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Basic Knowledge
5.2 Fluid loss additive
  • Detailed description

When the slurry flow the permeable stratum under the pressure, the filter phenomenon will be produced. The water separated from the slurry and flow into the ground, and the cement particles are all leaved, this phenomenon is called fluid loss. If the fluid loss can’t be controlled, a serial of serious consequences will produced and lead to construction fail. Along with the volume of water phase reduction, the cement slurry’s density increases gradually, and that makes the properties of slurry (such as rheological behavior and thickening time, etc.) deviated from the original design. If there is too much water permeating into the ground, the slurry will be unable to pumped, and this is the common said irrigation sausage. That will make the cementing failure, and causing significant damage. The fluid loss additive is used to reduce the slurry filtration. The purpose is to 1. Prevent the slurry premature dehydration in the permeable stratum, especially in the tail pipe note cement.2. Protect the water sensitivity formation. 3. Improve cement squeeze work.
The fluid loss of the neat cement slurry in 30 minutes is usually more than 1500 ml. Generally, the fluid loss is adjusted according to the different type cementing job. To a casing cementing work, the fluid loss should be controlled below 250 ml, and to crowded cement work, fluid loss control is the key, the biggest must not exceed 150 ml. In order to guarantee the cementing quality, the requirements of cementing work to the fluid loss become more and more strict. Usually the fluid loss must to be controlled below 50 ml, even fewer than 30 ml.
Generally, the mechanism fluid loss additive can be explain as following
1. Improve cement slurry particle size distribution. The particle dispersion degree of the cement slurry has an important impact to the slurry fluid loss, and sometimes certain dispersant must be used, if not, no matter how much fluid loss additive, it is difficult to control the fluid loss.
2. It makes the cement slurry gap producing sticky, and also has adverse effect to the cement slurry, can increase the slurry viscosity and making the liquidity becoming poor.
3. There is a permeability membrane between the filter cake and the first few layer of porosity filter medium, such as polyvinyl alcohol kind of fluid loss additive which is to form a density film, and polymer kind of fluid loss additive is to form a resistance cement filter cake.
Fluid loss additive products can be divided into two categories: particle materials and water-soluble polymer materials.
In the particles materials, bentonite is the first used. The bentonite with its tiny particles setting into the filter cake and cements particles make the filter cake structure compact, and having a lower permeability. In addition, the latex also has a very good filtration performance. The colloidal particle size is smaller than cement particles, and has good elasticity. Latex with its good filtration, anti-fluid channeling and toughening performance, becomes the hot research advantage at home and abroad.
In the water-soluble high polymer materials, natural modified high polymer materials and water-soluble polymer has become an important component of fluid loss additive. In addition, the international also does many researches on environmental protection fluid loss additive.
The modified fiber is one of fluid loss additives which are used more in natural water soluble products. The modified fiber which can be used as fluid loss additive have HEC (hydroxyl ethyl cellulose) and CMHEC (carboxy methyl hydroxyl ethyl cellulose), etc. The HEC comprehensive performance is compatible, but the viscosity is too large in low temperature and the ash cement in is difficult. When the temperature is high, the viscosity is too low and fluid loss is not easy to control bellow the 50 ml, and settlement is serious. Now it is still in applications. The performance of the CMHEC is better than the HEC. The common faults of modified fiber are poorly water soluble, high viscosity when mixing in low temperature. And the low viscosity when the temperature is high make the slurry settlement seriously, temperature and salt resistant poor and set cement strength development delay, etc.
Synthetic polymer type fluid loss additive has wide variety and excellent performance, and some characteristics which can not be matched by natural products, thus it becomes the key of various countries' researchers racing to develop.

  1. Non-ionic polymer

PVA system is a kind of non-ionic fluid loss additive used earlier in home. PVA and borax cross linking agent and dispersant mix into a product. Generally PVA1788 is used to mix in the home. The using temperature of this system is up to 95, if the temperature is higher than it, the cross-linking will be destroyed, and filtration control performance will become poor. In addition, the system salt resistance (NaCl) performance is poor, salt resistance usually is no more 5%; and the adaptable for cement is poor. For different cement varieties, factory and batch, the performance can vary widely. At present there is a trend of most fluid loss additive is instead by AMPS polymer type.

  1. Anionic type polymer

Anionic type polymer fluid loss additive is researched the most widely at home and abroad and its type of products is also the most, its monomer including two kinds the ion and anionic monomer. The ion monomer mainly includes: AM (acrylamide), NVP (N-vinyl pyrrole), NNDMA (N, N-two methyl acrylamide), St (styrene), VI (ethylene sulfonated), VFA (ethylene armour amide), VP (ethylene pyridine) and VMAA (N-methyl-N-vinyl ethyl amide), etc. In these monomers, AM is easy hydrolysis, so its content in the copolymer should not be too much. The NNDMA introduces not easily hydrolyzed group, so its high temperature resistant performance enhanced obviously, but this monomer is expensive, in China it is still in the test stage. The anionic monomer mainly includes two kinds: one kind is sulfonic acid salt monomer, such as AMPS (2-acrylamide base-2-methyl c sulfonic acid) SS (styrene sulfonic acid salt) VS (ethylene sulfonic acid salt) PS (propylene sulfonic acid salt). Another kind is the carboxylic acid salt monomer, such as AA (acrylic) MAA (methyl methacrylate) HEA (hydroxyethyl acrylate) IA (itaconic acid), etc
AMPS has good high-temperature salt resistance ability, and its polymerization activity is high, the usage becomes wider, the early patent in the United States such as no. 4015991 pointed out that 2-acrylamide base-2-methyl sulfonic acid (AMPS) copolymer slurry can be used as fluid loss additive, but it will reduce the strength of the slurry and increases the thickening time. AMPS tripolymerization fluid loss additive has batch production in the domestic at present, but the tripolymerization fluid loss additive can only be used in half-saturation salt water, and heat resistance only can reach 150℃.Domestic and foreign has started the AMPS quadripolymer products research, the relevant application reports in foreign have appeared, such as patent number 5294651 of Phillips company patent is use AMPS/AM/AA/NVP as monomer, and through a variety of polymerization and get quadripolymer fluid loss additive (H-D344). Its using temperature is wide, high temperature resistant salt resistance performance is good, but the price is expensive. The kelioil engineering material and technology company CG610 series fluid loss additives belong to AMPS quadripolymer product (patent no. 201010108900.X). These CG610 series fluid loss additives can be used in a wide temperature range and can control the fluid loss effectively. The temperature resistant can reach to 150 ℃ and if adding some salt into the slurry the temperature resistant can reach to 180 ℃. The fluid loss can be controlled bellow 50ml and if the fluid loss additive dosage increases, the fluid loss will bellow 30ml. this product also has a good salt resistant property and can be used in the half-saturation or even saturated salt water if the dosage is increased properly.
The fluid loss additive technical requirements

    The evaluation method for oil cement additives-- part 2: fluid loss additive: has the following performance requirement to the slurry with fluid loss additive:

Table 2-1 the properties of cement slurry with CG series of fluid-loss additives

Item

Technical index

Initial consistency,Bc

≤30

Shape of thicken-time curve

Normal

Transition time from 40 Bc to100Bc,min

≤40

Fluid loss (18% brine),mL

≤150

Free fluid, %

≤1.4

24hcompressive strength,MPa

≥14

Table 2-2 the properties of cement slurry with CG series of fluid-loss additives in 18%brine system

Item

Technical index

Initial consistency,Bc

≤30

Shape of thicken-time curve

Normal

Transition time from 40 Bc to100Bc,min

≤40

Fluid loss (18% brine),mL

≤250

Free fluid, %

≤1.4

24hcompressive strength,MPa

≥14

The industry standard has set the minimum requirements of the fluid loss additive, at present stage, the fluid loss additive as a high quality product must meet the following requirements
1. Fluid loss: the fresh water fluid loss should be controlled bellow 50ml and even 30ml. the half-saturation salt water fluid loss should bellow 120ml and even 80ml.
2. The initial consistency: 10~25Bc is the best and the lower of the slurry initial consistency, the slurry liquidity is better and the pumping is easier. But if the initial viscosity is too low, the slurry system will be easy to settlement, and the free fluid will increase, so the initial consistency should be no less than 10 Bc. And the bigger the initial consistency, the slurry is more stable. But it is difficult to pumping; therefore the initial consistency generally can't more than 25 Bc.
3. The shape of thicken-time curve: the shape should be normal. When the slurry began to thicken, the consistency can reach to 100 Bc soon. The thick time of quality fluid loss additive products should be within 10 min when the consistency increasing from 40 Bc to100 Bc.
4. The influence to the set cement strength: usually, the fluid loss additive can affect the cement hydration reaction. And some fluid loss additive can reduce the set cement strength. But good fluid loss additive has little or no affection on the set cement strength.
5. Apparent viscosity. Usually, the fluid loss additive is high polymer. The viscosity of its water solution is high. This is hard for the liquid product to be pumped into the barrel. So the apparent viscosity of liquid products should be small, usually 1000 ~ 6000 cP is good, and should not be more than 20000 cP.
6. Dosage. The bigger the dosage of the fluid loss additive, the smaller of the fluid loss is. But, increasing the dosage will increase the initial consistency. And so the above several indexes have the contradiction, and many times cannot achieve the best at the same time. Usually, the dosage of pure fluid loss additive should be bellow 2% (by the cement weight) and liquid product should not bellow 5% (by the cement weight).