Inhibition and Removal of Low-pH Fluid-Induced Asphaltic Sludge Fouling of Formations in Oil and Gas Wells

Abstract

Abstract Many low pH fluids are used in the Petroleum Industry to treat oil and gas wells. These include acid fracs, pickling acid, near wellbore acid jobs and scale inhibitor squeeze chemicals. All of these fluids can cause asphaltic sludge (rigid film emulsion) in some wells that can reduce production of oil and gas, plug a well or oil wet the near wellbore formation by reacting with the asphaltenes or asphaltic resins in an oil. This paper will provide information on how low pH fluids react with asphaltenes or asphaltic resins in oil, explain why the standard test results may be inaccurate in predicting these reactions and present a non-standard method which has been used for a number of years to stop the formation of asphaltic sludge in any well at any iron concentration. Case histories of removal treatment of asphaltic sludge from wells and inhibition treatment will be presented. Introduction Asphaltenes are defined by solubility test as those components in crude oil that are precipitated by large volumes of short chain n-alkanes. They are polar, benzene ring structured platelets containing carbon, hydrogen, oxygen, nitrogen, sulfur and may contain various metals. The asphaltene platelets are coated by layers of stabilizing resins (maltenes) that suspend the asphaltenes in the oil. The removal of these resins by large volumes of short chain n-alkanes will cause the asphaltenes to precipitate and agglomerate so they can be separated. It is interesting that the use of different n-alkanes will give varying weights of asphaltenes for the same crude oil1. The asphaltene, when precipitated, is a black or dark brown, brittle, amorphous solid which is dispersible in aromatic solvents. This dispersibility, in aromatic solvents, has led to the general belief that asphaltenes can be removed from anywhere in a well or formation with just the use of aromatic solvents2. This belief ignores the polarity of the asphaltene, which gives it the ability to adsorb onto any charged surface it encounters3. Asphaltenes will actually deposit from dispersion in aromatic solvents onto charged surfaces. For many decades the evidence has been mounting that asphaltenes are responsible for much of the wettability changes seen in the oil field formations5,6,7. The destabilization of asphaltenes in the presence of charged particles or surfaces is a major problem in most oilfields. Asphaltenes are destabilized by gas breakout (pressure drop), condensate treatments, gas or gas liquid injection (CO2, NGL floods), hydrochloric acid jobs, crude blending and high shear or streaming potential8,16. If this destabilization occurs in a formation that contains charged minerals the asphaltenes can adsorb and alter wettability and permeability. Wells damaged by asphaltic sludge (see Figure 1 for a picture of the rigid film emulsion) following an acid job are the most visible example of these problems9,16. Increased water production and loss of oil and gas are a frequent occurrence over the months following acid jobs in some wells. Laboratory studies have shown that asphaltenes will adsorb through water layers onto a charged mineral surface4. The charge bond between formation minerals and asphaltenes is so strong that aromatic solvents alone will not separate them4,10. Dolomite formations with large quantities of charged minerals are especially susceptible to damage11,12. The asphaltene can cause severe damage to wells by depositing on the formation minerals1,2,7,8,9,10,13,14. If asphaltenes are not removed before an acid job is performed, the acid will not be able to remove any inorganic damage coated with the asphaltenes. If aromatic solvents will not remove asphaltenes from formation minerals, acid jobs using aromatic solvents as preflushes are less effective. It is very important to find chemicals to remove these deposits and keep them from redepositing or to inhibit their destabilization and deposition on charged surfaces18. In this paper we will discuss other low pH fluids that have been observed to cause asphaltene problems in wells and tests to predict and prevent these problems. Laborabory Testing Various tests are used to determine if acid sludging or asphaltene precipitation occurs when various fluids are mixed with specific crude oils. The tests are used to determine if a specific crude oil is sensitive to various fluids, if formation minerals will have their wettability altered or if asphaltenes will be destabilized.