Study On The Pressure Conduction Law Of Depletion And Dynamic Characteristics Of Huff And Puff In Tight Oil Reservoir

With the decreasing of conventional oil and gas resources,tight oil reservoirs have been paid more and more attention in recent years as an important source of subsequent oil supply.Affected by the complex seepage,the production characteristics of depletion and subsequent huff and puff（H&P）of tight oil reservoirs are different from those of conventional oil reservoirs.How to accurately describe and characterize the development of tight oil,and further achieve accurate prediction,is still a key problem in tight oil development.In this paper,a systematic study of pore structure,nonlinear seepage,pressure conduction law of depletion,oil recovery mechanism and dynamic characteristics of H&P was carried out in tight oil,based on physical simulation,aiming to realize the fine description of depletion and H&P process in tight oil reservoirs.Based on the experimental data of mercury injection and seepage curve,the correlation between pore throat parameters and reservoir seepage capacity is analyzed.The results show that the uniformity of seepage curves in different blocks can be achieved by Reynolds number（R_e）and resistance coefficient（f）,and the critical Reynolds number R_ec=10^-4 can be used as the boundary condition for judging the nonlinear or linear seepage in tight reservoirs.Then,based on the fact that the liquid phase flow in tight oil reservoirs is generally in the low-velocity nonlinear stage,a simple and practical single-parameter modified nonlinear flow model is proposed.In terms of depletion,given the problem that the pressure conduction in tight oil reservoirs deviates significantly from the traditional elastic seepage model（Fourier model）,combining the long core pressure conduction experiments and the three-dimensional model depletion experiment,the essential reasons for the"L"-shaped rapid production decline in the tight oil depletion are clarified from the perspective of pressure conduction,which are as follows:（1）In the early stage of depletion（before the formation of boundary control flow）,the pressure conduction coefficient would drop rapidly(1.6×10^-5m²/s→9.8×10^-6m²/s),which leads to a rapid decrease in production;（2）In the middle and later stages of depletion（after the formation of boundary control flow）,the pressure conduction coefficient tends to be relatively stable and decreases slowly as the pressure gradient decreases,which results in the production being maintained at a lower level and slowly decreasing.Finally,a nonlinear elastic seepage model suitable for tight oil reservoirs is established based on the derivation of the nonlinear flow model,leading to a significant improvement in the prediction accuracy of the depletion process.In terms of the mechanism of H&P,using a segmented long core to carry out H&P experiments combined with nuclear magnetic resonance technology,it is pointed out that the limitation of sweep distance in water injection H&P would lead to a shortage of oil recovery capacity in the subsequent rounds,while the CO₂ injection H&P has a greater sweep distance.Meanwhile,CO₂ injection H&P has higher utilization of the oil in the large and medium pore,since the swelling and extraction after CO₂ injection will greatly reduce the blind end and cluster residual oil,which is confirmed by the results of high temperature and high pressure microscopic visualization experiments.In terms of the dynamic characteristics of H&P,based on the improved 3D model H&P physical simulation method,the simulation of the whole process of CO2 injection H&P in a staged fracturing horizontal well tight oil reservoir is realized.The experimental results show that the oil production of puff period presents a two-stage rising characteristic,which is in line with the characteristics of elastic gas flooding.Combining the dynamic parameters,the development process of CO₂ injection H&P can be divided into four stages:CO₂ backflow stage,gas production with attached oil stage,high-speed oil production stage and decay stage.The mechanism would gradually transform from the free gas flooding to the dissolved gas flooding in the puff period.Also,CO₂ injection weight and development rate are the sensitive factors affecting oil recovery.Large CO₂ injection volume and low development rate are conducive to enhancing oil recovery.Finally,combined with the swelling and molecular diffusion theory,a production prediction model for CO₂ injection H&P is established,and the quantitative splitting of flow sweep production and diffusion sweep production is realized.It is clear that the oil recovery of CO₂ injection H&P mainly comes from the diffusion sweep area.