The present invention relates to water-hydrolysable materials and, more particularly, to treatment fluids and associated methods relating to the hydrolysis of water-hydrolysable materials.
Water-hydrolysable materials are commonly employed in subterranean operations. For instance, water-hydrolysable materials may be used in subterranean operations as fluid loss control particles, diverting agents, filter cake components, drilling fluid additives, cement additives, and the like. In some instances, the water-hydrolysable material may be in a mechanical form (e.g., plugs, sleeves, and the like). In another instance, the water-hydrolysable material may be capable of releasing a desirable degradation product, e.g., an acid, during its hydrolysis. The acid released by certain water-hydrolysable materials may be used to facilitate a reduction in viscosity of a fluid or to degrade a filter cake, as well as for numerous other functions in subterranean operations.
Inclusion of a water-hydrolysable material capable of releasing an acid in a gelled (and optionally crosslinked) treatment fluid may be used to facilitate a reduction in viscosity of such fluid. Generally, these water-hydrolysable materials likely will hydrolyze over time due to contact with water present in the fluid, thereby releasing an acid. Upon its hydrolysis, the acid will function, inter alia, to reduce the viscosity of the gelled (and optionally crosslinked) treatment fluid, for example, by breaking the crosslinks in the treatment fluid, reducing the pH of the treatment fluid sufficiently to reverse the crosslinks therein, and/or breaking down the backbone of the gelling agent present in the treatment fluid. Typically, the acid released by the water-hydrolysable materials may breakdown the gelling agents at temperatures above about 150° F.
Water-hydrolysable materials capable of releasing an acid also may be used in the degradation of acid-soluble materials present in a subterranean formation, such as those present in or adjacent to filter cakes. Filter cakes commonly may be formed by a fluid (e.g., a drill-in and servicing fluid) on the face of a portion of a subterranean formation, inter alia, to minimize damage to the permeability thereof. The filter cake often comprises an acid-soluble component (e.g., a calcium carbonate bridging agent) and a polymeric component (e.g., starch and xanthan). Before desirable fluids, such as hydrocarbons, may be produced, the filter cake generally is removed. To facilitate the degradation of the acid-soluble component, a water-hydrolysable material capable of releasing an acid may be utilized. Filter cakes also may be removed using an acid where the filter cake does not contain an acid-soluble component, for example, by degrading the underlying carbonate adjacent, if the filter cake is present in a carbonate formation.
In one instance, the filter cake may be contacted by a treatment fluid that comprises the water-hydrolysable material. The resultant acid should interact with the acid-soluble component of the filter cake and/or the underlying carbonate adjacent to the filter cake in such a way as to facilitate their degradation. In another instance, the water-hydrolysable material capable of releasing an acid may be included in the fluid (such as the drill-in and servicing fluid) that forms the filter cake, such that the filter cake further contains the water-hydrolysable material. Subsequent contact of the filter cake with an aqueous fluid hydrolyzes the water-hydrolysable material thereby releasing an acid that acts to degrade the acid soluble component of the filter cake. Among other components, the aqueous fluid may contain oxidizing agents or enzymes suitable to facilitate the degradation of the polymeric component of the filter cake.
Use of water-hydrolysable materials capable of releasing an acid may be problematic, for example, if the water-hydrolysable material hydrolyzes too slowly or too quickly. For example, where used to facilitate a reduction in viscosity of a treatment fluid, the treatment fluid may need to have a desired viscosity for a requisite duration to ensure a desirable well treatment. In some instances, as the temperature in the well bore increases, the hydrolysis rate of the water-hydrolysable material increases, which may lead to an untimely or undesired reduction in viscosity of the treatment fluid. One method used to reduce the hydrolysis rate of the water-hydrolysable material may be to encapsulate it in a slowly soluble coating that can delay the hydrolysis of the water-hydrolysable material and thus delay release of the resulting acid. However, encapsulation of the water-hydrolysable material may add undesired expense and complexity. Further, where the water-hydrolysable material has a relatively small particle size, e.g., less than about 200 microns, encapsulation may not be practicable. Also, while it is possible to “tune” the water-hydrolysable material through various methodologies (e.g., initial choice of material, choice of plasticizers, molecular weight of the material, etc.), these methods may not be sufficient to extend or decrease the degradation time appropriately and/or may not be economical.