مجله ژئومکانیک نفت
سال چهارم شماره 1 (بهار 1400)

Wellbore stability modeling for drilling operations is primarily to create a safe mud weight (mud pressure) window. The designed mud density will be high enough to ensure borehole stability and low enough to not fracture the formation (i.e., mud losses do not occur) Therefore, the safe mud weight should be greater than the pore pressure gradient and shear failure gradient and less than the fracture gradient. In this article in order to investigate the effect of fracture in wellbore failure mechanism and determination of safe mud window, analytical and numerical approach was conducted. In order to shear failure analysis in wellbore in fractured formation, Mohr–Coulomb, Jaeger failure criterion was used. For numerical simulation the distinct element method was used. The simulation of a wellbore in Persian Gulf for fracture formation was implemented in order to understanding the effect of discontinuity in wellbore failure mechanism. The results showed, in a specific angle (β), that represents the direction of fracture orientation relative to the horizontal stress, the fractures do not have any effect on strength of the rock mass. Furthermore, in fractured formation, shear failure do not occur in the direction of minimum horizontal stress, but the orientation of discontinuity specifies the failure and slips direction in wellbore. In this article the maximum mud pressure to prevent failure in wellbore occurs in (β=45+ϕ_w/2=52.25). Also in the range of (17.5≤θ≤46.5) the fractures do not have any effect on strength of the rock mass. The practical mud pressure which was used in drilling operation in real condition in Siri oil field was (40.55 Mpa) and the mud pressure determine in analytical solution is (36.92 Mpa). The numerical result (distinct element method) shows, the overall condition of normalized yield zone, shear failure slips along the fractures and tensile failure in two different pressure (36.92 - 40.55 Mpa) are almost similar. Parametric study results showed that in higher horizontal stress ratio (σ_H/σ_h ), wellbore is more instable and in an isotopic stress regimes the shear and slips along the discontinuity was eliminated.

The general use of the stabilizer is in build, hold and drop in directional drilling. In this paper, it was tried to use it for hole cleaning study. Some experimental tests were performed by a closed flow loop to investigate the effect of the presence of drilling stabilizer in different positions of BHA and different hole inclinations. To achieve this goal, a stabilizer with four straight blades was designed and made in the experimental scale. Some experiments were performed with installing the stabilizer in two different positions of BHA (Near Bit and Far from Bit) in different hole inclinations. Finally, it was tried to construct a predicting model in FLUENT software to study the behavior of cuttings along the annulus in the presence of a stabilizer in near bit condition. According to the results, the presence of a drilling stabilizer has a significant effect on improving the cutting transport and subsequently hole cleaning process in critical inclinations.

Ignoring thermal effects in many applications of geomechanics including hydraulic fracturing of deep oil and gas reservoirs and geothermal energy extraction may result in significant errors in output. Production and stimulation of unconventional oil and gas reservoirs is highly dependent on performance of hydraulic fracturing (HF) jobs. These jobs create a network of fractures which are responsible for elevated hydraulic conductivity of the reservoir formation around the wellbore. The fractures increase inflow of hydrocarbons into the wellbore especially in low permeability reservoirs. A sound understanding of HF’s behavior and its relation to the increased production rate, decreases high costs of HF jobs. Thermal effects on HF propagation and mechanism are studied in this paper using the displacement discontinuity method. Firstly, the thermal effects on a thermoelastic model with a thermal source was studied. Models showed that boundaries of the geometrical model should be place farther from what was expected in elastic analyses to avoid boundary effects. The thermal effects were observed far away from the thermal source in the models.Then, thermal effects on a hydraulic fracture was modeled. It was shown that using a cold fluid for HF can decrease the required HF pressure for propagation. The HF width was also increased compared to an elastic model. This is an important parameter in determining hydraulic conductivity of the formation. The lower required pressure for HF propagation reduces the cost of equipment needed for the job, since they are not required to work at very high pressures.

Determining the directions and the values of in-situ stresses is of great importance as the most fundamental parameter needed to evaluate the borehole wall's stability. One of the methods for determining the directions and values of in-situ stresses is the borehole breakout phenomenon, which is developed by an increase in shear stress due to the concentration of compressive stress around the borehole and leads to the failure and collapse of the drilled borehole wall. In the present article, the width and depth of the failure were obtained by combining the stress relations around the borehole based on the theory of elasticity and through the Mogi-Coulomb failure criterion. Then, the minimum fluid pressure inside the borehole was evaluated to prevent damage to the borehole wall, and the effect of physical properties of materials, in situ stress ratio, and fluid pressure on the breakout dimensions was investigated. In the second section of this article, 215 breakout analyzes for different mechanical properties and in situ stresses were carried out for the selected sandstone. The objective of the present study is to evaluate the relationship between the width and depth of failure. The correlation coefficient between the width and depth of failure was equal to R=0.74 using simple linear regression. Moreover, the correlation coefficient's value was calculated to be R=0.82 based on the gene expression programming (GEP) method. It means that there is a moderate to strong correlation between these two parameters, and if the values of in situ stresses σ_h, σ_H to be obtained based on the breakout geometry, only the value of one of these stresses can be obtained due to the high correlation between width and depth of failure.