Daqing Oilfield is a water flooded oilfield, the injected water is usually the produced-water. It reduces the injection cost and performs a good environmental effect as well. The formation of tertiary infill wells are usually of a effective thickness less than 0.5m, and a permeability less than 0.050 Darcy, and the permeability in 60% of the formation is less than 0.010 Darcy, this kind of formations are vulnerable to be damaged, it requires a higher quality of injected water,. In tertiary infill wells, reinjection of produced-water does a more serious damage to the formation, the water absorbing thickness decreases from 76.4% of the secondary infill wells to that of 59.3% of tertiary infill wells, this result in a serious adverse waterflood development.
Through a considerable research and analyses of the injection operation, this paper suggests a finely treatment of produced-water by decreasing the suspended particle to less than 7–8mg/l, and increasing the water absorbing thickness of tertiary infill wells to 81.2%, this performs a better water absorbance. This makes the water absorbing thickness reach 66% in those formations with a permeability less than 0.010 Darcy, and the producing thickness in those formations with a permeability less than 0.010 Darcy reach to 61% in oil well, this eases the damage degree of produced-water to low permeable formations, thus performs a good waterflood effect in low permeable formations.
The Sazhong Area of Daqing Oilfield covers an area of 161.3 square kilometers with an OOIP of 1227.5 million tons, the current production degree is about 36.63%, its annual injected water is 140 million cubic meters. This zone has been undertaken a water injection development since early 1960s'. The injected water was simply separated from the produced fluid, and it provided a good effect. But with the oilfield development, infill wells have been drilled in previous decades, a tertiary infill wells had been drilled since 1992, the adjusted layers have moved from the high permeability layers to those with a low permeability. The simply separated water does not provide a satisfied effect any more in low permea-bility layers; the simply separated water damaged the target layers of the tertiary infilled wells and unfavorably restricts its development effect. Since 1999, the produced-water treatment system has been improved to dispose the produced-water finely so as to meet the required water quality in low permea-bility layers. This performed a good effect.
Presently there are two mainly applied produced-water disposal methods in Daqing oilfield, one is simply separated, the other is finely disposed. The simply separated produced-water methods includes gravitation type disposal process and pressure type water disposal process, they are described as follows:
As shown in Figure 1, in gravitation type process, water is separated from oil by natural sedimentation and a second time flocculating setting, after a gravitational filtration; finally sterilization and pressurization are performed for injection.
As shown in Figure 2, oil and water are separated by initial sedimentation and dissolved air flotation, filtration is performed afterwards, and finally sterilization and pressurization are performed for injection.
The above two processes can process the simply separated produced-water, and the water quality index obtained is almost the same. The oil content is not more than 20mg/L, suspended substance content is not more than 10mg/L with a median grain diameter of solid grain not more than 5µm, this kind of the simply separated produced-water (SSPW) is called common sewage, and it's suitable for rejecting into layers with a medium and high permeability.
Finely disposal of produced-water (FDPW) is a further treatment of the above two processes, after two times filtration, it is sterilized and pressurized for rejection. (See Table 3)
The water quality index of finely disposed water suited well for injecting into the target layers with a permeability of 0.010 Darcy, it's oil content is less than 8mg/L, suspended substance content is less than 3mg/L with a median grain diameter of solid grain not more than 2µm.