Abstract
In most oilfields, many wells produce in pseudo-steady-state period for a long time. Because of large reservoir pressure drop in this period, fractured reservoirs always show strong stress sensitivity and fracture closure is likely to occur near wellbores. The primary goal of this study is to evaluate productivity of vertical wells incorporating fracture closure and reservoir pressure drop. Firstly, a new composite model was developed to deal with stress sensitivity and fracture closure existed in fractured reservoirs. Secondly, considering reservoir saturation condition, new pseudo-steady productivity equations for vertical wells were derived by using the proposed composite system. Thirdly, related inflow performance characteristics and influence of some factors on them were also discussed in detail. Results show that fracture closure has a great effect on vertical well inflow performance and fracture closure radius is negatively correlated with well productivity. In this composite model, the effects of stress sensitivity of the inner and outer zone on well productivity are rather different. The inner zone’s stress sensitivity affects well productivity significantly, but the outer zone’s stress sensitivity just has a weak effect on the productivity. Strong stress sensitivity in the inner zone leads to low well productivity, and both inflow performance and productivity index curves bend closer to the bottom-hole pressure axis with stress sensitivity intensifying. Meanwhile, both maximum productivity and optimal bottom-hole pressure can be achieved from inflow performance curves. In addition, reservoir pressure is positively correlated with vertical well productivity. These new productivity equations and inflow performance curves can directly provide quantitative reference for optimizing production system in fractured reservoirs.
Highlights
In fossil reservoirs, the investigation of well inflow performance is an important method to evaluate and predict well productivity, which belongs to the category of well test and is widely used in oilfields for its simplicity and practicality [1, 2].rough a series of numerical simulation, Vogel first presented a dimensionless inflow performance curve suitable for dissolved gas drive reservoirs and laid a solid foundation for the study of inflow performance [3]
Mathematical Problems in Engineering performance analysis and provide quantitative basis for the optimization of production system, especially for different well types. All these equations have some implicit assumptions, such as that reservoir is homogeneous and reservoir pressure remains constant, which means these equations do not take stress sensitivity and reservoir pressure drop into account. us, it is less likely to incorporate fracture closure caused by stress sensitivity into these equations. erefore, the above equations have some limitations and deficiencies, which may lead to large error in field applications, especially in fractured reservoirs having severe stress sensitivity and fracture closure in late-time production [8]
All the above inflow equations have some limitations to some extent. They may not consider stress sensitivity and fracture closure in fractured reservoirs, or not consider oil-gas two-phase flow, or assume steady-state flow happened in reservoirs, or assume reservoir pressure remains constant during production
Summary
Considering reservoir saturation condition, new pseudo-steady productivity equations for vertical wells were derived by using the proposed composite system. Results show that fracture closure has a great effect on vertical well inflow performance and fracture closure radius is negatively correlated with well productivity. In this composite model, the effects of stress sensitivity of the inner and outer zone on well productivity are rather different. Strong stress sensitivity in the inner zone leads to low well productivity, and both inflow performance and productivity index curves bend closer to the bottom-hole pressure axis with stress sensitivity intensifying. Reservoir pressure is positively correlated with vertical well productivity. ese new productivity equations and inflow performance curves can directly provide quantitative reference for optimizing production system in fractured reservoirs
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