Connectivity Identification And Characterization Of Well Connectvity In Fracture-cavity Resevoir Using Tracer Method

The space complexity and formation heterogeneity of fracture-cavity carbonate reservoirs make identification and characterization of the interwell connecting structures difficult,which thus results in uncertainty for waterflooding and the distribution of the residual oil as well as the lack of efficient methods.Given this issue,it is highly important to conduct comprehensive studies addressing the identification and characterization of the interwell connectivity to improve the waterflooding technique in fracture-cavity carbonate reservoirs.In this work,2-D and 3-D visual fracture-cavity physical models derived from some typical well groups in Tahe oilfield were first utilized to perform tracer tests.The curve features and factors of tracer tests under different connecting structures were examined.The results indicated that the connecting structures in series were characterized by unimodal curve,and the structures connected in parallel presented two-mode or multiple-mode curves.In addition,as the size of the connecting spaces increased,the tracer's breakthrough time and output time prolonged,and the peak value of the mass concentration curve gradually decreased until it leveled off(the peak of fractures was high and steep,while that of caverns is low and gentle);meanwhile,the dimensionless cumulative production curve slowly approached the line where y=x and the area between the curve and x-axis diminished(the curve of fractures gets away from y=x and that of caverns gets closer to the line).With the advance velocity of tracer increasing,the breakthrough time reduced and the peak of tracer's curve moved forward,and the peak value increased(the secondary peak of channels that are in parallel levels off).The first half part of the corresponding dimensionless cumulative production curve became steep and the area between the curve and x-axis increased.Moreover,different injection-production relations can cause a differentiation in the efficiency of tracer's diffusion and dilution in the connecting spaces,and the tracer's output curve tended to indicate the fractured-cavity structures of the injection end.Bottom aquifer diluted the tracer and reduced the output concentration.The dimensionless mass concentration curve was close to y=x and the area between the curve and x-axis decreased.The clustering evaluation method of tracer's field cumulative production curve was proposed in this work based on similar precision.It combined the typical characteristics of the experimental curves and classified them into two main types(in series and in parallel)and 11 subcategories followed by their characteristic parameters as well as theoretical curves presentation.On the basis of pipe flow theory,a mathematical model of quantitatively characterizing the change in tracer's output concentration was first proposed.Through particle swarm optimization,the interpretation model of tracer in fractures and caverns reservoirs was computed and programmed a software for interpreting the curves of tracer.The trace method was validated using field production data suggested by the high consistency and reliability.Simulating the waterflooding process under a series of connecting structures between wells,and injection-production relations as well as patterns,the distribution regularity of residual oil after waterflooding was recognized.As for the connecting structures in series,in vertical dimension,the residual oil after waterflooding existed as "attic oil" and accumulated at the top of fractured-vuggy structures.Regarding the structures connecting in parallel,the residual or immobilized oil concentrated in the secondary channel.The experiments of improving waterflooding demonstrated that the oil recovery efficiency could be enhanced by tuning the injection and production velocities,relations and patterns.The results of gas injection suggested that the recovery of attic oil for the structures connecting in series could be enhanced bu injecting gas,while WAG hindered gas channeling improving the gas injection efficiency effectively.The experiment of channel adjustment suggested that using channel adjustment in parallel connecting structures could plug the main channel changing the water flow direction of injection well and increasing the sweep area,and consequently improving the waterflooding efficiency.Based on all aforesaid findings,a method was proposed to improve the waterflooding efficiency under different interwell connecting types in Tahe oil field,which has been validated in the oilfield.The study aimed to identify and characterize the interwell connecting structures and improve the efficiency of waterflooding.The results provide theoretical basis and technical guidance to waterflooding for fracture-cavity carbonate reservoirs,which are great of importance for both theory and practice.