An independent fracturing water-flooding development method for shallow low-permeability thin oil layers in multi-layer sandstone reservoirs

Abstract

Lots of multi-layer heterogeneous sandstone reservoirs have reached extreme high water content and high recovery after long-time water-flooding development, and their low-permeability thin oil layers (LPTOLs) have gradually been the main potential reserves. Therefore, taking the geology of LPTOLs into account, an independent water-flooding development (IWD) method with stratified hydraulic fracturing (SHF) for LPTOLs is put forward. The method combines undeveloped LPTOLs as an individual strata series in one injection-production system, and the HPTOLs even with low water content are abandoned in order to reduce the reservoir vertical heterogeneity. To realize economic benefits, mature stratified hydraulic fracturing is necessary to increase production. The key of IWD is selecting and combining appropriate LPTOLs in an individual strata series.Two development modes, OW-OL and WW-OL, are designed by different wells' perforations and the oil layers' distribution. The difference between these two modes mainly depends on whether the oil-wells perforate the HPTOLs connected with the LPTOLs. Upon an embedded fracture model for simulating hydraulic fractures, the oil production and water content of one LPTOL are calculated with different physical factors of reservoir and fractures. According to the cumulative oil production under different factors, their impact degrees are evaluated by gray relative analysis method. Permeability, oil layer thickness and oil viscosity are three main factors influencing the development of LPTOLs, and flow coefficient is a comprehensive index composed of these three factors. Therefore, the flow coefficient variation degree is a core indicator to evaluate the vertical heterogeneity of an individual strata series, and IWD will get better water-flooding effect with lower flow coefficient variation degree. In order to analyze the effect of IWD, 30 cases are designed with different flow coefficient variation degree, different thickness of HPTOL and different IWD modes. Oil production and water content variation of two development modes are studied by simulation. OW-OL mode is superior to the WW-OL mode on water-flooding recovery. That is because the oil is mainly produced from layered-LPTOLs with less water content, the vertical water-flooding efficiency is higher at the production side, and the whole displacement is more balanced in the whole OW-OL strata series. IWD of LPTOLs is not only suitable for Chinese sandstone oilfields, but also has a general significance for other multi-layer sandstone reservoirs owing high-potential LPTOLs.