Approximate expressions for the pressure response and flow rates of individual layers are given when a well produces with a constant production rate from a infinite multilayer reservoir with and without interlayer crossflow, kh product and skin factor of each layer can be determined by one drawdown test if the common wellbore pressure and the rate of each layer can be measured at the same time. The behavior of the crossflow, caused by skin factors, is discussed. For the purpose of determining the vertical permeabilities of the shales between layers, a new method is developed, which uses the crosspoint of the two straight lines of drawdown or buildup curves. The new method avoids to measure the wellbore pressures of the producing interval and the closed interval at the same time, as suggested earlier by the new drawdown test.
A large number of reservoirs consist of two or more layers with non-permeable or low permeable shales between them. It is important in practice to get a correct estimation of physical properties of each layer, such as permeability, porosity, thickness and so on. For noncommunicating stratified reservoirs. the usual way to get these parameters is to complete well tests for each layer. Lefkovitz et al(1) provided a basis for pressure test analysis of a well producing from commingled zones. They showed that only average parameters of the multilayer reservoir can be obtained from drawdown or buildup tests. Raghavan et al(2) extended the pressure buildup analysis method for a two-layered no-crossflow system to including the effect of the thickness of each zone for a wide range of permeability ratios. It also gave methods for estimating the permeability of the individual layers. Earlougher et al(3) showed that the buildup curves can be rather different for different closed multilayer systems. Larsen(4.5)gave a method for determining skin factors and flow capacities of individual layers in a two-layered reservoir. All these studies confirm that there is no way to get the formation parameters of a individual layer by analyzing the straight line portion of the drawdown or buildup curve at the infinite-acting period.
The pressure response of a well producing at a constant rate from all the layers in a multilayerreservoir with interlayer crossflow was studied by many author, (6–13)). Russell and Prats(7) pointed out in their study of two layer cases that the flow in a multilayer reservoir with crossflow behaves as though there were no crossflow between layers when time is short and behaves as though the reservoir had only one single layer when time is large. The straight line portion of the drawdown and buildup curves can determine the total kh product of the multilayer reservoir, These conclusions were found also by other authors(8,11). Pendergrass and Berry(8) studies the cases of more than two layers and concluded that the influence of the order of layering on pressure responses negligible in transient well tests when crossflow exists.