Abstract
Similar to conventional medium-high permeability reservoirs' water-flooding process, quite a number of tight low-permeability sandstone reservoirs (average permeability<10mD) in China have reached a high water-cut ratio of over 60%, after decades of water-flooding. Despite of the high WC, these reservoirs' recovery remains less than 14%. Heterogeneity caused by matrix and fracture is desperately in need to be characterized to curb the drastic production decline.
A systematic reservoir description workflow of C61 delta-front formation, Ansai oil field, was run, based on outcrop, densely-spaced well cores and loggings. 3 types of single sandbodies and 9 types of single sandbody stacking patterns were recognized and summarized, from the perspective of matrix heterogeneity. Also, hydraulic fractures near producers and dynamic fractures near injectors deriving from fracture reopening by injection pressure accumulation, are descripted. A fine-scale geologically conceptual model embracing information of single sandbody stacking patterns, calcareous barriers and fractures near wells, was then established to quantitatively evaluate the water-displacement differentiation between tight and conventional reservoir, and the impact of stacking pattern, barrier and fracture on tight oil sandstone reservoir recovery.
Simulation results indicate that, (1) water-flooding front in tight sandstone matrix runs slowly but relatively homogeneously compared with in conventional reservoir matrix, especially showing no conspicuous vertical tonguing or fingering; (2) these 9 stacking patterns differ by petrophysical property, barrier frequency and mutual erosion extent, the first 2 factors of which exert dominant controls on water-flooding; (3) calcareous barriers inside composite sandbodies deteriorate vertical displacement homogeneity, and block water-front near erosion interfaces to certain extent; (4) connectivity of these stacking patterns were then evaluated comparatively based on petrophysical property and barrier frequency; (5) heterogeneity derived from fractures, prevailing matrix, triggers oriental flooding along the in-situ maximum principal stress direction.
Fracture, especially the inevitably dynamic fracture in water-flooding, predominates in tight oil sandstone heterogeneity. How to harness fracture with matrix and design compatible well pattern would be the primary issue to be handled for tight oil sandstone water-flooding.
