Abstract

ScaleSoftTM is a new software package developed with a consortium of production and service companies for the prediction, control, treatment, and management of scale deposits in oil and gas fields. This software has been tested extensively with numerous well data and yields results more accurate and realistic than any other known scale control programs, whether in public domain or commercial. The software reflects the state of the art in science and technologies in scale prediction and treatment. In addition, it requires only information which is commonly available in the field. For example, if data are not available (e.g., pH or gas phase CO2 content), an estimation of missing data can be made. It is written in Microsoft ExcelR to be easy to use and to perform what-if type of scenarios, upgrade, and store databases of wells, inhibitors and fields with automatic graphical output. This is expected to be a significant contribution to the scale control, treatment and management for the oil and gas industry. A set of twelve field case studies is presented and recommended for use as quality control test data for screening other scale control programs used in the oil and gas industries. The following tasks can be performed with ScaleSoftTM:

  1. Calculation of scaling tendencies (in terms of saturation indices) for common inorganic scale deposits such as calcite, barite (including NORM scales), gypsum, anhydrite, hemihydrate, celestite (others can be added), in various locations of the production system from the reservoir, downhole tubings, wellhead, to surface facilities due to either changes in temperature or pressure or mixing of different sources of fluids;

  2. Selection of optimal inhibitor among commercial inhibitors and inhibitor blends (e.g. phosphate esters, phosphonates, polyacrylates, and polysulfonates) and prediction of the minimum inhibitor concentration required for a production system;

  3. Design and prediction of the inhibitor return based on inhibitor chemistry and specific formation and production information, and

  4. Data storage, recommendations for the scale control management and re-squeeze strategies.

Introduction

Formation of mineral scales from produced waters/brines in oil and gas wells and pipelines is an old but persistent problem facing the petroleum industry. Lost production, formation damage, and operational expenses due to scale deposits cost hundreds of millions of dollars a year. Lately, environmental concerns with scales containing naturally occurring radioactive materials (NORM) raise regulatory and liability issues.

The most common scale minerals found in the oilfield include calcium carbonates (CaCO3, mainly calcite) and alkaline-earth metal sulfates (barite BaSO4, celestite SrSO4, anhydrite CaSO4, hemihydrate CaSO4 1/2H2O, and gypsum CaSO4 2H2O). The cause of scaling can be difficult to identify in real oil and gas wells. However, pressure and temperature changes during the flow of fluids are primary reasons for the formation of carbonate scales, because the escape of CO2 and/or H2S gases out of the brine solution, as pressure is lowered, tends to elevate the pH of the brine and result in super-saturation with respect to carbonates. Concerning sulfate scales, the common cause is commingling of different sources of brines either due to breakthrough of injected incompatible waters or mixing of two different brines from different zones of the reservoir formation. A decrease in temperature tends to cause barite to precipitate, opposite of calcite. In addition, pressure drops tend to cause all scale minerals to precipitate due to the pressure dependence of the solubility product.

Progress has been made toward the control and treatment of the scale deposits, although most of the reaction mechanisms are still not well understood. Often the most efficient and economic treatment for scale formation is to apply threshold chemical inhibitors via the "inhibitor squeeze". Threshold scale inhibitors are like catalysts and have inhibition efficiency at very low concentrations (commonly less than a few mg/L), far below the stoichiometric concentrations of the crystal lattice ions in solution. There are many chemical classes of inhibitors and even more brands on the market.

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