines.
Feng, Y.. (University of Texas at Austin) | Arlanoglu, C.. (University of Texas at Austin) | Podnos, E.. (University of Texas at Austin) | Becker, E.. (University of Texas at Austin) | Gray, K.E.. E. (University of Texas at Austin)
Summary In the drilling of deep horizons, the mud-weight window between pore-pressure gradient and fracture gradient often narrows because of rock properties and underground-stress state. Nonproductive-time events such as lost circulation, wellbore instability, kicks, underground crossflow, and pipe sticking are more likely. Such problems greatly increase drilling costs. Bridging pre-existing natural fractures or drilling-induced fractures with lost-circulation materials (LCM) is often performed to increase fracture gradient and widen the mud-weight window. This technique, referred to as wellbore strengthening, includes the stress cage, fracture-closure stress, and fracture-propagation-resistance methods. Although these methods are often used, several aspects of these approaches are still not thoroughly understood. To reduce the risk of lost circulation while drilling in formations with narrow mud-weight windows, such as pressure-depleted reservoirs and deepwater formations, a good understanding of the mechanisms of wellbore strengthening in different downhole scenarios helps engineers to optimize the design of drilling fluids and operational procedures. This paper discusses the mechanism of wellbore strengthening, with a focus on hoop stresses at and near the wellbore, in elastic and poroelastic models, by use of the finite-element method to evaluate wellbore and near-wellbore stresses during fracture creation and propagation, and after plugging fractures with LCM. Factors affecting fracture behavior, such as horizontal-stress anisotropy, LCM-bridging location, initial pore pressure, and fluid leakoff, are investigated. A better understanding of the several interacting events local to the wellbore and near-wellbore regions can result in improved operational practices related to lost-circulation prevention and mitigation.
Abstract This article attempts to summarize measures that are required to be adopted in order to have safe drilling practices mainly in unstable shale formation and have a smooth landing section in target reservoir below the shale. For the last three years in North Kuwait, the number of geo-steering jobs increased significantly and some encountered problems while drilling Wara shale that may end up in losing tools and eventually the hole itself. This paper focuses on information regarding problems encountered in geo-steering jobs and the best practices followed to achieve goals. Stratigraphically, Wara shale lies underlying Tuba and overlying Mauddud carbonates. It has thickness variation in between 80 to 110 feet TVD. Wara shale in North Kuwait is generally unstable and very sensitive to mud properties and creates instability of the wellbore. A wide range of drilling data from well placement jobs were documented and analyzed in order to get an idea of the behavior of shale section, how to drill safely across this layer and mitigate possible risks that may harm the well as well as the tool. Detailed scrutiny was done to compare the drilling parameters, log responses, inclination and trajectory direction between stable and unstable wells. The results were integrated with regional tectonic regime in the area. The methodology and learning curve continuously improved with assimilation of new drilling data and information. Integrated study on all aspects in drilling that involving geo-steering engineers, drilling team, geologists and field engineers on-site has enhanced the ability to understand the situation in the hole and ultimately able to provide faster decision. It was observed that there is strong relationship of building inclination, direction of the well and mud weight in some wells when crossing Wara shale. Wells that have East - West or Northwest - Southeast direction tend to be unstable when compared to wells in North - South or Northeast - Southwest direction. Low incident angle (high inclination) will mostly cause more problems in drilling than high incident angle due to longer section of exposure as well as overburden. Compressional tectonic regime of the area dictate the recommendation of well trajectory should go towards maximum horizontal stress. Recommended to cross it with low inclination and consequently approaching the target reservoir with high dogleg severity tool was to maintain the stability of the wellbore. These findings will help service provider and client to make proper planning, adopt best practice, take proper decision and thereby successfully execute well operations. Indeed saving rig and delivery time that fulfill business objectives as planned.