Experimental investigation on water flooding and continued EOR techniques in buried-hill metamorphic fractured reservoirs

Abstract

Fractured reservoirs are complicated due to the coexistence of matrix and fracture systems. In particular, the matrix of buried-hill metamorphic fractured reservoirs (BHMFRs) has plenty of micro-fractures and a few dissolution pores rather than common pores in sandstone. The storage and heterogeneity features, such as the storage ratio between matrix and fractures, and the distribution of fractures, significantly affect the performances of waterflooding and EOR techniques. With the objective of finding effective waterflooding mode and EOR techniques in such reservoirs, the BHMFR of JZ25-1S in Bohai Bay in China was taken as an example. First, static imbibition experiments using a novel imbibition cell were conducted. Then, 3D large-scale models of BHMFRs using outcrops were designed based on the similarity criterion and employed to conduct the experiments of waterflooding and followed EOR techniques. The results show that co-current imbibition is principal and gravity is the dominant force of imbibition for BHMFRs. Because imbibition is slow, most of the yield comes from the macro-fractures. Both water-free recovery and waterflooding recovery increase as the decrease of the storage ratio of matrix to fractures. Pulsing cyclic water-injection is better than intermittently cyclic water-injection. In BHMFRs with heterogeneity of fracture density, injection in the area with low fracture density and production in the area with high fracture density can achieve better effects than the reverse scenario for both waterflooding and followed EOR techniques. If gas flooding is selected as the EOR technique, producer should be perforated at the lower layers of the reservoir. Suitable gel particles not only get a good EOR effect by effective plugging, but also can create a more favorable condition for surfactant flooding. In our experiments, another 2–4% of OOIP was produced by PPG flooding. Surfactant can enhance the oil recovery by both removing the adsorbed oil on the fracture surface and promoting imbibition in matrix. 3–5% of OOIP was recovered by surfactant flooding and soaking in our experiments.