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COMPARATIVE SIMULATION OF WATER-BASED MUD-FILTRATE INVASION IN HIGH-PERMEABILITY AND TIGHT SANDSTONE RESERVOIRS USING LARGE-SIZED FORMATION MODULES

Mud filtrate invasion is a complex and time-dependent process. During the process, a zone of ﬁnite size around the wellbore (invasion zone) in which a portion of the initial pore ﬂuids have been displaced by the mud ﬁltrate is gradually generated. As a result, the petrophysical and fluid properties of the formation in this zone will be inevitably altered, and sometimes tend to be quite different from their initial values. Petrophysicists and logging analysts have long considered mud filtrate invasion as a nuisance due to its troublesome effect on formation properties and logging measurements, especially on resistivity logging measurements. Note that even deep reading resistivity logging may not see deep enough (beyond the invasion zone), and need to be corrected. Therefore, simulation of mud filtrate invasion under near reservoir conditions is crucial for an in-depth understanding of its physics and effects on logging measurements, and hence for logging interpretation and formation evaluation. Otherwise, this will produce substantial errors in determining initial formation properties, and estimating hydrocarbon reserves and well productivity. To date, most researchers have done a number of works on mud filtrate invasion on the basis of physical simulation at core plug scale. However, conducting invasion experiment on core plug has intrinsic limitations. Firstly, the cylindrical shape of core plug determines that the seepage form of mud filtrate within it (horizontal linear flow) is completely different from that (plane radial flow) in the actual downhole environment, thereby causing a poor representation of the filtration law observed in the experiment. Secondly, due to the small size of core plug, it is almost impossible to monitor the radial resistivity variation for reflecting the dimension and geometry of the invasion zone. To overcome the limitations, a large-sized formation module (sectorial block structure, 55.9 cm in radial depth, and 10 cm in thickness) made by sandstone outcrop was introduced in this paper. Compared with core plug, as a novel type of experimental equivalent, the formation module is larger in size, greater in saturation capacity, and much more similar to the in-situ formation. Its structure can ensure the seepage form of mud filtrate within it is exactly the same as that in the actual downhole environment. Its large size is able to provide enough space and radial distance to follow the entire invasion process from beginning to dynamic equilibrium. The dynamic processes of long-term water-based mud filtrate (WBMF) invasions were duplicated realistically in laboratory. During the whole experimental period, the dynamic invasion data (including radial formation resistivity profile and filtration rate) can be uninterruptedly real-time acquired, thereby investigating and comparing the phenomenon of WBMF invasion in the formation modules with different physical properties. Finally, by combining physical and numerical simulation, the invasion characteristics of WBMF in high-permeability and tight sandstone reservoirs under in-situ formation conditions were quantified. The results obtained in this paper provide an experimental basis and theoretical support for enlightening novel simulation methodologies of mud filtrate invasion, revealing invasion mechanisms, and establishing invasion correction model for electric logging, etc.