The effect of stress and pore pressure on formation permeability of ultra-low-permeability reservoir

Low-permeability reservoirs have attracted increased attention from most oil companies due to increased demand and limited reserves in conventional reservoirs. Most scholars agree that worldwide hydrocarbon production decline from conventional reservoirs will be compensated for by the development of low-permeability reservoirs to satisfy growing demand. During the production life cycle of a low- permeability reservoir, permeability at any given location may change in response to the change of stress due to depressurization. Waterflooding is an important method used to develop low-permeability oil reservoirs. Therefore, it is necessary to study whether and how a stress state change can affect formation permeability. However, it is seldom seen the relevant research and report. This article presents a laboratory procedure to simulate the stress state change of ultra-low-permeability formation during injection and production applying cores from Changqing oilfield and evaluate the effect of stress on formation permeability under reservoir conditions using brine as the fluid medium. This work focuses on how formation permeability changes with stress change during injection and drawdown and discusses the difference in permeability change between injection and production. The analysis is not only on the basis of selecting zero pore pressure but also in situ pressure as the starting point; that is, a reference point. The results indicate that reservoir permeability changes during both drawdown and injection are caused by stress variation. Microcracks are the main factor resulting in this permeability change. This experimental method simulated formation stress state and flow behavior as truly. The results are meaningful for analyzing oil well production and reservoir development for ultra-low-permeability reservoir. Copyright © Taylor & Francis Group, LLC