Study On Oil Based Cement Slurry As A Selective Water Shutoff Material

Fossil oil crisis is coming with the rapid economy development and oil consumption both in China and the world. Improved Oil Recovery is the key to reprieve the predicament. Water production is popular in most inland oil fields, which led to poor water sweep efficiency, high oil production cost and low oil recovery factor. Water shutoff agents with high sealing strength, availability and selectivity are demanded especially in high temperature and high salinity formations. Environmental friendly inorganic water shutoff materials are the momentous parts of water control agents for its high sealing strength. Oil based slurry is emerged for its superior selective water shutoff ability and high sealing strength. But systematic study of its working mechanism including cement hydration is rare both home and abroad.Selection and compounding of the wetting and dispersing agent are studied for the basis of preparation of the oil based slurry and liquid and solid state properties on the relationship of the composition and performance for selective water shutoff.Preparation of oil based cement slurry with qualified suspension stability and adequate fluidity is the fundamental step for water shutoff job using oil based slurry. Oil based slurry is a suspension of cement in oil phase which needs a water-in-oil surfactant while the wetting state reversion process needs an oil-in-water surfactant for a selective water shutoff agent. Therefore, appropriate balance of the hydrophilicity and hydrophobicity is vital for the preparation of the oil based slurry.Suspension stability, fluidity and oil-water displace efficiency are the three main parameters for performance check, which are at variance for the effect of surfactants. Thus the coordination of those three parameters is the difficult point for selection and compounding of the wetting and dispersing agent. A novel wetting and dispersing agent (OBS16) is verified on carefully evaluate and optimize the liquid and solid-state properties. OBS16 is compounded by a non-ion surfactant and other surfactants. Oil based slurry prepared with OBS16 exhibit the maxim water-oil displace efficiency up to 70% under room temperature and other acceptable properties such as suspension stability and fluidity for water shutoff jobs.The oil based slurry reacts with water and forms a hard cement stone, whose compressive strength is critical for the shutoff performance. The cement stone formed from hardened slurry at rest shows hiberarchy whose properties vary greatly fromdifferent layers. The cement stone from hardened homogeneous mixed slurry then exhibit even compressive and other mechanical properties. Results from later is much more reasonable, creditable and test time consumption is less. So it is used as the standard procedure for oil based cement stone preparation. The oil based slurry can react with water and form hard cement stone under wide water to slurry ratio and the cement stone show the highest compressive strength while water to slurry in the range ofO.20toO.30.A numerous miscellaneous admixtures and additives is used to improve the sealing strength of oil based slurry. Accelerating agent used for ordinary cement is not effective for oil based slurry. The retarder is effective with longer age curing. Silicon fume is not effective alone while it became effective by compounding with FeCb When they were added some expanding agent, the compressive strength is further improved.The hydration process of the slurry and the main hydration products are analyzed using hydration heat method, XRD, SEM, MIP, TG-DSC, et al. The hydration and harden mechanism of oil based slurry is discussed comparing with ordinary cement slurry.Hydration heat evolution rate of oil based slurry is much slower than ordinary cement slurry, with almost half of the peak hydration heat evolution rate. The delayed period in oil based slurry is much longer than ordinary cement slurry and increase with the water to slurry ratio. The hydration degree of hardened OBS at 0.5d age is 69.68% for ordinary oil well cement and 72.27% for fine oil well cement. The hydration degree turned to be 88% for ordinary oil well cement and 80% for fine oil well cement at the age from Id to 7d. Fine cement exhibits rapid early hydration rate but the later hydration degree is lower than ordinary oil well cement slurry.The main hydration products are CSH with high Ca/Si and calcium hydroxide. CSH with high Ca/Si will transform to CSH with low Ca/Si(C2SH, C4S5H and C5S6H) in later age with an increasing tendency in compressive strength. The temperature range from 50 °C to 80°C less affects kinds of hydration products.The absolutely hydration degree is studied using the loss on ignition and thermal analysis. The hydration degree is 33%~66%% with water to slurry ratio range in 0.10 to 0.50. It increases with temperature average 2% to 3% when rising curing temperature from 50 °C to 80°C (7d age). Cement fineness can increase hydrationdegree with a average of 3%~4% from ordinary oil well cement to fine oil well cement.Hardened oil based cement slurry in 7d at 80°C exhibits higher porosity, almost double the porosity of ordinary cement stone with W/C=0.50 at the same age. The porosity of hardened oil based slurry is mainly affected by the residual oil in the matrix. W/S has little influence to total porosity but greatly affect the pore distribution.Core simulation of water shutoff is prior to the oil field water shutoff operations. The squeezing instrument is scarce inland for grain plugging materials. A novel squeezing instrument for grain plugging materials is designed and made for the oil based slurry shutoff performance evaluation. Results shows the oil based slurry can plug off the high permeable water zone with water phase permeability decrease above 90%. After flowback with 100 PV diesel oil, the oil phase permeability decrease is less than 20%. The agent shows outstanding water selective shutoff performance.