Study Of The Low-degree Formation Damage Cement Slurry Of The Clastic Rock Reservoir

As a systematic project,the reservoir protection involves exploration,drilling,completion,development and down-hole operations and other links.It is directly related to whether the oil field can be discovered timely and appraised and exploited effectively,also it is related to whether oilfields can achieve higher oil recovery efficiency.As an important part of oil and gas well construction,the cementing process,has long been focused on ensuring safe pumping of cement slurry and long-term effective interlayer sealing ability of cement ring,neglecting the importance of reservoir protection during cementing.However,many years of practice have shown that due to the high pressure difference between annulus and formation during cementing and the high filtration loss and high solid content of cementing slurry,the damage caused by cementing slurry to reservoirs can not be ignored.Based on a thorough investigation of the current situation at home and abroad,this paper takes the typical clastic rock block of an oil field in western China as the background,and conducts an experimental evaluation of the damage of the cement slurry to the reservoir.The damage of the cement slurry to the reservoir under different working conditions and different formation conditions was studied.Under no-lost working conditions,due to the protection of drilling mud cakes,the cement slurry has a relatively low degree of damage to the reservoir.In the case of leakage of pore formation,the filtrate contamination depth of cement slurry is in the near-wellbore zone of 10?40cm.Although it is within the perforation range,it will still affect the seepage field of the near-wellbore zone.The reservoir damage caused by the filtrate cannot be ignored.In the case of the leakage condition of the pore-fractured formation,the solidification blockage after the cement slurry enters the micro-fracture is the main cause of reservoir damage.The damage mechanism of the cement filtrate to the reservoir was studied through experiments and microscopic analysis methods,which were mainly reflected in the compatibility of the filtrate with reservoir rocks and fluids.Filtrate into the reservoir will cause a certain degree of alkali-sensitized phenomenon,but also lead to volume expansion of the reservoir core,making the clay particle volume increases,the porous stratum containing clay mineral internal expansion,resulting in porosity and permeability Lower,causing damage to the reservoir.The consistency of the cement filtrate and formation water is poor.After entering the reservoir,it is in contact with the formation water,causing fluctuations in temperature,pH,and ion concentration,leading to the precipitation of inorganic scales.Ca(OH)2 and CaCO3 have a large amount of scale,followed by Mg(OH)2 and CaSO4.The precipitation of inorganic scales will cause blockage in the pore throat of the reservoir,causing a decrease in reservoir permeability.In addition,the high molecular weight polymer contained in the filtrate cross-links with the high-valence cations in the formation water to form a flocculation-like precipitate,resulting in darmage to the reservoir.The damage mechanism of solid particles of cement slurry on the reservoir mainly shows the blockage of the pore throat of the reservoir after the infiltration of solid particles.Under the condition of the loss of cement slurry in the pore formation,the solid particles of the cement slurry will invade the reservoir 2-3 cm,and the solid particles of the cement slurry will undergo hydration and consolidation,and the reservoirs caused by the backwash cannot be released.damage.Under the condition of pore-fractured formation loss,solid particles of the cement slurry enter into the micro-fractures of the reservoir and are consolidated by hydration,which blocks the micro-fracture of the reservoir.Due to the low acid-soluble rate of conventional cementite,conventional acid dissolution plugs can only corrode the surface of the cementite,and it is impossible to effectively relieve the blockage of the cementite to the microcrack of the reservoir.After clarifying the damage mechanism of the solid phase and the filtrate,the performance of the cement slurry is improved by the preferred calcium carbonate and retarder.When 20%light calcium carbonate(QZ-A)was added to the cement,the acid-soluble rate of the cement reached 71.1%,the macrostructure of the cement stone was destroyed by the acid solution,and the acid dissolution effect was good.The retarder GY-21 can effectively reduce the ion concentration of filtrate filtrated fluid slurry,thereby reducing the amount of harmful ions entering the reservoir,and has a better filtrate optimization effect.After optimization,the conventional engineering properties such as fluidity,compressive strength,fluid loss,and thickening time of cement slurry system all meet the needs of cementing construction.At the same time,in the case of the loss of the porous formation and the loss of the pore-fractured formation,the degree of damage to the reservoir is significantly reduced.