Lost circulation is one of the significant challenges encountered while drilling in a depleted reservoir. Downhole mud loss problems get accentuated while drilling through highly permeable or fractured reservoirs or drilling with inadequate mud weight. Worldwide, the expenses due to mud loss can be significant in the drilling of a well. The presence of fractures can act as conduits for mud losses into the formation in a depleted reservoir. Hence, a comprehensive petrophysical and geomechanical evaluation was needed to identify and characterize these fracture networks.
Though high-resolution image logs are the industry standard for identifying the presence of fractures, their shallow depth of investigation limits their information near the wellbore, and the extent of fractures in the far field couldn't be determined. Here, seismic, acoustics, and petrophysical data can shed information on the fractures at different levels. Understanding the stability of the detected fractures with the current-day stresses is vital in ascertaining its potential to support the flow of reservoir fluids. Thus, a collaborative workflow linking the high-resolution logs and deeper depth of investigation logs was devised for exhaustive characterization of the fractured reservoir.
Drilling through the depleted formations of multiple reservoirs was challenging because of the lost circulation problems, with mud losses going as high as 140 bbls/hr. Detailed analyses of different acquired data were conducted to understand and evaluate this naturally fractured reservoir. Image interpretation showed the presence of fractures, vugs, and dissolution features in different densities across various encountered formations. Acquired acoustic monopole, dipole, and Stoneley data were studied in diverse domains to understand other properties: Stoneley reflection and transmission analysis provided information on the openness of the fractures in the near wellbore. Since an extensive fracture network creates intrinsic anisotropy in a formation, anisotropy analysis and sonic waveform dispersion analysis were carried out to identify and characterize the acoustically anisotropic zones. A forward modeling approach incorporating image interpretation and acoustic data were used to model and interpret acoustic anisotropy associated with geological features such as beddings and fractures. It provided a consistent solution, differentiating open fractures from closed ones. Acoustic reflection survey analysis delivered insight into laterally extensive fractures penetrating as deep as 20 m. Detailed geomechanical analysis hinted at current-day pore pressure and stresses acting on different formations and was used further for fracture stability analysis.
This paper aims to describe how an integrated evaluation using geological, petrophysical, acoustic, and geomechanical analysis help delivers invaluable information on the laterally extensive, critically stressed fractures acting as primary culprits for severe mud losses, thus helping in optimizing the drilling of future wells to avoid mud losses in depleted fields.
