New BIGCCS PhD – Dr. Sohrab Gheibi

Mr. Sohrab Gheibi successfully defended his PhD thesis at the Norwegian University of Science and Technology on February 12. The title of his thesis was:

Impact of Faults and Fractures in Geomechanics of Reservoir Pressurization: A Numerical Approach.

His trial lecture was titled: Induced seismicity by fluid injection and depletion: Observations, mechanisms and possible mitigation

The Assessment Committee consisted of: Professor Panos Papanastasiou (University of Cyprus), Ph.D. Bård Bostrøm (Statoil, Norway), and Adjunct Professor Andreas Bauer (NTNU, Norway). Supervisors were Professor Rune M. Holt, Department of Geoscience and Petroleum, NTNU (main supervisor), and Senior Research Scientist Alexandre Lavrov, SINTEF Industry (co-supervisor).

BIGCCS congratulates and wishes Dr. Gheibi good luck in his future endeavors!

From left: Professor Panos Papanastasiou (University of Cyprus), Dr. Sohrab Gheibi, Dr. Bård Bostrøm (Statoil, Norway), Adjunct Professor Andreas Bauer (NTNU)

Summary of thesis:

The importance of geomechanical modeling has risen due to the increase in the concerns about the possible seismic activities and leakage problems due to the reactivation of faults/fracture in the reservoir-caprock system during fluid injection, especially in very large scale CO2 sequestration projects. Global warming and its consequences from one side and the geomechanical risks associated with CO2 storage from the other side have forced the geomechanics research community to study every possible source of these risks and the associated advanced methods of investigation. Throughout this thesis work, it was attempted to investigate the geomechanics of fluid injection problem from several prospective using Modified Discrete Element Method (MDEM).

Stress path evolution and the likelihood of fault/joint reactivation during reservoir pressurization were investigated. Stress path is perturbed in the presence of faults due to the elastic shear deformation of faults. This perturbation can destabilize some parts of the system in the reservoir-caprock interface region. Effect of parameters such as fault shear stiffness, Poisson’s ratio, and reservoir aspect ratio were studied on the quantity of the stress path perturbation.

Stress path hysteresis was also investigated in the thesis. A reduction in the stress path was observed due to damages occurred during reservoir depletion in the reservoir flanks. The stress path hysteresis implies a less stable fluid injection into depleted oil and gas reservoirs.

New developments in MDEM were presented in the thesis. A Linear Elastic Fracture Mechanics (LEFM) based methodology was developed. By this methodology, MDEM can calculate stress intensity factors, KI and KII using the contact forces of particles. It is able to be used in complex boundary condition and geometrical configuration such as curved and interacting multiple cracks with acceptable accuracy. The methodology has been also used in reservoir scale to study the rupture likelihood of faults and fracture. Mode I and II Stress Intensity Path (SIP) coefficients were introduced in the thesis as the change of stress intensity factor per change of reservoir pore pressure. SIPs can be used to investigate the likelihood of faults rupture growth in the reservoir-caprock system. Using SIPs for a given fault, some relationships were proposed to find the critical overpressure to prevent propagation of reactivated fault/fractures in mode I and II.

The elastic-softening Crack Band mesh size independent model was also embedded into MDEM. The code is now able to model nonlinear behavior of quasi-brittle materials including and excluding preexisting cracks based on fracture energy.