The Theory And Method Of Well Test Interpretation For Complex Fractured Low Permeable Reservoirs

With further progress in China oilfields'development, more and more low permeable reservoirs are being placed on production. Well test data can provide effective information of depicting the formation dynamic characters, which helps to develop the reservoir reasonably and effectively, however, the current methods can only consider some single influencing factor when interpreting the test data from low permeable reservoir. The complex fractured low permeable reservoirs are the ones which characterizes at their starting pressure gradient and the deformation of the media. Based on the analysis of the fluid flow mechanism, the theoretical well test models for both dual-porosity and dual-permeability media in low permeability reservoirs with complex fractures are derived, which allow for the influence analysis of both non Darcy effect and the deformation of the media. The models are solved implicitly by numerical method and semi-log and log-log curves for the pressure responses are plotted. Results of the sensitivity analysis show that the pressure responses for both Darcy flow and non-Darcy flow conditions are the same for the following factors, including well-bore storage, skin factor, ratio of storativity, inter-flow coefficient and the circular non-permeable boundary. Pressure responses are similar for constant pressure circular boundary for non-Darcy flow and the circular sealing boundary for Darcy flow. Both the starting pressure gradient and permeability modulus can make the pressure curve upward obviously in the mid-late period and the pressure derivative curve won't show the radial flow character of the horizontal line section with value 0.5 after the transition phase while the former has more influence. The characters of the pressure curve are quite complicated because of the compound effects from starting pressure gradient, permeability modulus, storativity , interporosity flow coefficient and the outer boundary, which causes more trouble for the interpretation. In low permeable reservoir of dual-permeability media, the starting pressure gradient of the fracture system has more influence on both the permeability ratio and the pressure response than that of the matrix system while for the later case the character of deformation for both systems are considered. Therefore, the starting pressure gradient and the permeability modulus for matrix system can be ignored when interpreting the real well test data. The developed well test interpretation models which allow for the analysis of influence from both starting pressure gradient and deformation of media are solved numerically. A well test interpretation software for complex fractured low permeable reservoir is developed, in which auto-matching technique using dual-population genetic algorithm is incorporated. The real well test data from Shengli oilfield are analyzed by the software. The results verify the proposed well test interpretation theory and its applicability.