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Summary. Scale deposited within production systems associated with high volumes of produced brine can pose serious problems to the operator both in field economics or in total loss of production. Seven barrels of oilfield brine are produced for every production. Seven barrels of oilfield brine are produced for every barrel-equivalent of energy obtained in the United States each y ear and with the advent of enhanced recovery methods, this ratio can be as high as thirty or fifty to one. Many inhibitors have been in use for over fifty years, but no quantative explanation for their effectiveness and mechanisms of action are presented and two mechanisms based on diffusion rates are proposed with associated equations. Many scale inhibitors are currently on the market primarily based on phosphonates, copolymers or polymaleates. An inhibitor evaluation apparatus developed at Rice University has gained wide acceptance in the petroleum industry and will be described. The results of inhibitor evaluation tests in various brine systems will be discussed in conjunction with the theory of inhibitor effectiveness. Results are presented for CaSO4 and CaCO3.

Introduction

Seven barrels of oilfield brine are produced for every barrel-equivalent of energy obtained in the United States each year. With enhanced recovery of hydrocarbons this ratio can be as high as thirty or fifty to one. In addition, refineries process about 24 barrels of water for each barrel of crude oil. Hundreds of millions of pounds of chemical are added each year to water in industrial treatment plants and energy producing systems. The total cost of those chemicals is estimated to be about two and one half billion dollars per year. Scale deposited within production systems associated with high volumes of produced brine can pose serious problems to the operator. controlling scale affects the economics of problems to the operator. controlling scale affects the economics of the field and a scale problem left untreated can result in the total loss of production or costly replacement of facilities, sometimes in a matter of days. Scale, whether referred to as fouling, sediment, sludge, gyp, incrustation, calculus or stone, can be a problem soon after water production begins and must be treated effectively, efficiently and economically throughout the remaining life of the field. Scale formation can be prevented either by the addition of stoichiometric amounts of acid or sequestrant or by the addition of low concentrations of special chemicals called "threshold inhibitors". Rosenstein was one of the first to knowingly use threshold inhibitors. He was able to prevent calcite precipitation in an irrigation project by the addition of only 0.5 to 1.0 mg/1 of phosphate. Numerous authors have reviewed the use of threshold phosphate. Numerous authors have reviewed the use of threshold inhibitors in various fields of study, but most conclude that there is still no generally accepted mechanism of inhibitor action. Most threshold inhibitors fall into one of four chemical classes:

  1. low molecular weight polycarboxylates, primarily polyacrylates and polymaleates;

  2. inorganic polypohosphates, both polyacrylates and polymaleates;

  3. inorganic polypohosphates, both cyclic and linear;

  4. phosphate esters; and

  5. phosphonates.

Reviews and patents related to each chemical class can be found in Cowan and Weintritt. Both the inorganic polyphosphates and the phosphate esters tend to hydrolyze in water and gradually lose phosphate esters tend to hydrolyze in water and gradually lose effectiveness.

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