Abstract: The object is to provide a subterranean formation processing composition which has a highly stable viscosity against change in temperature and a high water stopping property. A subterranean formation processing composition comprising cellulose fibers wherein the composition contains ultrafine cellulose fibers having 0.14 to 2.5 mmol/g of a substituent is used. The substituent is preferably an anion group, particularly a phosphoric acid group. The present invention relates to various fluids which are used in well processing. The present invention relates to a method for processing a subterranean formation, for example, drilling of an exploratory well or a wildcat, an appraisal well, an exploratory well or an exploration well, a delineation well, a development well, a production well, an injection well, an observation well, and a service well; cementing; fracturing; and a method for producing petroleum resources.

Abstract: Objects of the present invention are to provide a phosphoric acid-esterified fine cellulose fiber of which slurry shows superior transparency, and to provide a method for producing a phosphorylated fine cellulose fiber showing superior transparency with good efficiency and high yield. According to the present invention, there is provided a phosphoric acid-esterified fine cellulose fiber, of which 0.2 mass % aqueous dispersion shows a solution haze of 15% or lower.