The Research Of Calculation Methods For Determination Of Average Reservoir Pressure

Well test is one of key tools to characterize oil and gas reservoirs. Build-up test is still widely employed by field engineers. Many parameters such as average reservoir pressure could be achieved through such impletion. Average reservoir pressure is a kind of important information to describe reservoir performance and characteristics, calculate initial geological reserves and monitor well operation during primary recovery, secondary recovery and pressure- maintenance projects.This paper presented image- inversion results for one well bounded in Hexangular, rectangle, right triangle, and rhombus reservoirs. Reservoir pressure distribution along with deliverability equations would be obtained after the use of the image- inversion method and the principle of superposition. At the same time, MBH function for reservoirs with all kinds of regular geometry boundary is also determined.Further derivation is done based on the Russell’s build- up model of one well in the circular closed reservoir. This analytical model verified Muskat’s experienced conclusions. Application results also showed a perfect precision.Control equations for unsteady-state flow or pressure distribute function is integrated according to the definition of average reservoir pressure. Results from these two different integrations are actually identical for pressure drop of circular closed reservoirs.Average reservoir pressure for circular closed reservoirs in the build- up process is obtained by use of the principle of superposition. With the increase of pressure build-up time, average reservoir pressure increased, but slowed down gradually. It will remain unchanged when the pressure reached to a certain degree. The longer the production time, the smaller the corresponding average reservoir pressure.According to the definition of average reservoir pressure, unstable-state flow control equations are integrated. Then use the Green Theorem to achieve average pressure variation of conventional gas reservoirs. In the early stage of unsteady flow, average pressure kept the same. With time increasing, average pressure decreased. The larger the boundary, the later the time of the boundary flow occuring. When pressure reached the bo undary, entire reservoir pressure falled because of the closed boundary. Deliverability entirely comed from pressure drop of fluid and rock volume expansion due to release of elastic energy. After a period of time, the rate of pressure drop kept the same, which was equal to average pressure drop rate of the discharge zone. It reached the pseudo-steady flow.According to the definition of average reservoir pressure, unstable-state flow control equations are integrated. Then use the Leibniz formula to achieve average pressure variation of low-permeability reservoirs. With time increasing, average reservoir pressure decreased, but slowed down gradually. The greater the threshold pressure gradient, the later the physical boundary reached. After reaching the boundary, average reservoir pressure and time kept a linear ralationship. The greater the threshold pressure gradient, the smaller average reservoir pressure.