| dc.description.abstract | Gas and oil production performance in shale oil wells is fundamentally different from the conventional reservoirs. The difference is more pronounced in the producing gas-oil-ratio (GOR) trends. In the conventional case, when flow occurs in a permeable reservoir and in radial geometry, the GOR is a function of average reservoir pressure: when the pore pressure is high enough, the produced GOR stays constant and equal to the solution gas-oil ratio of the reservoir oil; when the pressure in the system is reduced below the bubble point pressure of the reservoir oil, gas phase appears in the reservoir and causes increase in gas saturation. Increase in the gas saturation beyond critical gas saturation leads to flow of gas and, hence, an increase in the producing GOR. A well with fracture, on the other hand, experiences a different transient flow behavior. In horizontal shale wells with multi-stage hydraulic fractures, in particular due to ultra-low permeability of the rock matrix, the dynamic GOR behavior will have an extended time. Further some of the producing shale oil wells have GOR remaining flat, regardless of rate of the depletion. Although the literature has already attributed the peculiar GOR trends of the unconventional oil wells to several factors, it is still an unclear area with further debate. In this thesis I considered single-well production – in the absence of neighboring well interference-and analyze the effects of stresses on the flow and production. Using reservoir flow simulation modeling approach, I show that the permeability reduction caused by hydraulic fracture closure and pore volume change due to stresses reduce the gas saturation inside the fracture and in the matrix near the fractures, hence, producing GOR stays relatively low. The higher the fracture closure and the pore compressibility, the lower the gas saturation, and lower the gas released to the wellbore. Flow, on the other hand, becomes stress-sensitive because of the stress-dependency of the permeability field. As the reservoir is depleted, the reservoir experiences larger effective stress, which, in turn, reduces the permeability of the matrix and the permeability of the fractures. Because the in-situ stresses could be anisotropic, I also investigated the impact of stress anisotropy on the producing GOR.
The more dependent the permeability is on the stress of the system, the flatter the GOR curve is. When hydraulic fractures are considered to be initially at infinite-conductivity flow, then pore compressibility in the matrix is the reason for GOR flatness, but if the flow is controlled by the finite-conductivity of the fractures, then the reduction in the fracture’s permeability caused by effective stress is the reason for GOR flatness. When the matrix permeability is in the micro-Darcy range, the stress effect on the permeability leads to a lower GOR. The stress anisotropic study showed that the difference in the overburden stress and the minimum horizontal stress did not have a large impact on the GOR. The stress anisotropy study did find that the stress dependency in the hydraulic fractures contributes more to the GOR trend than the stress dependency in the matrix. The stress effects on the shale oil reservoir systems can have large implications on the drawdown strategies for the operators. | |
