In the middle and late stages of oilfield development, replenishing stratigraphic energy by water injection is the main measure adopted by most oilfields in China. Due to the heterogeneity of the oil layer, the phenomenon of “protrusion” and “turbulence” of water in the oil layer may occur, which seriously affects the development effect of the oil field. In order to improve the water injection effect and the ultimate recovery rate of the oil field, it is necessary to take timely measures of water shutoff and profile control.
First, the concept of water shutoff profile control
(1) Water absorption profile and profile control
For water injection wells, due to the heterogeneity of the formation, the water absorption of each layer of the formation is unbalanced, and the water absorption of each part of each layer is different, which is reflected in the water absorption section.
The unevenness of water absorption in the formation, in order to increase the sweep coefficient of the injected water, it is necessary to block the high permeability layer with strong water absorption capacity, which is called profile control.
(2) Production liquid profile and water blocking
For oil wells, due to the heterogeneity of the formation, the oil production and water content of each layer and the different parts of each layer are not necessarily the same, and the liquid production profile is uneven. Blocking high-yield water layers and improving the liquid production profile is called water shutoff. Water blocking can increase the sweep coefficient of the injected water. The success rate of water shutoff often depends on the success rate of finding water. In addition to directly measuring the production fluid profile, well water logging can be used to determine the water horizon.
Second, water blocking profile control method
(1) Mechanical card seal
The use of downhole tools to seal the superabsorbent layer or the high-yield water layer is called a mechanical seal. The scope of mechanical sealing is limited to the wellbore range, but due to its simple construction and low cost, it is often the preferred method of water blocking.
(2) Chemical water shutoff
The chemical agent is injected into the ground, and the high permeability layer or the high-yield water layer is blocked by the chemical agent or the reaction product thereof, which is called chemical water shutoff.
(1) Single liquid method and two liquid method:
From the construction process, chemical water shutoff can be divided into single liquid method and two liquid method. The single-liquid method is to inject a working fluid into the oil layer, and the material carried by the working fluid or the subsequently changed substance can block the high-permeability layer. The two-liquid method is to inject two kinds of working fluids (or working fluids) which can generate blocking substances after encountering the formation. When injected, the two working fluids are separated by an isolation wave, but as the working fluid moves outward, the spacer fluid becomes thinner and thinner. When extrapolated to a certain extent, that is, the spacer liquid is thin to a certain extent, it will not be isolated, and the two working liquid phases will produce substances that block the formation. Since the high permeability layer draws in more working fluid, the plugging mainly occurs in the high permeability layer to achieve the purpose of profile control.
(2) Selective water shutoff process:
Selective water shutoff is performed after the water is removed by using test data such as the product profile. For the lower part of the water, seal and block the upper part, use the packer to separate the upper and middle parts of the oil production section, and then block the lower part of the water. For the middle part of the water, seal the upper and lower parts with a two-stage packer, and then block the water in the middle part of the water.
For the upper part of the water, seal and block, use the packer (or cement plug) to seal the lower part, the middle part, and then block the upper part of the water. For the upper and lower parts of the water, seal the upper and lower parts, use a two-stage packer to separate the middle part, and then block the upper and lower parts of the water. In the oilfield production process, the corresponding plugging pipe column can be selected according to the specific conditions. (3) Formation selectivity:
Since the plugging agent always enters the high-permeability part first, the high-permeability part is the target part that we want to block, which is called the selectivity of the stratum.
(4) Selectivity of blocking agent:
The selective plugging agent utilizes the difference between oil and water or the difference between the oil layer and the water layer to achieve the purpose of selective water shutoff. Selective blocking agents can be divided into three categories, namely water-based blocking agents, oil-based blocking agents and alcohol-based blocking agents, which are respectively formulated with water, oil or alcohol as a solvent or dispersion medium. Here are some of the more commonly used selective plugging agents:
1HPAM (water-based): HPAM has obvious selectivity for oil and water. It reduces the permeability of rock to oil by no more than 10%, while reducing the permeability of rock to water can exceed 90%. In oil wells, the selectivity of HPAM plugging is: it preferentially enters the formation with high water saturation; HPAM entering the formation can be adsorbed by hydrogen bonds on the surface of the formation exposed by water scouring; the unabsorbed part of HPAM molecule can be Stretching in the water reduces the permeability of the formation to water. The stretching of the HPAM in the water greatly increases the resistance to water flow. Although HPAM also exerts some resistance to the flow of crude oil, it can form a water film that reduces the flow resistance on the rock surface, so that the resistance of the plugging agent to the oil flow does not change much.
2 yin and yang non-terpolymer (water-based): if copolymerized and hydrolyzed by acrylamide (AM) and (3-acrylamido-3-methyl)butyltrimethylammonium chloride (AMBTAC), A yin and yang non-terpolymer. Such blocking agents have anionic, cationic and non-ionic links in the molecule. When the aqueous solution of the plugging agent is injected into the formation, its cationic links will be firmly adsorbed on the surface of the negatively charged rock, while the anionic and non-ionic links will extend into the water to increase the resistance of the water flow, thereby selectively blocking water.
3 Foam (water-based): Foam with water as the dispersion medium can preferentially enter the effluent layer and be stably present in the effluent layer. The water is blocked by the superimposed Jiamin effect. In the oil layer, the oil can be emulsified to form a three-phase foam in the dispersion medium of the foam. Some of the oil beads in the dispersion medium cause damage to the foam, so the foam entering the oil layer does not block the oil layer. Therefore foam is also a selective plugging agent.
4 active oil (oil base): This is a heavy oil with emulsifier dissolved. The emulsifier is a water-in-oil emulsifier (such as Span 80) which produces a highly viscous water-in-oil emulsion when the heavy oil is exposed to water. Since heavy oil contains a considerable amount of water-in-oil emulsifiers such as naphthenic acid, colloidal, asphaltenes, etc., heavy oil can be directly used for selective water shutoff. The oxidized bitumen can also be dissolved in oil to form an active heavy oil. This bitumen is both a water-in-oil emulsifier and a thickener for oil.
5 water-packed heavy oil (water-based): This plugging agent is an oil-in-water emulsifier to emulsifie heavy oil in water. Since the emulsion is an external phase of water and has a low viscosity, it is easy to enter the water layer. In the water layer, since the emulsifier adsorbs on the surface of the formation, the emulsion is destroyed, and the oil beads are aggregated into a highly viscous heavy oil, which generates a large flow resistance and reduces water effluent. The emulsifier for the heavy oil of the water is preferably a cationic surfactant because it is easily adsorbed on the surface of the negatively charged sandstone, causing the destruction of the emulsion.