Overview of on-site application of fluoroboric acid【1】
Fluoboric acid was recommended as a workaround for soil acids by Thomas and Crowe (1981). It does not contain significant amounts of hydrofluoric acid under any given condition and is therefore less reactive. However, when HF is consumed, more HF can be produced through autohydrolysis. Therefore, its total dissolving capacity is comparable to that of earth acid. Fluoboric acid can be used as a pre-flush before soil acid treatment (sensitive formations); this avoids particle instability and subsequent pore clogging. They are also used as the only treatment for damage removal in sandstone bedrock containing little carbonate cement or in fractures containing many clay particles. Fluoboric acid is also used as a post-flush after soil acid treatment to remove the near-wellbore zone (up to 1 foot), thereby allowing the fluoboric acid solution to easily penetrate (several feet). The use of fluoroboric acid is also particularly recommended when the sandstone contains potassium minerals, to avoid damage to the sediment and in cases of particle migration due to its particle stabilizing properties.
Acidification mechanism of on-site application of fluoroboric acid【2】
Fluoroboric acid (HBF4) is a slow acid that can slowly hydrolyze to form HF. Its reaction speed is lower than that of conventional earth acid, so it can penetrate deep into the oil layer to a wide range before the acid is exhausted. In addition, HBF 4 can also cause chemical agglomeration of clay particles. The agglomerated particles are cemented in place, which limits the migration of dimethyl sulfoxide particles caused by increased flow after treatment. After fluoroboric acid (HBF4) enters the formation, it is hydrolyzed in the aqueous solution and slowly produces hydrofluoric acid (HF). HF can interact with clay and other siliceous particles. The reaction formula is as follows.
ＨＢＦ ４ ＋Ｈ 2 Ｏ→ＨＢＦ ３ ＯＨ＋ＨＦ
ＨＦ＋Ａ1 2 Ｓｉ４ Ｏ 1６（ ＯＨ ） 2 →H 2 ＳｉＦ 6 ＋ＡAlF ３ ＋Ｈ 2 Ｏ
Because HBF4 has the characteristics of slow reaction, its action distance in the formation is much longer than that of earth acid, and it also has a strong ability to dissolve clay, reduce the cation exchange capacity of clay, inhibit clay swelling, and has the ability to It has the advantages of less damage to the oil layer skeleton.
(1) The relationship between the hydrolysis rate of HBF4 and temperature
Hydrolysis of HBF4 is a multi-stage reaction, that is
HBF 3 OH+H 2 O→HBF 3 (OH) +HF (slow)
HBF 3 (OH) 2 +H 2 O→ HBF 2 (OH) 2 +HF (fast)
HBF 2 (OH) 2 +H 2 O→HBF (OH) 3 +HF (fast)
HBF (OH) 3 +H 2 O→HBO 3 +HF (fast)
Because its first-stage hydrolysis reaction is very slow, it is the control step of the entire hydrolysis reaction, which limits the generation rate of HF in the acid solution. At 25°C, the equilibrium constant of the first-stage hydrolysis reaction is:
K = [HFB 3 OH] [HF] [HBF4] = 2.3×10-3
As the temperature increases, the equilibrium constant of the first-stage hydrolysis reaction also increases. The relationship between the equilibrium constant of the hydrolysis reaction and temperature in this step is shown in Table 1. It can be seen from this that as the temperature increases, although the hydrolysis constant of the first step also increases exponentially, it is still a very small value. When 12% HBF4 was kept at a constant temperature of 80°C for 6 hours, the experimental result of the degree of hydrolysis was 6.8%, and the calculated value using the equilibrium constant was 7.03%. The concentration of hydrofluoric acid was only 0.186%, which is far lower. HF concentration in soil acid.
(2) HBF 4 reaction speed with clay and ability to dissolve clay
The reaction speed of HBF 4 with the core, especially the reaction speed with the core at higher temperatures, is a key issue whether it can be used for deep acidification. Therefore, the reaction rate of HBF4 with cores at higher temperatures was studied and compared with the reaction rates of soil acid and cores (Table 2).
It can be seen from this that after 1 hour of reaction, the reaction between earth acid and rock core has basically ended. However, the reaction between HBF4 and the core continued until 10h. Therefore, even at a temperature of 80°C, HBF4 has a much slower reaction rate than earth acid, which indicates that it can be used for deep acidification treatment. This also proves that although the increase in temperature accelerates hydrolysis, due to the small concentration of HF in the HBF 4 solution, the reaction speed between it and the core is still much slower than the reaction speed between earth acid and the core.
It should be particularly pointed out that although HBF4 reacts slowly with rock core, its ability to dissolve rock core is no worse than soil acid. The complete dissolution rate of soil acid reaction is 54.6%. After 12 hours of reaction, the dissolution rate of HBF4 to the core is 50.0%. These two values are very close. Therefore, HBF4 is an acid with slow reaction speed and strong solubility.
（3��ＨＢＦ４ Stabilizing effect on clay and formation particles
Fluoroboric acid has the effect of stabilizing clay and formation particles. This effect has both chemical and physical factors. It is mainly because the HF hydrolyzed by fluoroboric acid destroys the nature of clay minerals and reduces the cation exchange capacity of the residual soil. At the same time, the boron in its hydrolyzate replaces the aluminum in the clay, and together with the clay, forms a layer of insoluble borosilicate covering, connecting the particles to the rock skeleton.
Field application of fluoroboric acidField application【2】
Based on the literature and the characteristics and pollution conditions of Shenyang Oil Dimethylpyridine Reservoir, the following acid solution construction technical measures were formulated: ① Use 10% to 12% low-concentration hydrochloric acid in the pre-fluid to prevent damage to the cement sheath and calcium barrier. Channeling phenomenon occurs in the reservoir; ② The main acid liquid is mainly 6% ~ 8% fluoboric acid + 13% ~ 15% HCL, so as to achieve the purpose of relieving damage without seriously damaging the reservoir structure; ③ Properly extend the shut-in reaction time , give full play to the role of fluoboric acid in stabilizing formation particles and preventing formation sand from being acidified. From 2008 to 2009, fluoroboric acid acidification was carried out on-site for 22 wells, with a cumulative oil increase of 6860t that year. The typical production well section 45-65 in front of the well is 2332.6~2651.1m. Due to multiple well cleaning operations, the oil layer was contaminated, causing blockage in the near-well area and reducing the permeability. Moreover, the well produced sand, which may cause acidification. Sand production intensifies. For the above reasons, 10% hydrochloric acid was first used for pretreatment, and then fluoroboric acid was used to remove the oil layer. The well was constructed on April 9, 2008, with a daily oil increase of 3 tons, and a cumulative oil increase of 342 tons throughout the year. Judging from the statistical results of the 22 wells that have been constructed, most of the wells did not affect the normal production of the oil wells due to sand production after the blockage was released, proving that fluoroboric acid can not only relieve the blockage of the oil layer, but also have a stabilizing effect on formation particles.
References
[1] (U.S.) M.J. Economides, K.G. Nolty, Reservoir Production Increase Measures (Updated Edition), Petroleum Industry Press, June 1991, 1st edition, page 574
[2] Qin Wei. Application of fluoboric acid plugging technology in Shenyang Oilfield [J]. Petroleum Geology and Engineering, 2011, 25(S1): 65-66.
