Views: 6 Author: Site Editor Publish Time: 2022-01-22 Origin: Site
Hydroxyethyl cellulose (HEC) is widely used in many industrial sectors for its excellent properties of thickening, suspension, dispersion and water retention. Especially in the oil field, HEC has been used in drilling, completion, workover and fracturing processes, mainly as a thickener in brine, and in many other specific applications.
HEC properties to the use of oil fields
(1) Salt tolerance:
HEC has excellent salt tolerance for electrolytes. As HEC is a non-ionic material, it will not be ionized in water medium and will not produce precipitation residue due to the presence of high concentration of salts in the system, resulting in the change of its viscosity.
HEC thickens many high concentration monovalent and bivalent electrolyte solutions, while anionic fiber linkers such as CMC produce salting out of some metal ions. In oilfield applications, HEC is completely unaffected by water hardness and salt concentration and can even thicken heavy fluids containing high concentrations of zinc and calcium ions. Only aluminum sulfate can precipitate it. Thickening effect of HEC in fresh water and saturated NaCl, CaCl2 and ZnBr2CaBr2 heavy electrolyte.
This salt tolerance gives HEC the opportunity to play an important role in both this well and offshore field development.
(2) Viscosity and shear rate:
Water-soluble HEC dissolves in both hot and cold water, producing viscosity and forming fake plastics. Its aqueous solution is surface active and tends to form foams. The solution of medium and high viscosity HEC used in general oil field is non-Newtonian, showing a high degree of pseudoplastic, and the viscosity is affected by shear rate. At low shear rate, HEC molecules are randomly arranged, resulting in chain tangles with high viscosity, which improves viscosity: at high shear rate, molecules become oriented with flow direction, reducing resistance to flow, and viscosity decreases with the increase of shear rate.
Through a large number of experiments, Union Carbide (UCC) concluded that the rheological behavior of drilling fluid is nonlinear and can be expressed by power law:
Shear stress = K (shear rate)n
Where, n is the effective viscosity of the solution at a low shear rate (1s-1).
N is inversely proportional to shear dilution. .
In mud engineering, k and n are useful when calculating effective fluid viscosity under downhole conditions. The company has developed a set of values for k and n when HEC(4400cps) was used as a drilling mud component (table 2). This table applies to all concentrations of HEC solutions in fresh and salt water (0.92kg/1 nacL). From this table, the values corresponding to medium (100-200rpm) and low (15-30rpm) shear rates can be found.
Application of HEC in oil field
(1) Drilling fluid
HEC added drilling fluids are commonly used in hard rock drilling and in special situations such as circulating water loss control, excessive water loss, abnormal pressure, and uneven shale formations. The application results are also good in drilling and large hole drilling.
Due to its thickening, suspension and lubrication properties, HEC can be used in drilling mud to cool iron and drilling cuttings, and bring cutting pests to the surface, improving the rock carrying capacity of the mud. It has been used in Shengli oilfield as borehole spreading and carrying fluid with remarkable effect and has been put into practice. In the downhole, when encountering very high shear rate, due to the unique rheological behavior of HEC, the viscosity of drilling fluid can be locally close to the viscosity of water. On the one hand, the drilling rate is improved, and the bit is not easy to heat up, and the service life of the bit is prolonged. On the other hand, the holes drilled are clean and have high permeability. Especially in hard rock structure, this effect is very obvious, can save a lot of materials. .
It is generally believed that the power required for drilling fluid circulation at a given rate is largely dependent on the viscosity of the drilling fluid, and the use of HEC drilling fluid can significantly reduce hydrodynamic friction, thus reducing the need for pump pressure. Thus, the sensitivity to loss of circulation is also reduced. In addition, the starting torque can be reduced when the cycle resumes after shutdown.
HEC's potassium chloride solution was used as a drilling fluid to improve wellbore stability. The uneven formation is held in a stable state to ease the casing requirements. The drilling fluid further improves rock carrying capacity and limits cuttings diffusion.
HEC can improve adhesion even in electrolyte solution. Saline water containing sodium ions, calcium ions, chloride ions and bromine ions is often encountered in the sensitive drilling fluid. This drilling fluid is thickened with HEC, which can keep gel solubility and good viscosity lifting ability within the range of salt concentration and weighting of human arms. It can prevent damage to the producing zone and increase drilling rate and oil production.
Using HEC can also greatly improve the fluid loss performance of general mud. Greatly improve the stability of mud. HEC can be added as an additive to a non-dispersible saline bentonite slurry to reduce water loss and increase viscosity without increasing gel strength. At the same time, applying HEC to drilling mud can remove the dispersion of clay and prevent well collapse. The dehydration efficiency slows down the hydration rate of mud shale on the borehole wall, and the covering effect of long chain of HEC on the borehole wall rock strengthens the rock structure and makes it difficult to be hydrated and spalling, resulting in collapse. In high permeability formations, water-loss additives such as calcium carbonate, selected hydrocarbon resins or water-soluble salt grains may be effective, but in extreme conditions, a high concentration of water-loss remediation solution (i.e., in each barrel of solution) may be used
HEC 1.3-3.2kg) to prevent water loss deep into the production zone.
HEC can also be used as a non-fermentable protective gel in drilling mud for well treatment and for high pressure (200 atmospheric pressure) and temperature measurement.
The advantage of using HEC is that drilling and completion processes can use the same mud, reduce the dependence on other dispersants, diluents and PH regulators, liquid handling and storage are very convenient.
(2.) Fracturing fluid:
In the fracturing fluid, HEC can lift the viscosity, and HEC itself has no effect on the oil layer, will not block the fracture glume, can fracture well. It also has the same characteristics as water-based cracking fluid, such as strong sand suspension ability and small friction resistance. The 0.1-2% water-alcohol mixture, thickened by HEC and other iodized salts such as potassium, sodium and lead, was injected into the oil well at high pressure for fracturing, and the flow was restored within 48 hours. Water-based fracturing fluids made with HEC have virtually no residue after liquefaction, especially in formations with low permeability that cannot be drained of residue. Under alkaline conditions, the complex is formed with manganese chloride, copper chloride, copper nitrate, copper sulfate and dichromate solutions, and is specially used for proppant carrying fracturing fluids. The use of HEC can avoid viscosity loss due to high downhole temperatures, fracturing the oil zone, and still achieve good results in Wells higher than 371 C. In downhole conditions, HEC is not easy to rot and deteriorate, and the residue is low, so it will basically not block the oil path, resulting in underground pollution. In terms of performance, it is much better than the commonly used glue in fracturing, such as field elite. Phillips Petroleum also compared the composition of cellulose ethers such as carboxymethyl cellulose, carboxymethyl hydroxyethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose and methyl cellulose, and decided that HEC was the best solution.
After the fracturing fluid with 0.6% base fluid HEC concentration and copper sulfate crosslinking agent was used in Daqing oilfield in China, it is concluded that compared with other natural adhesions, the use of HEC in fracturing fluid has the advantages of "(1) the base fluid is not easy to rot after being prepared, and can be placed for a longer time; (2) the residue is low. And the latter is the key for HEC to be widely used in oil well fracturing abroad.
(3.) Completion and workover:
HEC's low-solid completion fluid prevents mud particles from blocking the reservoir space as it approaches the reservoir. The water-loss properties also prevent large amounts of water from entering the reservoir from the mud to ensure the reservoir's productive capacity.
HEC reduces mud drag, which lowers pump pressure and reduces power consumption. Its excellent salt solubility also ensures that there is no precipitation when acidizing oil Wells.
In completion and intervention operations, HEC's viscosity is used to transfer gravel. Adding 0.5-1kg HEC per barrel of working fluid can carry gravel and gravel from the borehole, resulting in better radial and longitudinal gravel distribution downhole. The subsequent removal of the polymer greatly simplifies the process of removing workover and completion fluid. On rare occasions, downhole conditions require corrective action to prevent mud from returning to the wellhead during drilling and workover and circulating fluid loss. In this case, a high-concentration HEC solution can be used to quickly inject 1.3-3.2kg of HEC per barrel of water downhole. In addition, in extreme cases, about 23kg of HEC can be put into each barrel of diesel and pumped down the shaft, slowly hydrating it as it mixes with rock water in the hole.
The permeability of sand cores saturated with 500 millidarcy solution at a concentration of 0. 68 kg HEC per barrel can be restored to more than 90% by acidification with hydrochloric acid. In addition, the HEC completion fluid containing calcium carbonate, which was made from 136ppm of unfiltered solid adult seawater, recovered 98% of the original seepage rate after the filter cake was removed from the surface of the filter element by acid.