Summary

Chemical scale inhibitors are commonly used to prevent or inhibit scale formation in production. The most economic treatment of scale inhibitor is normally through chemical squeeze. However, there is little agreement regarding the primary mechanism by which the threshold scale inhibitors are retained in producing oil or gas formations as a result of squeeze procedures. Recent advances in phosphonate/rock interaction research at the Rice U. Brine Chemistry Consortium have significantly improved our knowledge of what controls inhibitor placement in the formation. It is commonly suggested that reservoir type determines how an inhibitor is retained in a formation. Our research suggests that the pill chemistry is also an important determinant for retention of carbonate reservoir. Acidic pills are mostly retained near the wellbore, while more neutralized pills move farther into the formation. Three calcium nitrilomethylenephosphonate (NTMP) solid phases, an amorphous phase, and two crystalline Ca2.5HNTMP phases with pKsp = 22.6 and pKsp = 24.2, are particularly important for inhibitor retention. The relative sizes of these solid phases formed are governed by the pill composition and acidity. These results can be explained by a solution-phase-controlled sequence of reactions. All of this information has been incorporated into a new squeeze-design software program, Squeeze SoftPitzer.

Introduction

Threshold scale inhibitors, such as the phosphonates, polyacrylates, and polymaleates have been used to control scale since 1936 (Rosenstein). Preferable to sequestering agents, which must be added in stoichiometric amounts to inhibit precipitation, chemical threshold scale inhibitors generally require the addition of only a few milligrams per liter to inhibit scale. Inhibitors are commonly applied to production wells through an inhibitor squeeze. An inhibitor squeeze is performed by pushing scale inhibitor solution into a producing formation and fixing the inhibitor in the formation. When well production begins, inhibitor is produced along with the formation water. Following the squeeze, scale inhibitor flows back at low concentrations, which increase and peak rapidly to some value and then decline within a few days to a low plateau concentration, which comprises the bulk of the squeeze duration. The plateau flowback inhibitor concentration is normally sufficient to inhibit scale.

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