Geomechanical Effects of Reservoir Pressure Drop and Drilling Fluid Temperature on Wellbore Stability Conditions

The instability of the wellbore has always been one of the major problems during drilling. In order to maintain the wellbore stability during drilling, the weight of drilling fluid is normally estimated to be decreased in the initial condition of reservoir. However, after reservoir production, the pore pressure of these layers decreases in case of having no pressure support. This change affects the magnitude and direction of in-situ stresses. In this situation, in order to maintain the wellbore stability for drilling new wells, it is necessary to consider the effect of reservoir pressure drop to determine the stability conditions and optimum well trajectory. Other factors affecting the stability of the wellbore are osmotic effect and drilling fluid temperature. In this research, one of the reservoirs in the Southwest of Iran was investigated. This reservoir underwent a pressure drop of 11 MPa during the production. The impact of reservoir pressure drop, osmotic effect and drilling fluid temperature are investigated on the wellbore stability condition and the optimum drilling trajectory. In the initial conditions of this reservoir, the minimum collapse pressure is in the range of 28.4–34.2 MPa, and the most stable drilling trajectory is a well with inclination of 45˚ in direction of minimum horizontal stress. After a pressure drop of 11 MPa that causes changes in induced stresses around the wellbore, the minimum collapse pressure for the stability decreases to the range of 21.7–26.4 MPa, and the most stable drilling trajectory, with 10˚ reduction in inclination, is a well with inclination of 35˚. In addition, including the effect of drilling fluid temperature, the minimum collapse pressure in the well is negligibly impacted to be in the range of 21.4 -26 MPa. Also, chemical effect of drilling fluid on the formation clay content increased minimum collapse pressure to the range of 22-26.7 MPa. Finally, considering both osmotic effect and fluid temperature estimates minimum collapse pressure in the range of 21.8-26.5 MPa. Therefore, the reservoir pressure drop has a significant effect on the wellbore stability condition, and other factors have negligible impacts on this reservoir.