The Study On Reasonable Capacity Of Fractured Low Permeability Reservoir

With the rapid development of society, the demand of energy,especially the petroleum, grows faster and faster. But recoverable reserves of medium and high permeability declines gradually, and the yields decline rate continues to grow as well. So the low permeability reservoir has become the focus of the current exploration and development. The rocks of low permeability reservoir are tight, and fragile. During the diagenesis and tectonic movement, the tectonic force had made some kinds of fault and fracture (collectively referred to as the fractures), making the reservoir become fractured low permeability oil reservoir. The storage medium of the reservoir has a variety of types including:fractures, pores, solution caves etc. This kinds of storage medium have very big difference and can communicate with each other, which makes its heterogeneity very strong. At the same time, due to the starting pressure gradient and pressure-sensitive effect, which exists in low permeability reservoir generally, enlarge production pressure difference too much in order to magnify productivity, will leads to formation permeability decreased, and production capacity decline on the contrary. It is very difficult to exploit the fractured low permeability oil reservoir. So It is very meaningful to study the percolation theory of this kind reservoir, and to determine a reasonable yields. It can ensure such oilfield developed stably and efficiently, and improve the development effect as well.This paper analyzed the characteristics of percolation in the fractured low permeability reservoirs, and investigated research status of the starting pressure gradient, pressure sensitive effect and the anisotropy of reservoir at home and abroad. In consideration of crackes, this paper chose the equivalent continuous medium model to characterize the fractured low permeability reservoirs. Due to the low permeability reservoir needs to be fractured, considering starting pressure gradient and pressure sensitive effect, the productivity equation of a vertical fractured well was derived and solved, in case of steady and unsteady, and then analyzed the rationality of its capacity.There is a certain difference between flowing bottom hole pressure and formation pressure, when the fluid begin to flow, due to the existence of starting pressure gradient. The larger the start-up pressure gradient, the greater the difference, and the shift of IPR curve downwards more. In addition to pressure-sensitive effect, an inflection point will appear on the IPR Curve, if the coefficient of pressure-sensitive is large enough. Formation pressure distribution is similar to the distribution of permeability, both of which damaged seriously near wellbore, and aggravated as the coefficient of pressure-sensitive grow.On occasion of unsteady, reservoir can be divided into agitated area and un-agitated area. In the agitated area, formation pressure and permeability will change with time, however in the un-agitated area, formation pressure and permeability are equal to the original value. Along with the formation fluid being continuous mined, the boundary of agitated area extends outwards gradually. Something can be found as followed:if the producing pressure drop is equal to a constant, with agitated area boundary gradually expanding outward, oil production capacity will continue to decline. This is most obvious in the early time, then it tends to zero gradually. It Indicates that the control area of wells in fractured low permeability reservoirs is limited, It necessary to make the well spacing reasonable; if the yields is equal to a constant, the flowing bottom hole pressure is needed to reduce continuous. When the agitated boundary extended to the reservoir boundary, curves of formation pressure distribution and permeability distribution are an upward convex curve, of which damage will be exacerbated. The reservoir elastic recovery has a lot to do with the formation pressure sensitivity. The reservoir elastic recovery increases with the increase of formation yields, when the Pressure sensitive coefficient is relatively small. But with the increase of the pressure sensitive coefficient, the relationship of elastic recovery and formation yields is no longer a linear relationship. With yields growth, the reservoir elastic recovery will rise slightly, but the magnitude of the growth become smaller and smaller, the curve upwards slightly. When the pressure sensitive coefficient increased to a cert value, the curve shows an inflection point. The formation of elastic recovery rate reaches the maximum value.Synthesize steady seepage and unsteady seepage flow, the pressure sensitive coefficient effect is the biggest influence factor of the fractured low permeability reservoir development. When it reaches a certain value, there is an inflection point on the IPR Curve, whether the steady seepage or unsteady seepage flow. Now here is a reasonable bottom-hole flowing pressure. With the agitated boundary extending outward, the change of reasonable bottom-hole flowing pressure value is small, when unsteady seepage flow.Using the above theory, was analyzed the reasonable formation yields of Chang6formation Bai115wells area. The results showed as follow:before the agitated boundary extended to the reservoir boundary, the well productivity should be controlled less than the maximum capacity, the bottom hole flowing pressure should be higher than pressure corresponding to the maximum capacity. While the agitated boundary extended to the reservoir, if there exists a supply boundary, raise yields would be raise because of the energy supplement. Now, the well should work at its maximum capacity.