Effect Of Salt Precipitation On Rock Physical Propreties Of Tight Sandstone Gas Reservoir

Salt precipitation and its induced problems are increasingly serious during exploitation of deep and ultra-deep tight sandstone gas reservoirs with high salinity formation water.Salt precipitation usually occurs in the reservoir and wellbore within the range of 5 m in the near well zone.Salt crystals is very easy to block the pore throat and reduce the effective flow channel,which leads to the decrease of the production of tight sandstone gas wells,which affects the efficient development of the tight sandstone gas reservoir.Rock physics is the basis of geophysical exploration of oil and gas reservoirs,is an important bridge between the oil and gas reservoir geological knowledge and development.Therefore,based on geological and engineering factors controlling salt precipitation of tight sandstone gas reservoirs,accurate understanding of the changes of rock physical parameters after salt precipitation is of great significance for the potential of salt precipitation tight sandstone gas reservoirs,the rate of drilling,the optimization of drilling fluid and completion fluids,and the efficient development of gas reservoirs.Taking the K area of Tarim Basin as an example,the geological and engineering factors controlling the salt precipitation of sand stone gas reservoirs are analyzed.Gelogical factors include gas reservoir temperature and pressure,fluid characteristics,reservoir physical properities,pore structure,reservoir fracture development characteristics and sensitive mineral types.Engineering factores include working fluid type and production pressure difference.The rock samples of typical tight sandstone blocks in China are selected as the research objects,and the test experiments of physical parameters of tight sandstone before and after salt precipitation are carried out,and the influence mechanism of salt precipitaion on the physical parameters of tight sandstone is revealed.The experimental results show that the mass and the velocity of ultrasonic wave increase after salt precipitation,and the porosity and permeability decrease.After many salt precipitation,the mass and velocity of ultrasonic wave are decreasing,the porosity changes little,but the permeability is remarkably improved.Using high-pressure mercury injection,we analyzed the change of pore size distribution of tight sandstone before and after salt precipitation,when the size of the salt crystalline is closer the pore size distribution of the tight sandstone,the more easily the salt crystalline of the cracks and the pore throat are blocked,which will cause the porosity and permeability of the rock to be significantly recuced.Combined with SEM,we observed the morphology and occurrence state of salt crystals within the pore,the occurrence state of the crystalloid salt within the pore and throats can be divided into three models in tight sandstone gas reservoirs:superposition growth along with the intergranular pore-fractures/natural micro-fractures.lamellar growth attached to the surface of the hydrophilic mineral like I/S interstratified mineral and the individual particles located in the corner of pore.The effect of salt precipitation on the parameters of tight sandstone under different main control factors is explicated.The experimental results show that the higher the fluid mineralization,the higher the saturation,the higher the development of the cracks,the longer the time,the greater the displacement pressure,the more serious the damage to the permeability of the sandstone rock.Compared with formation water,salt precipitation of working fluid damages the permeability of rock samples more seriously.Experiments on rock strength and stress sensitivity before and after salt precipitation of tight sandstone samples were carried out.The experimental results show that repeated salt precipitation causes rock strength reduction and sand production,The lower the permeability of tight sandstone is,the more sensitive the stress is after salting out.Compared with the base rock samples,the stress sensitivity coefficient of the fractured rock samples after salt precipitation is reduced,because the crystalline salt plays a supporting role in the cracks.Based on the salt precipitation phenomenon and function during the development of tight sandstone gas reservoir,the mechanism of salt precipitation blockage in the pore seams of tight sandstone,the mechanism of salting out crack growth,the strength of salting out and weakening rock strength and the mechanism of sand production are clarified by literature investigation and theoretical analysis,and the advantages and disadvantages of salt precipitation in the process of development are accurately recognized,and the precise regulation and control are carried out.We should make use of the effect of expansion and seepage and supporting cracks of crystalline salt to avoid the problem of reservoir damage caused by salting out and sand production.