Optimization of Spatial Placement of Horizontal Wells in Stacked Tight Limestone and Shale Reservoirs

Abstract

Block M, with a high water-saturation thick limestone reservoir stacked over a lower thin shale reservoir, had been developed via multi-stage volumetric fracturing with large spacing horizontal wells at the early stage. When infilling well pattern to further increase reserve recovery degree and production rate, issues such as interlayer interference and planar interference, and spatial matching between well pattern and artificial fractures should be taken into consideration. Well deployment of this kind of reservoir has been designed via a comprehensive investigation to optimize horizontal section length, well spacing, spatial configuration and artificial fracture parameters. The investigation combined evaluation of early stage production performance, micro-seismic data analysis, fracturing design simulation and reservoir numerical simulation. The results show the optimal well deployment strategy is to place 5 horizontal wells in parallel in one unit area of 2.56 km2, to develop the upper and lower reservoirs together. Three of those wells are placed in the upper limestone reservoir with 550 m (1,800 ft) well spacing; meanwhile the other two wells are placed in the lower shale reservoir with 800 m (2,600 ft) well spacing. Wells in the upper and lower reservoirs are placed alternately. The optimized horizontal section length is recommended to be 1,500 m (close 5,000 ft), and the proposed number of fracturing stages is 13. Field implementation results show that the above optimized well placement design can enhance recovery by 2.5 times and effectively inhibit early water breakthrough. This study solved the synergetic development problem encountered in such kind of stacked reservoirs with large differences in reservoir lithology, physical properties, rock brittleness and water saturation.