Assessment of wellbore stability involves several parameters for which data that may not be readily available. The rock mechanics input data required for analysis are complicated and costly to acquire. Nevertheless, wellbore stability assessment plays an important role in the design of drilling and production of oil and gas wells; therefore, a methodology for bridging the data gap is needed. This paper presents a methodology for wellbore stability assessment with limited data, using common reservoir data. The methodology provides correlations for estimating in-situ stress regimes based on the regional data and rock mechanics parameters based on its lithology. Afterwards, in-situ stress and rock mechanics parameters are used to investigate the effect of stress anisotropy on the mechanical stability of the borehole for various inclination angles. Three failure criteria are reviewed to assess borehole stability, namely Mohr-Coulomb, Drucker-Prager, and modified Lade criteria. These failure criteria are combined with linear and isotropic rock mechanics behavior. Evaluation of stress behavior of a variety of rock lithologies was performed, using failure criteria. Results indicate that the modified Lade criteria tend to be more realistic than the Mohr-Coulomb and Drucker-Prager criteria for these evaluations. Furthermore, this study confirms that, for any type of rock, an increase in borehole inclination angle increases the risk of borehole instability. Sensitivity analysis, based on various reservoir parameters, is performed and confirms the reliability of the correlations reviewed. The methodology presented here can be used as an indicator of borehole instability risk before drilling. Subsequently, the results can be used to optimize drilling design and enhance safety in drilling operations. Introduction Accurate planning before commencement of a drilling operation is necessary to achieve a cost-saving and stable borehole. One of the most crucial aspects of a drilling operation is wellbore stability; wellbore instability can jeopardize the process and achievement of drilling goals. Depending on the source of the problem, wellbore instability is classified as either mechanical or chemical. Chemical wellbore instability, often called shale instability, is most commonly associated with water adsorption in shaly formations, where the water phase is present and can cause borehole collapse. In contrast, mechanical wellbore instability is caused by applying mud of insufficient weight, which will create higher hoop stresses around the hole-wall. Hoop stresses around the hole-wall are often excessively high and result in rock failure. The most rapid remedy for this instability is to increase mud weight and/or adjust the well trajectory for high-angle wells. The mechanical instability occurs as soon as the new formation is drilled, but chemical instability is time dependent because shales are subject to strength alteration once exposed to different drilling fluids. A series of experimental studies led to the conclusion that shale strength decreases with time when the shale is exposed to most drilling fluids.1 Despite the tendency of shale to experience chemical instability, it can also experience mechanical instability simultaneously, which can lead to a more complex problem. In this paper, only mechanical wellbore instability is considered. This study has three objectives. The first objective is to present the extensively used correlations for estimating rock strength and in-situ acting stresses, based on the limited input data and conservative assumptions to proceed with the wellbore stability analysis. The second objective is to demonstrate the use of risk analysis theory in wellbore stability, which incorporates the uncertainties of the rock strength and in-situ stress parameters. The risk analysis theory allows for stochastic modeling in assessing the outcome of wellbore stability analysis. The third objective is to extend the stochastic wellbore stability analysis in evaluating the borehole collapse risk in high-angle or horizontal wells in underbalanced drilling operations.
Controlled pressure drilling (CPD) is an emerging, multi-component drilling application that can improve drilling time and reduce previously recorded lost time on offset conventionally drilled wells-when applied appropriately. In addition, specific drilling techniques offered within CPD technology not only improve the time it takes to reach total depth (TD), but also have been proven to optimize reservoir section productivity and maximize economically recoverable reserves. CPD is composed of three main techniques: air drilling, underbalanced drilling, and managed pressure drilling. This paper provides an updated discussion of CPD techniques, the nonproductive time that it can address and screening methodology.A methodology to properly screen CPD techniques to reduce failure/misapplication and align objectives with expectations had been lacking. This paper addresses the latest refinements in an expert system developed to better understand and screen options for CPD operations. The Internet-based selection tool provides guidance by using key indicator questions, beginning with the primary objective(s) for the given wellbore section. If the specific CPD technique is known, the user can activate the online screening tool. The online screening tool considers a range of economic and technical parameters, which are applied to algorithms and logic rules, to provide a relative ranking for each CPD candidate. This paper will also address modifications that have been made to reduce cost related sensitivity when screening air and managed pressure drilling, which previously made the screening process less consistent. Sensitivity analysis can also be performed to determine the impact of key uncertainty parameters. Additional expert-user screening and selection can be incorporated upon the completion of the online CPD candidate selection process. Lastly, the paper will also include supporting case histories.
In addition to reducing formation damage and improving drilling performance, underbalanced drilling (UBD) can improve field productivity and recovery via real-time reservoir characterization. A UBD characterization tool to interpret the production associated with the drawdown applied during UBD has been developed using variable rate well-testing theory.A new well that is drilled in a depleted field is likely to encounter differentially depleted zones. It is important to know what the pressures are so that an underbalanced condition can be maintained. Additionally, the identification of a permeability distribution is relatively easy when the pressures of the different zones are known independently. In classic well testing, only the average permeability and pressure of the whole formation is usually identified; but with UBD, it is possible to identify a permeability distribution, enabling the detection of high-permeability layers or other similar objects, such as fractures and compartments. Other critical aspects-such as production interval length, optimal well length and ways to eliminate/reduce well testing costs-also can be obtained when evaluating the data.Many of the theories pertaining to this subject seek an integrated, transient model of the formation and the wellbore. In the approach adopted for this program, the problem may be decoupled with independent models for the two parts of the system. This paper will expand the methodology used and will present case studies and results.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.