Study On Fracturing Fluid's Imbibition Of Chang-7 Source Based On Nuclear Magnetic Resonance

Unconventional tight sandstone reservoirs have become the top priority of China's energy industry.In general,China's unconventional oil and gas resources are more abundant.At present,there are more objective oil and gas resources in the Chang 6 and Chang 7 Member groups of the Yanchang Formation,Lucaogou Formation and Quantou Formation.Some scholars believe that the total amount of unconventional tight oil in China may be around 10 billion tons.If reliable,then unconventional tight oil will be China's future development of oil and gas exploitation of the most important.Relative to the traditional fracturing,the recovery rate of fracturing fluid is very low during the fracturing of tight oil reservoirs.a large amount of fracturing fluid stays in the reservoirs.However,after shut-in for a period of time,the productivity increases.It aroused widespread concern.At present,it is generally accepted that this phenomenon is caused by the imbibition."Seam fracturing + oil-water displacement" has become a new attempt to effectively develop tight reservoirs.Compaction of reservoirs and differences in fluid properties of reservoirs provide the conditions for fluid replacement.Fracturing fluid is not only a carrier for seaming and sand-carrying,It is a replacement oil displacement tool.For the study of the imbibition phenomenon,previous studies mainly used the experiment to investigate the cause of the retention of fracturing fluid in the reservoir.However,the retention of fracturing fluid into the reservoir has not yet been studied in depth.Traditional imbibition experiments,such as volumetric method,have some shortcomings such as inaccurate measurement,slow speed and inability to image.In order to obtain more convincing data,more accurate experiments.Based on nuclear magnetic resonance(NMR)technology and high-pressure mercury injection,rock thin sections,scanning electron microscopy and X-ray diffraction,the core of sandstone in the Ordos Basin Chang-7 tight sandstone was studied.The influence of imbibition on reservoir seepage is analyzed,the distinction between imbibition and displacement is distinguished,and the visual exploration is made.Different stages of core imbibition are distinguished,and the main pore throat radius of imbibition is described.The results are as follows:The core of Yanchang-7 Formationin Heishui area of Ordos Basin is very tight with relatively low porosity and very low permeability;The pore structure is very complicated;The small pore absorbs fluid and medium pore discharges fluid;The imbibition process is outside and inside,not only on the surface of the core;there is no significant surface enrichment phenomenon in the process of imbibition;the main control of the core imbibition is the capillary force;the imbibition process can be divided into different stages:initial stage,medium term,later stage;there was imbibition in the cores of tight sandstone;The lower the permeability of core,the larger the amount of fracturing fluid retained,the lower the porosity and the higher the amount of fluid retained;The pore-throat range of imbibition motive force is 0.002?m-0.04?m,the pore-throat range of discharges fluid is 0.04?m to 0.81?m.