Wellbore Stability Analysis in UBD Wells of Iranian Fields (2007)

Wellbore Stability analysis in UBD Wells of Iranian fields saeed salehi; saeed salehi University of Calgary Search for other works by this author on: This Site Google Scholar Geir Hareland; Geir Hareland U. of Calgary Search for other works by this author on: This Site Google Scholar Keivan Khademi Dehkordi Keivan Khademi Dehkordi NISOC Search for other works by this author on: This Site Google Scholar Paper presented at the SPE Middle East Oil and Gas Show and Conference, Manama, Bahrain, March 2007. Paper Number: SPE-105155-MS https://doi.org/10.2118/105155-MS Published: March 11 2007 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Get Permissions Search Site Citation salehi, saeed, Hareland, Geir, and Keivan Khademi Dehkordi. "Wellbore Stability analysis in UBD Wells of Iranian fields." Paper presented at the SPE Middle East Oil and Gas Show and Conference, Manama, Bahrain, March 2007. doi: https://doi.org/10.2118/105155-MS Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex Search nav search search input Search input auto suggest search filter All ContentAll ProceedingsSociety of Petroleum Engineers (SPE)SPE Middle East Oil and Gas Show and Conference Search Advanced Search AbstractDrilling underbalanced is often used to prevent formation damage, avoid lost circulation, and increase rate of penetration. However, it is also risky and may lead to wellbore collapse due to lack of positive support provided by the hydrostratic wellbore fluid column. Hence, the application of underbalanced drilling (UBD) should be evaluated thoroughly through the use of in-situ stresses and rock mechanical properties to estimate under what hydraulic drilling conditions the wellbore is stable.This paper presents a mechanical wellbore stability analysis for two depleted Iranian fields, named herein as field A and B. Depleted Iranian fractured carbonate fields are suffering from severe wellbore stability problems and lost circulation during overbalanced drilling conditions. The application of UBD in these fields with a pressure less than formation pore pressure brought on new wellbore stability problems like risk of shear failure and collapse of borehole wall. Using good geomechanical model description matching field characteristics in conjunction with rock failure criteria in some cases may lead to a good prediction for avoiding wellbore stability problems and choosing the optimum mud weight window. By analyzing cores, log and triaxial rock mechanical data, an elastoplastic model combined with a finite-explicit code was used in the wellbore stability analysis to estimate an optimum Equivalent Circulating Density (ECD) for these fields. Compared to some actual field data it was observed that using an elastoplastic constitutive model would be sufficient to analyze mechanical wellbore stability in these fields.IntroductionUnderground formations are subjected to a vertical compressive stress caused by the weight of the overlying strata and horizontal stresses due to the confining lateral restraints. Under the action of these in situ stresses, prior to drilling a borehole, the rock mass is in a state of equilibrium that will be destroyed by the excavation. When a borehole is drilled, the load carried by the removed rock is then taken by the adjacent rock to re-establish equilibrium. As a result, a stress support pressure introduced into the borehole, failure in the concentration is produced around the well. If there is no hydrostatic support pressure introduced into the borehole, failure in the formation may take place. Therefore, maintaining equilibrium in the field to prevent rock failure requires the use of a support pressure which is usually provided by the drilling fluid. In order to evaluate the potential for wellbore stability a realistic constitutive model must be used to compute the stresses and strains around a borehole (1, 2).Out of the numerous published models, the linear elastic model (LEM) is the most common approach. This is due to its simplicity and less required input parameters compared to other models. However, using these simple models in some cases underpredicts the wellbore stable ECD. The LEM based models do not adequately explain the fact that, in many cases the borehole remains stable even if the stress concentration around the borehole exceeds the strength of the formation.Alternatively, elastoplastic models offer the ability to asses the mechanical integrity of a borehole more realistically. Westergaard (1940) (3) published one of the early works contributing to the knowledge of stress distribution around a borehole, in which an elastoplastic model was developed. Later works using elasto-plastic models have been published (e.g., Gnirk, 1972; Risnes and Bratli, 1981; Mitchell et al., 1987; Anthony and Crook, 2002) (2). Table 1 shows a summery of the current wellbore stability models (4).An elastoplastic model for assessing wellbore stability analysis in two depleted carbonate fields are presented in this paper. Based on stability analysis results compared with behavior of the drilling of two UBD horizontal wells in these depleted fields proved the feasibility and accuracy of using an elastoplastic model to predict the operational ECD windows. Although considering behavior of only two wells is not sufficient to conclude that implementing this model for all the carbonate fields will yield similar results, it is suggested that the result of this study can be expanded for use on other fields with similar characteristics.Fields Background90 % of the discovered fields in Iran are in carbonate reservoirs putting Iran as one of the largest carbonate producers in the world (5). Field A and B are located in southern part of Iran. Geological studies combined with the core samples taken from these fields, represented a highly fractured limestone reservoirs with low formation pore pressure gradients. Because of the confidential nature of the studies undertaken in these fields the names are not revealed.Lost circulation is one of the most severe drilling problems in these fields. So the determination of proper ECD is challenging. Keywords: wellbore integrity, strength, borehole, ECD, Upstream Oil & Gas, cohesion, overburden stress gradient, Wellbore Design, elastoplastic model, critical value Subjects: Wellbore Design, Reservoir Characterization, Wellbore integrity This content is only available via PDF. 2007. Society of Petroleum Engineers You can access this article if you purchase or spend a download.