Imbibition Effects Of Tight-Low Permeability Reservoirs And Related Laws

Imbibition has been a hot issue in the frontiers of petroleum exploration for a long time,especially in the production of tight-low permeability reservoirs.The effect of imbibition is particularly important for the extraction of crude oil in the matrix.With the gradual deepening of research,many new problems have emerged in the theory and technology of oil permeation,absorption and drainage.In this context,this dissertation has carried out research methods and related laws of oil absorption and drainage.Several key difficulties in the research method of oil imbibition have been overcomed.A global quantitative simulation technology for the wettability of the reservoir physical model was developped—the wettability index of the prepared reservoir physical model can be quantified and controlled within the range of-1.0 to 1.0.By applying this technology,physical models of strong oil-wet to strong water-wet reservoirs can be made according to reservoir conditions and research needs,and the wettability of the model can be kept constantly throughout the experiment.Compared with the method of changing the wettability using the principle of adsorption of silicon oil on the surface of rock particles,the wettability control method developed in this paper can not only realize the global quantitative simulation of the wettability index,but also eliminate the impact of uncontrollable wettability changes during the experiment.Using experimental method of rock core circumferential sealing,we solved the problem that the traditional imbibition experiment cannot distinguish between the end face and the circumferential imbibition;With the imbibition recovery on single imbibition surface as the characterization parameter,a unified and standardized evaluation method of imbibition effect was establishedThe correlation laws between the effect of oil imbibition and core permeability and wettability were reached.Using the core developed in this paper and the improved experimental method,the simulation experiments of core permeability 0.5×10^-3μm²～200×10^-3μm²and wettability index-0.7～0.9 were carried out.The experimental results show that with the increase of permeability or wetting index,the critical time of high-speed oil drainage and the critical time of oil drainage termination are shortened,while the imbibition oil drainage speed and the ultimate imbibition oil recovery are increased.Under the experimental conditions in this paper,the ultimate imbibition oil recovery and permeability satisfy the semi-logarithmic empirical formula,and the ultimate imbibition oil recovery of strong hydrophilic and tight core is only about 50%of that of medium-high permeability core,which is about 14%.The ultimate imbibition oil recovery of the oil-wet core（wetting index-1.0～0）is 0,and there is a strong positive correlation between the ultimate imbibition oil recovery of the water-wet core（wetting index 0～1.0）and the wetting index.The imbibition experiment method with core circumferentially sealed was adopted to measure the relationship curve between the total oil imbibition volume and the core length,and the core length corresponding to the"limit oil imbibition volume"of the total seepage,absorption and oil displacement no longer increased with the core length on this curve was determined as the effective action distance of oil imbibition.By using this method,the basic law of positive correlation between the effective imbibition action distance,permeability and wetting index is obtained.It is recognized that the effective distance of oil absorption and drainage in low permeability reservoirs is very limited,only on the order of centimeters.The non-monotonic variation law of water displacement efficiency with core wetting index is determined,and the water displacement efficiency of neutral wetting core is the highest extreme value.It is found that the characteristic parameters of water flooding are obviously different in cores with different permeability:the breakthrough speed and water cut rising rate of water flooding front of ultra-low permeability cores are significantly higher than those of medium-high permeability cores,and their waterless oil displacement efficiency and ultimate oil displacement efficiency are obviously lower than those of medium-high permeability cores.Compared with medium-high permeability cores,the oil displacement efficiency of ultra-low permeability cores is more affected by wettability during water flooding.The ratio of imbibition oil recovery to water displacement efficiency is used to characterize the contribution of imbibition.The imbibition contribution rate of low permeability cores is equivalent to that of medium and high permeability cores.Considering that the original oil-water flooding in low permeability（especially tight）reservoir matrix is particularly difficult,the dependence of low permeability reservoir on imbibition effect to improve the recovery of matrix crude oil is much higher than that of medium-high permeability reservoir.The feasibility of using imbibition agent to strengthen imbibition and oil discharge effect was explored.For the neutral wetting core（WI=0.12）,the imbibition agent can improve the imbibition and oil displacement rate,but the effect is not obvious.For weak oil-wet cores,the imbibition agent can greatly improve the imbibition oil displacement rate.