Well- and Formation-Damage Removal With Nonacid Fluids

Broaddus, Gene, SPE, Halliburton Services Introduction Well damage, in the form of various impediments to fluid flow through the formation rock, can reduce or completely eliminate production or injection rates of the wall. Prevention and/or removal of well damage have consequently become a major field of study and co ion within the petroleum industry. Although acid solutions have been used for well-damage removal since before the turn of the century, there are situations in which acid is not the best choice for damage removal. The most obvious condition under which acid is not applicable is where the damaging mechanism is not appreciably soluble in acid solutions, Use of acid hem would be a waste of time and material. Detrimental Effects The other reason for not selecting acid for well stimulations is that the acid may damage the formation and decrease production rather than stimulate it. Carbonate, sandstone, and shale formations are susceptible to acid damage. Some siliceous formations contain carbonate and clay cementing materials. When acid is used to stimulate these formations, fine particles can become unconsolidated and migrate to form plugs that reduce formation permeability. Even when the permeability is not reduced, production of formation particles may damage the production equipment. Sonic sandstones contain iron-rich chlorate clays, the iron, magnesium, and aluminum portions of which are soluble in acid solutions. Removal of these components destroys the clay structure and leaves the silica portion of the clay free to migrate and cause formation plugging. Another determinant to production occurs when acid solutions are applied to soft chalk formations, which tend to flow chalk particles after acid application. Formations with the following damaging materials and mechanisms should be stimulated only with nonacid solution: gyp scales, paraffin, asphaltenes, oil-based mud, water-based mud lost to naturally fractured carbonate reservoirs, bacteria and bacterial slimes, polymer damage, pump lubricants and oil carry-over in injection wells, water or emulsion blocks, and surfactant and corrosion inhibitor adsorption. In the remainder of this paper, each of these mechanisms and means of their removal am discussed. Gyp Scales. Gyp scale is calcium sulfate (CaSO4.2H2O) formed by pressure drop, cooling, mixing of incompatible waters, or a combination of these factors. These scales can form in tubulars, perforations, or in the formation. Zones with natural fractures or with induced fractures are likely to have scale deposits in the zone because the pressure drop occurs as the fluid enters the fracture. Induced fractures might extend vertically out of the zone of interest into a water zone and cause scale formation when the water of the two zones and/or the fracturing fluid am incompatible. Removal by mechanical means, such as hydrajetting, may be, the best method when scale deposits are only m the goods, but scale in die perforations or fractures requires a chemical means of removal. In the chemical removal of gyp scales, scales are either converted to acid-soluble form or disintegrated and circulated out of the hole. Basic solutions of potassium acetate, potassium glycolate, potassium citrate, or potassium hydroxide are the most commonly used converter/disintegrators. Although the disintegrated material can be c out, an HCl solution is most often used to dissolve the converted gyp scale. Chelating agents such as tetrasodium EDTA or trisodium nitrilotriacetic acid are used to dissolve gyp deposits. These compounds do not require subsequent acid treatments, and time required for scale dissolution is less than with converter/disintegrators; however, the volume of gyp scale removed per gallon of fluid is lower. Paraffin Deposits. Since the beginning of the petroleum industry, paraffin has been a source of trouble for the producer. Paraffin is a waxy crude off deposit. As in gyp-scale deposition, the mechanisms for paraffin deposition are cooling and pressure reduction. During winter months in cold climates, cold stimulation fluid injection causes paraffin depositions back into the formation. Deposition occurs in perforations, on tubulars, and in surface flowlines. Paraffin is commonly removed bysniping,hot-oil treatments, orapplication of solvents. Tubing and casing are scraped to move paraffin by lowering a metal scraper into the well on a wireline and pulling the scraper back to the surface. The scraper is designed so that it enters the well by its own weight, but upon being lifted, expands to contact the walls of the tubulars to scrape off paraffin and "swab" it back to the surface ahead of the scraper. Scraping is the fastest, most economical means of removing paraffin, but can be used only above the perforations; use of scrapers m the perforated interval would fill the perforations with paraffin. In the hot-oil method of paraffin removal, crude oil is heated by service companies specializing in the technique and then pumped down the annular space between the casing and tubing and out through the tubing, removing the paraffin. This method can be used safely only when paraffin is deposited above perforations because the heated crude oil can carry paraffin into the formation when it can be cooled by the formation and deposited in the pore space. Paraffin from perforations or the formation can be removed by aromatic paraffin solvents, such as xylene, toluene, or a mixture of one of these with a straight-chain hydrocarbon. JPT P. 685^