NAF Filter Cake Removal Using Microemulsion Technology

Abstract

Abstract Traditional chemical methods for removal of non-aqueous fluid (NAF) filter cake and internal formation damage use acids, solvents and mutual solvents mixed in clear brine. In some cases, this conventional technology gives good results; but, in other instances, the results can be disappointing. A new generation of single-phase microemulsion stimulation fluids has been developed for removal of filter cake and formation damage in wells drilled with NAF systems. The development work included the design of microemulsion formulations, determination of the destruction rate of the filter cake with additives that prevent massive completion fluid losses, and evaluation of return permeability for injector and production wells. The driving mechanism of this technology is to obtain a near-zero free energy state between the novel microemulsion and the NAF filter cake or damaged formation face which promotes incorporation of the oil into the microemulsion and spontaneously reverses the wettability of the filter cake particles, disaggregating them so they are no longer damaging. The laboratory results described in this paper show that the novel microemulsion design delays its spontaneous action long enough to permit well completion operations without experiencing massive losses to the formation. Permeability data obtained demonstrate that this new technology is highly effective for enhanced water-injection and productivity. Introduction The reservoir sections of oil and gas wells drilled with non-aqueous fluids require efficient removal of the filter cake to maximize their estimated production potential, or water injection potential in the case of injector wells. This is more critical in the case of open hole completions, including stand-alone and expandable-screen applications, where the screen can be easily plugged by filter cake residue. An emerging solution for cleaning NAF filter cakes is to use a novel adaption of single-phase microemulsion (SPME) technology.1 The SPME cleans the NAF by a solubilization process, where the oil is incorporated into micelles to form swollen micelles or bicontinous microemulsions (Figure 1), 2–4 and the solid particles change from oil-wet to water wet. The single-phase microemulsion can be formulated with the conventional acid packages used in oil and gas well operations. The combination of the single-phase microemulsion with an acid allows complete destruction of the NAF filter cake by removing the oil of the filter cake, reversing of the wettability of the filter cake solids, and simultaneously dissolving the acid-soluble particles. This paper presents results of experiments with the SPME which demonstrate its efficiency to clean-up of NAF fluids, to reduce skin damage, and to increase water injection rates. Background The term microemulsion was introduced by Schulman and co-workers more than 60 years ago.5 Since then, many research groups have been conducting fundamental studies related to the structure of ME phases, phase behavior and applications of these complex fluids.6–20 A microemulsion is defined as a thermodynamically-stable clear fluid typically composed of a non-polar oil phase, a water phase, and surfactants. In some cases, additional components, such as co-surfactants and electrolytes are used to formulate microemulsions.13 They are macroscopically homogeneous and, at the microscopic level, heterogeneous, consisting of individual domains of the non-polar oil phase and polar water phase, separated by a monolayer of surfactant (amphiphile).2, 3, 8, 9 These fluids may be classified into three categories according to Winsor definitions.10 Winsor I microemulsions consist of oil-swollen micelles in a water phase in equilibrium with excess oil. A Winsor II microemulsion consists of water-swollen reverse micelles in an oil phase in equilibrium with excess water. A Winsor III microemulsion is a middle-phase microemulsion, with excess water and oil. The Winsor III microemulsion systems can be understood as an accumulation of swollen micelles, so numerous that they touch one another, forming a perfectly bicontinous structure.11, 12 Figure 2 shows a photograph with Winsor I, Winsor III and Winsor II microemulsion. A single-phase microemulsion (Winsor IV) is obtained by increasing the surfactant concentration of a Winsor III microemulsion fluid. 2, 3, 21