Linearly Supported Radial Flow -- A New Flow Regime Identified in Dual-Porosity Reservoirs

Abstract

Abstract Modern welltest analysis and interpretation techniques are based on match between actual reservoir pressure response and analytical models. In most well and reservoir configurations the analytical techniques are sufficient. However, recent technology advances in drilling and completion, and reservoir heterogeneity detected by modern pressure gauge have identified complex well and reservoir configurations where existing analytical well test models are not adequate for retrieving reservoir properties. This paper presents case studies where a new flow regime was indicated in two hydrocarbon reservoirs when the two wells were tested. Log-log plot of pressure derivative versus time is called diagnostic plot in welltest analysis. Special slope values of the derivative curve are usually used for identification of reservoir and boundary models. These slopes include 0-slope, ¼-slope, ½-slope, and unity slope, etc. However, in many cases the derivative curves do not exhibit slopes of these special values, and it can be assured that the non-special slopes reflect certain flow patterns in the reservoirs. One type of these reservoirs has been observed to be thick reservoirs with either bottom or down-dip water drive. The slopes of the pressure derivative curve for this type of reservoirs are normally below ½ and not equal to ¼. A slope of as low as 0.2 has been observed in a gas reservoir. This paper demonstrates that these low slopes can be attributed to a new flow regime namely Linearly Supported Radial Flow (LSRF). The LSRF may exist in the drainage area of a vertical well where a radial (normally horizontal) flow prevails in a high permeability layer and a linear (normally vertical) flow into the high permeability layer dominates in a low permeability layer. The LSRF may also exist in the drainage area of a horizontal well after pseudo radial flow in the high permeability layer is reached. The linear flow in the low permeability layer may be supported by an aquifer. Analyses using a numerical welltest simulator support this LSRF theory.