Acid Removal of Scale and Fines at High Temperatures

Abstract

Abstract With the onset of water production, fines are usually mobilized to the wellbore, resulting in screen plugging. Fines-plugged screens and/or gravel pack can then accelerate scale precipitation, especially in high-temperature formations. Due to the type of metallurgy, long acid contact times and high acid sensitivity of the formations, removal of the scale with HCl acids has been largely unsuccessful. In high-temperature wells, the choice of acid must be done such that the formation and the completion are not damaged, but the scale and the fines are removed. A series of tests conducted on screens and clay-rich cores show that a new organic acid system, which is highly biodegradable, can successfully remove the calcium carbonate scale and fines to stimulate production. Core flood testing demonstrates that this organic acid system can effectively remove calcium carbonate scales and fines at temperatures up to 204°C(400°F). In addition to its reactivity, the acid system, when combined with corrosion inhibitor, exhibits very low corrosion at high temperatures. Corrosion tests show that at 177°C (350°F), the corrosion rate caused by this organic acid is 0.00049 g/cm2(0.001 lb/ft2) on 22-Cr for 16 hours. Acid soaking to completely remove scale prior to removal of fines is now possible at temperatures in excess of 121°C (250°F). Successive removal of plugging fines from screens can then proceed without further damage to the integrity of the screen. Introduction Carbonate scale mixing with fines is a major problem in some high-temperature wells in the North Sea1. Fines migration and mineral scale deposition that plug near-wellbore region, including gravel pack screens, are the main reason for reduced production, especially after water breakthrough in water-drive reservoirs. As water ensues, fines are released due to changes in viscosity, velocity and ion exchange. Mobilized fines plug gravel pack screens and formation adjacent the well. The fines can then act as a nucleation site to precipitate scale. In turn, the precipitated scale captures more fines, and finally, the scale mixed with fines totally chokes the production zone. This process is aggravated by high temperature and high developed pressure drops across the screens. EDTA and other chelating agents have been used to remove such scale in high-temperature wells2,3. But low solubility with acid and low compatibility with corrosion inhibitors limits their application for removing scale that is mixed with fines in high-temperature wells. Acid treatments have been used effectively for the removal of fines and scale. However, removal of these fines and scale with HCl-based acid fluids poses problems such as high reactivity, high corrosivity, and sludging tendencies when the acid contacts crude oils, and the HCl sensitivity of some formations4–6. These problems are intensified by high temperature and high pressure. Subsea flow lines, especially in deepwater fields, present a large-scale corrosion problem with pipelines that may extend for several miles7. Some corrosion problems may be alleviated by the use of a corrosion inhibitor, but the adsorption of corrosion inhibitors on the inside of pipe surface may remove the inhibitor and reduce the protection to corrosion caused by the live acid on the downhole tubulars. The adsorption of the inhibitors on the rock may block the pore space, reducing water wettability and therefore, reduce the relative permeability to oil or gas8,9. A need is, therefore, demonstrated to find a new acid fluid, which has low corrosivity, environmental acceptability, and suitability for high-temperature reservoirs.