Abstract
Effective stress state has great influence on permeability of tight oil reservoirs, which is one of the most important properties to evaluate a reservoir. We present a fully coupled poro-elastic finite element production prediction model with consideration of stress induced permeability evolution of tight oil sandstones. Based on deformation and failure modes of tight oil sandstones, stress induced permeability evolution model was considered as three types under different stress states and described independently. Using finite element method based on the poro-elastic theory and stress induced permeability evolution model included, we can get the production prediction. The accuracy of the model is verified using some wells’ production data.
The objectives of this study are to find an appropriate numerical model to simulate the production of tight oil reservoirs. It is essential for the successful planning and management of oil and gas reservoirs.
With the depletion of conventional oil resources, unconventional resources become more and more important. Today, the world's energy structure is conventional oil, heavy oil, low permeability oil and natural gas, shale gas and so on (Sun Fengrui et al., 2017a, b). Since the permeability of unconventional reservoirs such as tight oil reservoir is usually low, reliable permeability data input into the reservoir simulator of production is essential for the successful planning and management of oil and gas reservoirs (M. Jin. et al., 2000). Although the permeability is determined by original geometric properties of the formation, these properties are changed due to variations in stress when the pore pressure is drawdown during the production (Samuel Ojagbohunmi et al., 2012). Stress-dependent permeability has become a significant issue in petroleum industry in the last three decades.
There are two common features obtained from previous experimental studies on tight oil reservoir formation. Firstly, there is a large variation of permeability with pressure appears at very low pressure. Permeability declines approximately exponentially with increasing confining pressure (Fatt. et al., 1953) and the reduction of permeability would be relatively more severe when it comes to a lower permeability matrix. (Vairogs, J et al., 1971) Secondly, either depletion in reservoir pore pressure due to production or unreasonable production pressure differential can cause significant deformation, which can lead to porosity compaction and permeability loss (M. Jin. et al., 2000). Since the oil production of tight reservoir depends largely on the permeability of the rock matrix, stress induced permeability evolution should be considered in tight oil reservoirs production simulation to get an accurate prediction for longterm oil production.
