Experimental Investigation Of Tight Oil Pore Characteristic And Depletion

Exploration and development of tight reservoirs is a hotspot in petroleum engineering research.Horizontal drilling technology and hydraulic fracturing technology provide conditions for commercial development of tight reservoirs.At present,the development mode of tight reservoir is depleted production after horizontal drilling and hydraulic fracturing reforming reservoir.The tight reservoir has low production because of its tight lithology,high seepage resistance and poor pressure conductivity.Investigation shows that most of the research of tight reservoirs is based on field and numerical simulation methods by pressure depletion.There is almost no experimental research report,therefore.From the standpoint of experiment,this paper will reveal the mechanism of tight reservoir and determine the recovery degree by pressure depletion.The pore structure of reservoir not only affects the oil and gas reserves and the percolation law of pore fluid,but also has an important influence on the productivity and ultimate recovery of oil&gas wells.Because of its complex structure,tight reservoirs cannot be fully characterized by conventional physical properties.Therefore,gas adsorption,CT,mercury injection,nuclear magnetic resonance and other methods to characterize pore structure are used to characterize the pore structure of tight reservoirs at full scale.The lower limit of pore radius of tight reservoir core is 0.35 nanometer,and the pore throat radius above nanometer is mainly distributed around 150 nanometer.There are micro-fractures in the tight reservoir,and the fracture width mainly distributes in 300-800 nm.Submicron-micron pore size plays a dominant role in the seepage of tight reservoir rocks.Nuclear magnetic resonance(NMR)results show that the lower limit of available pore radius of crude oil in tight reservoir is 2 nanometers.After defining the pore characteristics of tight reservoir,we choose the Chang 7 tight sandstone reservoir is taken as the research object.A set of experimental platform and evaluation method for depletion mining of tight reservoir is established.We breaks through the full diameter physical simulation technology under reservoir conditions.Three full-diameter cores drilled along horizontal bedding corresponding to outcrop cores of Chang 7 tight reservoir are selected to conduct pressure depletion experiments.Seven pressure measuring points are arranged along the way to monitor pressure.The results show that the recovery factor of depleted production in tight reservoirs is about 3-11%.Formation pressure coefficient and whether the crude oil contains dissolved gas are important factors affecting recovery factor of depleted recovery.The higher the formation pressure coefficient is,the higher the recovery degree of crude oil reservoirs containing dissolved gas is.The recovery factor of tight reservoir has nothing to do with the way of depressurization.The depletion production of tight reservoir with dissolved gas can be divided into two stages.The first stage is that when the reservoir pressure is higher than the saturation pressure,the elastic energy is dominant,and the recovery degree is 2-3%.The second stage is that when the reservoir pressure is lower than the saturation pressure,the dissolved gas drive plays a major role,and the recovery degree is 5-8%.This paper clarifies the depletion recovery mechanism and recovery factor of tight reservoir by pressure depletion.The investigation provides theoretical guidance for predicting productivity and optimizing parameters of depleted production in tight reservoirs.