The Bubble Acidification Process is a novel acidification stimulation technique for low-permeability, low-pressure, and water-sensitive formations. By adding surfactants and foam stabilizers to conventional acid systems, and mixing them with nitrogen through a foam generator, a foam system is formed, which combines the properties of foam fluids and acidification capabilities. It is particularly suitable for acidification of multi-layer heterogeneous oil reservoirs, low-pressure and low-permeability wells, as well as for the repeated acidification of mature wells.
The foam has a strong carrying capacity, which facilitates transporting particles and debris generated by the acid rock reaction to the ground, with the amount of particles usually higher than that of regular acid.The foam acid is highly effective, with a concentration up to 7 times greater. Upon reaching the wellbore due to a sudden drop in pressure, the foam rapidly expands, creating high pressure within the wellbore and generating backflow, which facilitates the return of spent acid. The foam acid, rich in gas and low in liquid volume, is less likely to cause clay swelling, making it particularly suitable for acidification of water-sensitive clay reservoirs. Additionally, the shear-thinning characteristics of the foam fluid temporarily堵塞 the permeable layers, allowing the acid to redirect into lower permeability layers, distributing more evenly and effectively, achieving the goal of directional acidification. The foam acid is generally suitable for operations at well depths of 1500 to 2500 meters, requiring nitrogen generators, foam generators, and similar equipment. Its application has been more prevalent internationally, particularly for multi-zone matrix directional acidification.
I. Basic Properties of Foam Acids
The composition of the foam acid:
Foam acid is a composition consisting of both vapor and liquid additives, with the vapor phase making up a significant portion.52% to 90%, with the remainder being in the liquid phase. The general composition includes:
（1) In the gas phase: It can be N2, CO2, or other mixed inert gases, with N2 being commonly used.
（2) Liquid Phase: Can be hydrochloric acid or other inorganic and organic acids. The concentration of hydrochloric acid is generally 15% to 30%, with 28% being the most commonly used.
（3) Additives: Include foaming agents, defoamers, corrosion inhibitors, iron stabilizers, etc. Foaming agents are generally surfactants and are a key component of foam acid. The selection of foaming agents should be based on the type of acid fluid and the characteristics of the oil and gas reservoir, and must meet the following conditions: good foaming performance, good stability of the foam, good compatibility, and easy to burst under reduced pressure, etc. The general dosage is 0.5% to 1.0%.
2. The Acidification Principle of Foam Acid
Foam acid is a dispersion system of gas stabilized by a foaming agent in an acidic solution, with the gaseous phase supplied by a pressure blower's nitrogen and the liquid phase consisting of various acidic solutions based on the well conditions. This foam solution is pumped into high-permeability aquifers, gradually increasing the fluid flow resistance, leading to a gas-blocking effect in the throat. Under the compounded gas-blocking effects, foam acid is then used to react with the low-permeability strata and rocks, forming more dissolution channels to relieve pollution and blockages in the low-permeability layers and improve the oil well's fluid production profile. Subsequently, foam acid drainage fluid is injected to aid in the removal of residual acid and to promote induced production.
3. Characteristics of Foam Acid
Foam acid is a new type of acid fluid, compared to conventional acids, featuring low liquid column pressure, strong backflow capability, high viscosity, minimal filtrate loss, minimal damage to formations, long acid action distance, easy construction, lower overall costs, and higher economic benefits.
（1) High vapor phase ratio, low liquid phase ratio. Generally, the vapor phase occupies 60% to 80%, resulting in a lower density.
（2) The viscosity is relatively high, typically ranging from 10 to 100 mPa.s, which is beneficial for reducing the loss of acidic fluid and the rate of acid-rock reaction.
（3) Low filtration loss. In low-permeability reservoirs, it is two orders of magnitude lower than the general liquid filtration coefficient; in high-permeability reservoirs, it is comparable to the gel acid filtration coefficient.
（4) Low internal flow friction.
（5) Fast drainage speed. Generally, 90% of the liquid can be drained within 10 hours.
（6) High capacity for carrying non-acid-soluble substances.
（7) Deceleration results are impressive.
（8) Minimized reservoir damage
（9) The construction process is relatively complex, requiring the addition of nitrogen generation equipment and foam generators, etc.
4. Factors Affecting the Stability of Foam Acid
To achieve the desired acidification effect of the foam acid, the stability of the foam acid system is crucial. During the acid injection process, the foam acid should retain minimal water loss, allowing it to penetrate deeply into the formation.
（1) Increased Liquid Viscosity: Enhancing the liquid viscosity improves the stability of the foam acid system. This can be achieved by adding high molecular weight polymers that are resistant to both acids and temperature.
（2) Interfacial Film Strength: Selecting the appropriate surfactant and foam stabilizer can enhance the interfacial film strength, thereby increasing the stability of the foam.
（3) Foam Quality: The foam has high quality and excellent stability. The foam quality of commonly used foam acid typically ranges from 60% to 85%. Foam with too low or too high quality is prone to breakage and instability. The foam's liquid viscosity, filtrability, fluid return, and friction resistance are all related to the foam quality.
（4) Temperature: As temperature rises, gas molecule movement accelerates, leading to a decrease in foam stability.
（5) Surfactant Adjuvants: Some surfactant adjuvants, like ethanol and n-butanol, typically have defoaming properties, whereas others can stabilize foam. Therefore, compatibility tests are necessary to select the appropriate adjuvants.
5. Performance Evaluation of Foam Acid
Foam acidizing fluids generally exhibit good stability, high viscosity, excellent suspending sand performance, low friction, and low damage. The success of foam acidizing operations largely depends on the performance of the foam acidizing fluid used. To ensure the foam acid has desirable properties, extensive evaluation work is required. The performance evaluation of foam acid includes:
（1) Rheology. The rheology of the foam acid is generally described using a pseudo-plastic mathematical model.
（2) Stability. The stability of the foam acid is measured using the Ross-Mill instrument, typically characterized by the half-life.
（3) Foam Quality: The foam quality is determined by the volume of gas within the total foam volume at a specific temperature and pressure. Generally, a foam quality of 75% to 80% yields effective foam acid.
（4) Viscosity. The viscosity of foam acid is generally 10~100 mPa.s, depending on the foam quality and shear rate.
（5) Loss coefficient. The loss coefficient of foam acid can be measured using a foam acid loss test instrument.
（6) Frictional Loss. The flow of foam acid inside pipes is generally laminar, resulting in lower frictional loss, which is approximately 60% of that of clean water.
Section II: Foam Acid Aeration Process
The foam acid acidification process involves thoroughly mixing an acidic solution that has been added with surfactants, stabilizers, and other additives with nitrogen in a ground foam generator, creating a stable foam that is then injected into the well. This foam, combined with the high-pressure hydraulic action and the chemical dissolution of the acid, acts on the reservoir to transform it. Due to the gas in the foam acid aiding in drainage, this process is particularly suitable for transforming reservoirs with low pressure and low permeability, which are difficult to drain, as well as for tapping into old wells and increasing production in low-pressure wells. Additionally, it can effectively address the challenges of difficult drainage and water-sensitive reservoirs in special wells. The bubbles in the foam acid can preventThe transfer of H+ to the rock surface slows down the acid-rock reaction rate, and the acidification of foamed acid can achieve a greater effective distance for the acidic fluid.
Foam Acid Aeration Process Features
The Foam Acidification process is a novel acidification stimulation technology that has emerged in recent years, highly effective for low-permeability, low-pressure, and water-sensitive formations. Its process characteristics include:
（1) The foam itself is selective, allowing more acidic fluid to penetrate into the middle and low permeability layers and oil layers.
Foam in porous media exhibits a high apparent viscosity, strong profile control capabilities, and a shear-thinning characteristic. Its plugging ability increases with the permeability. This allows for temporary plugging of high-permeability layers, diverting acid fluid into lower-permeability layers, and even distribution, achieving the goal of acidizing redirection.
（2) Stronger assistance in drainage capability
Due to the high gas content in the foam acid, the wellhead pressure drops upon completion and draining, causing a rapid expansion of the gas, which accumulates a significant amount of energy. This leads to a large pressure difference between the wellbore and the formation. Additionally, the low back pressure of the foam liquid enhances the draining efficiency of the foam acid. Simultaneously, the strong carrying capacity of the foam liquid ensures that most of the acid-insoluble substances and other particles are carried to the surface, guaranteeing the acidification effectiveness.
（3) Slower reaction speed compared to general acidic solutions.
The reaction rate of acid rock is dependent on the mass transfer rate of hydrogen ions to the rock surface. Foam acid, as a surface emulsion system, has a higher viscosity and less filtrate, significantly slowing down the mass transfer rate of hydrogen ions to the rock surface. Consequently, the effective distance of foam acid is greater than that of general acid solutions.
2. Foam Acid Aeration Basic Field Construction Techniques
Uncover the production casing in the wellbore→ Injecting paraffin oil to the bottom of the oil layer → Injecting the prefoam section → Injecting the prefoam acid → Injecting the main foam acid → Injecting the post-foam replacement fluid, shutting in the well for 1-2 hours to react, performing a low-density foam backflush to remove residual acid, opening the well to vent, and discharging spent acid.
3. Selection Principles for Acidification of Foam Acid
（1) Foam acid possesses deep penetration capabilities, suitable for acid fracturing operations in large-scale carbonate reservoirs.
（2) Acidification of multilayer heterogeneous oil reservoirs.
（3) Repeated acidification of the old well. 4) Suitable for reservoirs with difficultly controlled filtrate loss.
（5) Low-pressure, low-permeability, or water-sensitive reservoirs.
III. Application Examples1. Domestic Application (1) Application of Acid Foam in the East Sichuan Region: After decades of development, many wells in the Sichuan Gas Field have reached the late stage of exploitation. The formation pressure has severely depleted, and some water-producing formations are even unable to extract oil and gas due to the extremely low formation pressure, despite the presence of the corresponding water column.
Section 3: Application Examples
1. Domestic Application Status
（Foam Acid's Application in the East Sichuan Region
After decades of development, many wells in the Sichuan gas field are in the later stages of exploitation, with significant depletion of formation pressure. Some water-bearing formations, due to ultra-low formation pressure, prevent the extraction of oil and gas even with the presence of water columns, severely limiting the development of the gas field. As a result, an enhanced acid stimulation method has been proposed to increase production. By mixing acid with nitrogen to form a liquid foam, this method not only slows down the corrosion rate of the acid but also boosts the energy for the return of residual fluids, avoiding the contradiction of low formation pressure that prevents the return of residual fluids. Acid stimulation effectively improves the near-well permeability of the reservoir, restoring and enhancing the productivity of individual wells.
SinceIn 1986, following the successful foam acid fracturing test at Zhang 10 well, foam acid treatments were conducted sequentially with an efficiency of 75%. By May 1992, the cumulative increase in production reached 976.77×10^4 cubic meters, adding 2.54 million yuan in value and 740,000 yuan in profit. With the introduction of advanced nitrogen gas operation equipment in recent years, the application of foam acid fracturing has also improved, particularly for the tapping and transformation of some aging wells in the eastern and southern regions of Sichuan. Foam acid fracturing has demonstrated its superiority, with an average net increase of approximately 3.5×10^4 to 4.5×10^4 cubic meters per well per day, showing a highly significant acidification effect.
（2) Application of Foam Acidification Technology in Oil Sand Mountain Oilfield
The Oil Sand Mountain Oilfield, a mature field developed in the 1960s and 1970s, has most wells with low production rates and severely contaminated oil reservoirs. Due to the decline in reservoir formation energy and severe losses, conventional acid stimulation measures have been difficult to yield effective results. The application of foam acid stimulation technology has been successful in the middle...The 237 well underwent acidification, increasing the daily oil production from the previously producing well to around 10 tons, achieving favorable economic benefits. The foam acid acidification technology effectively reduced the amount of acid used, lowered the acidification cost, and is easy to return, causing minimal damage to the formation. To date, Qinghai Oilfield has applied this technology in 20 well construction sites, with an effectiveness rate of 87%, and an accumulated increase in production of nearly 4,000 tons.
（3) Application of Foam Acidification Technology at the M84 Well in Wenmingzhai Oilfield
Before acidification, Well M84 produced 68.8 m3 of fluid per day and 2.0 tons of oil per day; after foam acid treatment, the daily fluid production increased to 78.2 m3 and the oil production to 12.0 tons, with a decrease in water content and an increase in the dynamic fluid level by 291 meters, accumulating an additional 2203 tons of oil with an effective period of 220 days.
（4) Foam acidification has been applied in seven wells at the Liaohe Oilfield, including Jin 2-14-5203, Qi 2-22-307, and Huan 2-11-2010.
Seven wells have shown significant results after undergoing foam acid fracturing, with an average increase in production.12.5 tons, average increase by 8.8 times, cumulative increase of 1.24×10^4 tons.
（5) Application of Foam Acidification Technology in Xinjiang Bai66 Well
Please provide the Chinese content to be translated.The 66th well experienced no liquid output after completion. On May 12, 1998, a foam acid fracturing operation was carried out on the well. Post-acidification, the well produced 4.7 tons of liquid per day and 1.7 tons of oil per day.
2. Hyphen Corporation's Application of Foam Acid Aeration
Hengye Company has collaborated with the Foam Fluid Research Center of Petroleum University to conduct extensive basic research and field application experiments on topics such as deep acid fracturing with foam acid, foam diversion acid fracturing, and low-density foam acid drainage. For the foam acid fracturing technology for blocking in heterogeneous reservoir sandstone, through formulating, performance evaluation, and rheological experiments of foam acid, the rheological characteristics of foam acid fluid within the construction conditions allowed were obtained, and the flow laws of foam acid fluid in the wellbore and annulus were studied. Simultaneously, through experiments on temporary plugging and diversion of rock cores, it was concluded that foam fluids are selective to both high and low permeability cores and selective within water and oil-bearing cores.
Developed a high-pressure, acid-resistant foam generator capable of producing a low-density, uniform foam fluid, meeting the design requirements for pressure and acid resistance, and suitable for on-site foam acidification construction processes. Created a parameter design software for on-site acidification construction, providing rational process parameters for foam acidification, offering guidance for on-site production.
The acidification process has been widely applied in the Victory Oilfield and has been promoted as the recommended technique for gas well acidification, achieving favorable application results.
（1) Zhuangxi Oilfield Well Zhuang 121-2 undergoes foam acid fracturing
The well was deep.3421.2m, two production layers of sandy dolomite: Layer 1, 3364.6m to 3369.6m, perforated from 3368.0m to 3371.0m, perforation thickness of 3m, clay content of 41.17%, porosity of 4.7%, permeability of 0.10×10^-3 μm², carbonate content of 59.3%; Layer 2, 3372.9m to 3379.3m, perforated from 3372.9m to 3378.0m, perforation thickness of 5.1m, clay content of 18.65%, porosity of 14.27%, permeability of 67.6×10^-3 μm², carbonate content of 59.3%.
The wellFollowing the low liquid volume of approximately 5 tons after November 2004, the current daily liquid production is 4.7 tons, with oil production at 1.6 tons per day, and a water content of 65%. The liquid level is at 1983 meters (currently mixed with water; without water, it's around 2200 meters), and the flow rate is insufficient. Analysis of production and reservoir data suggests implementing a foam acid acidification to clear the blockage. The reservoir rock type is sandy dolomite with a high mud content, which is prone to particle migration and blockage during production. Since 1996, the well has undergone three acidification treatments, resulting in daily oil increases of 14 tons, 4 tons, and 3 tons, respectively. Analysis indicates potential for increased oil production through acidification. However, to achieve a better increase in oil production with this round of acidification, it is necessary to further expand the acidification radius. Currently, the conventional acid solution used cannot increase the acidification radius, and the acid will enter the formation through the channels of the previous three treatments, reducing the effectiveness of acidification. Therefore, it has been decided to conduct a foam acid test in this well, utilizing the分流 characteristics of the foam liquid to allow the acid to enter the low permeability flow zone, while also taking advantage of the slow reaction rate of foam acid to expand the acidification radius.
During construction, foam precursor acid injection15m³, foam density 0.80g/cm³, flow rate 11.3m³/h, injection pressure 18MPa; injected main body foam acid 15 cu. m, foam density 0.75g/cm³, flow rate 8.2m³/h, injection pressure 21MPa. After well closure reaction, the well was opened to vent, using low-density foam liquid for backcirculation well washing and acid removal.
This construction utilizes the temporary plugging effect of foam in the formation. After the pre-positioned foam acid enters the formation, based on the foam...The "block big, not small" characteristic allows the high-permeability layer to temporarily seal off, redirecting the main acidic fluid to the lower-permeability layer for more efficient and even distribution, achieving the goal of turning acidification. This expands the acidification area and enhances the acidification effect. Additionally, the main foam acid reacts more slowly with the rock, increasing the acidification radius. By using the foam liquid as the backflow fluid after acidification, the wellbore forms a temporary negative pressure, facilitating the easy expulsion of spent acid from the formation. The high-viscosity foam liquid has strong carrying capacity, aiding in bringing the fine particles and rock fragments generated by the acid-rock reaction to the surface, with particle quantities exceeding seven times that of regular acid. Post-construction of the well, the fluid production increased to 18.6 cubic meters, with a daily increase in oil production of 8.2 tons, demonstrating significant production enhancement.
（2) Well 62-9 at the Five-Pile Oilfield undergoes foam acid stimulation
The well is deep.3,350 meters, perforation section S2 layer from 3,098.5m to 3,107.0m, thickness 8.5m, porosity 13.8%, permeability
13.5 x 10^-3 μm², clay content 12.7%, S3 Lower Layer from 3301.2m to 3306.0m, post-firing 4.8m, porosity 16.3%, permeability 21.9 x 10^-3 μm², clay content 5.7%.
The well atIn June 1996, the S3 was put into production. In November 2002, the S2 was combined with pump testing and air replenishment. Initially, the daily liquid production was 12.1 m3/day, oil production was 11.4 tons/day, and water content was 6.4%. By June 2005, the daily liquid production dropped to 4.5 m3/day, oil production to 2.8 tons/day, and water content to 38.1%. From June 15 to July 21, 2005, pump testing and CO2 injection were conducted. After the well was opened, it continued with low liquid and moderate oil production (approximately 8.0 m3/day and 3.4 tons/day of oil). In November 2005, after acidification, the daily liquid production reached 18.7 tons, daily oil production was 14.5 tons, and water content was 22.6%. In February 2006, the liquid production plummeted to 5.7 tons, with insufficient fluid supply in the performance chart and no liquid level display. As the CO2 injection's effective period exceeds 4-5 months, the main reason for the current decline in liquid production is believed to be particle migration and pore blockage during production. It is recommended to implement acidification to clear blockages. Since the oil layer is divided into two sections with varying permeability, the previous acidification may have only mobilized one section. It is suggested that this time's acidification use foam acidification technology, leveraging the slow-acting properties of foam acid to expand the acidification radius and the selective plugging effect of foam acid to achieve uniform improvement of the formation.
Please provide the Chinese content to be translated.On April 19, 2006, the well was subjected to foam acidizing, with a foam acid density controlled at 0.7 to 0.8 g/cm³. A 12% HCl pre-acid was injected at 9 cubic meters, followed by a displacement of 12% HCl pre-acid at 3 cubic meters, a displacement of 10% HCl treatment acid at 20 cubic meters, and a displacement of 10 cubic meters of produced water. The well was shut in for a reaction period of 1 hour at a pressure of 19.5 MPa. Post-shutdown, a low-density foam with a density of 0.65 g/cm³ was used for acid displacement washing.
Following the well construction, the fluid production has recovered well, and the daily fluid production has returned to normal.24.3 m3, with a daily oil increase of 7.7 tons, the increase in production is significant.
