Oil-based critical mud weight window analyses in HTHP fractured tight formation

Abstract

Abstract There are plenty of tight oil and gas reserves in western China. The depths of the target reservoirs are usually as deep as 6000–7000 m, which are featured by: HTHP drilling environments, low matrix permeability, well-developed natural fracture systems, and anisotropic in-situ stresses. Even using highly inhibitive oil-based fluids during drilling operation, mud losses and wellbore collapses still occurred frequently. Moreover, in many wells when the mud weight is added, borehole conditions become worse. Contrary to the traditional field experiences, collapse pressure itself has a window. The purpose of this work is to investigate the features of fractured tight formation and correspondingly build a model to explain the unusual drilling complexities. In this approach, the formation is divided into a tight member and a fractured member. 1. In the tight member, a model for pore pressure and stress distribution prediction around a borehole is used based on the poroelasticity theory. The calculation results show that in the near borehole region, instantaneously after the borehole is opened, an abnormal high pore pressure is induced due to the poro-mechanical coupling effect under anisotropic in-situ stress field. The induced pressure is harmful to borehole stability, and a mud weight lower bound is necessary to overcome it. 2. In fractured member, a 3D weak plane model is established considering natural fracture׳s (weak plane) orientation and mud invasion. The calculation results show that muds with hight weight will infiltrate into the aperture of fractures and decrease the normal stress on it, which may induce shear slip failure along the weak plane. Thus an upper bound of mud weight is derived. 3. The time dependent temperature drop at the bottom of a wellbore is also simulated and its impact on borehole stability of tight and fractured section is studied respectively, which helps constrain the mud weight auxilarily. By integrating the above-mentioned factors, a comprehensive critical mud weight window is obtained, which can serve as the theoretical basis for wellbore stability analyses in the fractured tight formation drilled with oil-based fluid.