wellbore stability

Wellbore instability and drilling fluid loss in fracture formation is one of the main issues in deep drilling. In order to determine an efficient drilling methodology it is necessary to investigate the effect of fracture on instability and fluid loss mechanism. In this article in order to evaluation of the vertical wellbore stability and fluid loss in fracture formation, three dimensional simulation of of a wellbore in the Persian Gulf was carried out using Discrete Fracture Network (DFN) and Distinct Element Method (DEM). Validation of the model and stability analysis of wellbore was carried out using maximum allowed movement, normalized yield zone radius criteria and caliper log data. The initial analysis of the model showed that the wellbore is in an unstable state for kakhdumi formation. In order to investigate the hydromechanical mechanism in fracture formation, drilling fluid was injected by rate of 25 BPH and viscosity of 1.08 cP to the wellbore. Slip in fractures, shear displacement and the volume of fluid loss was determined as main parameters for wellbore stability analysis. The effect of in-situ stresses ratio (σ_H/σ_h ) on instability mechanism and fluid loss was carried out based on six different scenarios for in-situ stresses ratio. By increasing in-situ stresses ratio and in an anisotropic (σ_H/σ_h =2) satat, slips and shear displacement along the discontinuity increased. In this case, for 25 BPH drilling fluid flow ratio the fluid pressure decrease along the discontinuities. The parametric study for five different fluid flow ratio showed that in (σ_H/σ_h =1.06) the fluid expansion in fracture increased. Moreover, tension failure and shear displacement decreased in low fluid flow ratio. In 5 BPH fluid flow ratio, the fluid pressure in fractures decreased compared with higher fluid flow ratio. This is because of less shear displacement and fluid expansion along fracture in lower fluid flow ratio.

Summary : Wellbore instability can increase drilling time and sometimes leads to wells being side-tracked. Appropriate estimation of the weight of drilling mud can be mentioned as the key designing factor to ensure the mechanical stability of wellbores. Considering the elastoplastic behavior of the medium, yield criterion plays a key role in this analysis and estimation. Applying the Mohr-Coulomb criterion leads to underestimation of rock strength due to ignoring the effect of the intermediate principal stress. On the other hand, the Mogi-Coulomb yield criterion is able to consider the effect of the intermediate principal stress, while preserves the simplicity of the Mohr-Coulomb model. In this study, the elastic - perfectly plastic constitutive model is developed based on the Mogi-Coulomb criterion. Model predictions for some triaxial tests are compared with experimental data, and reasonable agreement is achieved. The proposed model is also applied for analyzing a wellbore stability problem and results are compared with two popular models, Mohr-Coulomb and Drucker-Prager, to investigate the effect of the intermediate principal stress.
The simplicity of the Mohr-Coulomb criterion makes it as one the most popular models in geomechanical analysis. However, ignoring the effect of the intermediate principal stress can be mentioned as the main shortcoming of this model. On the other hand, the effect of the intermediate principal stress has been widely studied in experimental and analytical investigations. The Mogi-Coulomb yield criterion, not only overcomes this shortcoming, but also conserves the simplicity of the Mohr-Coulomb model. Alajmi and Zimmerman (2005) analytically showed the ability of Mogi-Coulomb criterion in considering the effect of the intermediate principal stress. This investigation showed that this criterion can introduce a more realistic estimation of rocks strength.
Methodology and Approaches : An elastic-perfectly plastic constitutive model is developed based on the Mogi-Coulomb yield criterion. The proposed model is implemented to “FLAC”. Verification is done by simulating some triaxial test data. Numerical example is also conducted to investigate the efficiency of the model.
Results and Results of the numerical study shows that the Mohr-Coulomb criterion results in underestimation of rocks strength by ignoring the effect of the intermediate principal stress. On the other hand, the Drucker-Prager criterion leads to overestimation of the strength by considering identical effects for both intermediate and minimum principal stress. While the Mogi-Coulomb criterion introduces a more realistic estimation of the strength by moderating the effect of the intermediate principal stress.

Well bore stability analysis is a procedure with the view to economically improving the drilling operation. Identification of diverse types of rupture in the well bore is crucial in stability analysis. Due to reduction I reservoir pressure and index of per، hydraulic fracturing well be performed in order to increase the permeability and production from well. During the aforementioned procedure، fluid is injected inside the well to induce the fracture in the walls. Hydraulic fracturing has received much recognition as a reliable method for the stimulation of oil reservoir. In this study، conducted on 400 m of limestone in fahlyian reservoir in darkhovein oil field، pressure of shear and compressional failure is determined by virtue of parameters including rock mechanic and information on well logging as well as in situ stress of vertical and maximum and minimum horizontal. Safety mud window was defined between 0. 84 and 4. 2. The minimum allowable mud pressure for preventing the shear failure was between 33. 51 and 52. 89 and the maximum range was determined between 47. 4 and 97. 55. Furthermore، minimal required mud pressure for inducing compressional rupture and hydraulic fracturing was estimated between 108. 87 and 151. 38.