Experimental research of the tracer characteristic curves for fracture-cave structures in a carbonate oil and gas reservoir

Due to the complexity of the connection between fractures and caves in a carbonate oil and gas reservoir, it is still difficult to recognize their production characteristics, which, in turn, significantly restricts the exploitation of such reservoirs as the Tahe oilfield, China. Therefore, in this work, five typical fracture-cave structures in the Tahe oilfield were modeled to investigate their tracer characteristic curves and to understand the differences in production. The experimental results indicate that the characteristic curves of the models are highly consistent with the curves obtained from the oilfield. For the signal fracture-cave connection, a “single-peak” behavior was observed on the tracer concentration curve; meanwhile, for the parallel fracture-cave structure, the tracer concentration curve shows a “double-peak” feature. For all models, a smaller internal space of the fracture or cave (especially for the fracture) corresponds to a s breakthrough time of the tracer (KBr), a higher peak value, and a faster mass concentration increase in the first stage after the breakthrough of the tracer. As the space increases, the dimensionless cumulative mass curves approach the line y = x, and the caves are closer to the line y = x than the fracture. The bottom water dilution, tracer injection rate, injection and production model impose significant effects on tracer curves. These results are highly useful in identifying the real fracture-cave structures in the reservoirs and may at least provide a reference for analysis.