Performance Evaluation of Ionic Liquids as a Clay Stabilizer and Shale Inhibitor

Abstract

Abstract Most oil and gas producing formations contain clay minerals that were deposited during the sedimentation process, formed during the interaction of heat, pressure, and time on the naturally present minerals in the formation, or precipitated from fluids flowing from one area of the formation to others. The oil and gas industry has extensively investigated the role of these clay minerals during production and the potential permeability damage that can occur with swelling and migration of these clays. For many years, the effective use of salts such as KCl and NaCl in workover fluids for temporary clay stabilization has been well established. However, due to the potential environmental issues and the logistics of using large quantities of salts for this application, many operators have begun to search for alternative clay stabilizer products. This project details the investigation and evaluation of ionic liquids as a clay stabilizer and shale inhibitor additive to prevent clay swelling and migration in comparison to the industry-standard clay stabilizers. Ionic liquids are compounds that are liquids at ambient temperatures and consist entirely of cations and anions, usually a organic cation and a inorganic anion. Screening studies to determine the state of flocculation of clay-bearing fluids and to screen for shale inhibition were conducted with Capillary Suction Time tests (CST), Clay Pack Flow tests and core flow studies. This paper summarizes the results of the clay stabilizer screening studies conducted with ionic liquids and details the effectiveness of the ionic liquids as KCl substitutes, clay stabilizer additives, and shale inhibitors. Introduction It has been reported that approximately 97% of all petroleum reservoirs contain clay minerals. An important consideration with these minerals in the production of oil and gas is to protect the water-sensitive clays and reduce their potential to damage permeability. The two major mechanisms by which clays can cause permeability damage are swelling and migration. When a clay-containing formation comes into contact with a treatment fluid or water that is not in ionic balance with the formation, the clays absorb water into the crystalline structure. This water absorption causes the clays to swell and the particles to increase in volume, plugging the pores in which they reside. In clay migration, the clays can be dispersed by contact with a foreign fluid, or can be dislodged by produced fluids, causing migration thru the formation until a pore throat restriction is encountered. This restriction will cause bridging of the clay particles across the pore throat and result in plugging of the capillary. Clay stabilizing additives can be classified into two categories: temporary and permanent. Temporary clay stabilizing additives are materials that will prevent swelling and migration of clays during drilling, completion, and fracturing operations but will be easily removed by the formation's produced fluids following the treatment. The most common temporary clay stabilizers are simple inorganic cations such as NaCl, KCl, ammonium chloride and calcium chloride. Solutions of 2% to 7% by weight KCl are frequently recommended in treating fluids to minimize the swelling of clays and the migration of nonexpanding type clays. The most recent advances in clay stabilization have been realized in the area of permanent clay stabilizing additives. The most common permanent clay stabilizers are quaternary amine polymers. A monomolecular film of these polymers tightly binds with the clay surface by cation exchange; this film is not removed by the formation's produced fluids following the treatment. The more permanent stabilization is claimed due to the bridging of multiple cationic sites along the polymer chain. These additives leave the formation water-wet, and due to their cationic nature they may be utilized in acidic, neutral, and basic conditions.