Experimental Study On Selective Water Blocking Mechanism Based On Gelatinization Theory Of Polymer Materials

After decades of waterflooding processes, some of Tarim Oilfield wells have already reached into the stage of high water cut and high degree of recovery, which is associated with the macroporous channels (wormholes). There are little effective theories on the selection of water shutoff agent and the development of its technology during a long period, which results from the limitations of the mechanism and factors of forming wormholes that we have, and also from the high temperature (>120℃) and high salinity (>2×105mg/L) reservoir in Tarim Oilfield. The solidified working fluid has been effectively applied in high temperature (140℃) well-washing in Donghetang area. A sharp decline in water production leads to a new discovery. The main research on its mechanism shows as follows:(1)By analyzing the characteristics of waterflooding sandstone reservoir in Tarim Oilfield, the research on mechanism of formingwormholes in this kind of reservoir has been performed. The results of physical simulation experiments show that:lower permeability of unconsolidated sandstone, higher bonding strengthbetween rock particles, higher water injection rate and higher viscosity of formation fluid contribute to the formation of large pore channels (wormholes).(2)According to the theory of starch gelatinization under high temperature, a water shutoff technology of solidified working fluid has been advanced creatively. Solidified water becomes into a plugging layer through its physical dehydration in large pore channels (wormholes), and then into a gelatinous one through chemical changes under high temperature (>120℃).(3)Dynamic filtration experiments in the wormholes indicate that:the leakoff coefficient and loss rate increase, while the initial leakoff reduces with an increase in temperature. High temperature is a more conducive environment for gelatine plugging layer.(4)Physical simulation tests of solidified water plugging effect indicate that:solidified water shows better plugging strength, elastic deformation ability and toughness in extra-high permeability reservoirs of which the permeability is about10μm2its expansion ratio is between5-7g/g with minerized fluid of NaCl (5×103mg/L), Ca2+and Mg2+(5×103mg/L); it shows good contractility in oil, which means it has the function of selective water plugging; also the size of particles has an influence on the ability of its injection and transportation in wormholes. With continuous injection of formation water, plugging rate maintains about98%.(5)Field application shows that high temperature of140℃is conductive to form a gelatinous layer with solidified working fluid, which is capable of plugging water with high strength and long effective period, increasing oil production and decreasing water cut.