Application Practice for Low Environmental Impact Deep Acidizing Process to Improve Injection Capacity of Water Injection Wells

There are problems of high injection pressure and water injection difficulties in some injection wells among more than thirty wells that have been injected in H oilfield, and the overall water injection is insufficient. This paper analyzes the reasons for the injection problem from the perspective of block characteristics: for the decline in injection capacity due to reservoir pore blockage caused by clay expansion and contamination by injected impurities, it is proposed to apply a low environmental impact deep acidification system to improve the injection capacity of water wells.


Analysis of the Current Situation of Water Injection Wells in Oilfield H
H oilfield has carried out water injection test since 2009, and has bet on 31 water wells, the ratio of short-injection/low-injection wells is more than 50%, and the ratio of wells with oil pressure more than 15MPa is more than 70%.In the blocks put into earlier, there are some water injection wells layer injection pressure is high, water injection difficulties, the overall amount of water injection is insufficient, the block injection and extraction ratio is low [1][2][3][4] .Affected by reservoir physical properties and pollution plugging, nearly 50% of the water injection wells oil pressure close to or exceeding the trunk pressure, resulting in injection difficulties, the new injection wells "well 6X" when the pressure at 23.5 MPa, the daily water injection is only about 2 m 3 /d.At the same time, through the investigation of water absorption profile and well group dynamic liquid surface shows that the corresponding wells are poorly affected by the effect, and the degree of interlayer mobilization is not balanced, which is also an important problem faced by H oilfield.

Reservoir Sensitivity
Reservoir rock is composed of solid skeleton particles and intergranular pores, the size of the reservoir permeability reflects the nature of the rock pores, in the rock skeleton particles, there are some very small scale particles, they are often some clay mineral components.Some of these clay mineral components, affected by external fluids, are prone to swelling or shrinking of their own volume.From the results of water sensitivity experiments of 24 samples from 6 wells, the water sensitivity index is between 0.52 and 0.78 showing moderately strong water sensitivity characteristics, while scanning electron microscopy shows that the authigenic quartz of the block rock is coeval with kaolinite, illite and illite/montmorillonite mixed layers, while the intergranular particles are filled with lamellar illite.

Reservoir Heterogeneity
Influenced by sedimentary factors, the thickness and physical properties of reservoirs in H field are highly variable and relatively non-homogeneous.According to the statistics of core analysis data, the longitudinal permeability variation coefficient is 0.69-0.89, the abruptness coefficient is 2.47-5.51,and the gradation difference is 209.39-942.86.According to land-phase sandstone reservoir non-homogeneity grading standard, the reservoirs of each oil group in the longitudinal direction of the block have strong non-homogeneity.

Reservoir Contamination
Influenced by the environmental regulations, the injection well block is mainly clean and dirty mixed injection, the injected water contains O2 , CO2 , at the same time, Ca 2+ , Mg 2+ , HCO3 -, CO3 2-, SO4 2- and other ions content is high, the local temperature is high, the bacteria multiply a lot in the pipeline, and other factors lead to the injection pipeline and the equipment fouling, clogging the pipeline column, contamination of reservoir, resulting in the problem of pressure of the injection.
Figure 3. Plugging of column packers and water distributors Tracking the corrosion and fouling of the tubing, it can be seen that some wells in Block A have obvious corrosion and fouling phenomena, and according to the analysis of corrosion and fouling products, it can be seen that iron scale and carbonate are the main products.

Research on Low Environmental Impact Deep Acidification System
Acidizing is a measure of reduce injection pressure increase quantity that is commonly used in various oilfields with small investment and fast reduction, and its mechanism is to improve the permeability of the formation in the near wellbore and reduce the resistance to fluid seepage in the near wellbore by lifting the epidermal injury of the near wellbore and dissolving part of the minerals (especially the clay minerals).The sandstone treatment fluid system usually consists of hydrochloric acid prelude and mud acid as the main acid fluid.For low permeability sensitive reservoir, due to the expansion of clay particles, dispersed transport caused by deep pore throat contamination blockage, in order to achieve good acidification effect, must increase the acidification treatment radius, due to the conventional mud acid is more acidic, faster reaction rate, most of the active acid consumption in 0.5m, the treatment radius is small, if the amount of too much may cause the rock skeleton structure is damaged and the well wall collapses, the reaction is insoluble precipitates.Generates are insoluble precipitation, easy to produce secondary precipitation and block the pore throat, in order to protect the effect of acidification need to be discharged as soon as possible, and the strong acidic discharge has a more serious impact on the environment, and is easy to cause the treatment system is unstable, need to improve the acid system to achieve the depth of the low environmental impact acidification injection, improve the overall effectiveness of the measures [5][6][7] .
Table 2 Retarded mud acid depth acidification system consists of hydrochloric acid, fluoroboric acid, multi-organic acid, chelating agent, corrosion inhibitor and multi-effective additives, long-lasting corrosion ability is better than conventional mud acid, the reaction speed of 40% of conventional mud acid, and at the same time, it can effectively chelate the precipitation of CaF2, NaSiF6, Fe(OH)3, etc., and it does not need to flowback after the acidification , so that it can effectively reduce the impact on the environment, extend the distance of the acid treatment.

Practice of Measures to Improve Injection Capacity of Water Injection Wells
Well X is an injection well of oil field H, with permeability 0.01-14.8mD,and layer 287# is a strong liquid absorption capacity layer, and the acidizing process measures will be implemented for this well in 2016,2019 and 2022 to effectively improve the suction capacity of layer 288-301, and to effectively avoid the well from single-layer surging, which will lead to the lowering of the driving efficiency, and to avoid the well from seeing the water and decreasing the production.

Figure 1 .
Figure 1.Scanning electron microscope images of rocks from well A1 (2555.49m,×2500; 2276.38m,×2000) At the same time, most wells in the short-injection block had a 90% drop in water injectivity index within the first month of starting injection, proving that the reservoir is more sensitive and prone to contamination plugging.

Figure 2 .
Figure 2. Variation of injectivity index in the first year of injection wells in Block A

Figure 4 .
Figure 4. Acid-Rock reaction speed comparison diagram

Figure 5 .
Figure 5.The change of liquid absorption capacity of each injection layer in Well X

Figure 6 .
Figure 6.Interpretation of the stabilized electric field injection boundary monitoring in Well X

Table 1 .
Core permeability non-homogeneity parameter statistics . Comparison of dissolution ability of acid system

Table 3 .
Precipitation inhibition of different acid systems