Abstract

As gas fields are developed around the world, new and interesting scale challenges are presented. In the United States, gas reserves are found in fields near Green River, Wyoming. This gas is produced from shale layers interbedded with the carbonate mineral trona, or, sodium sesquicarbonate, Na3H(CO3)2*2H2O. Well known carbonate minerals, such as calcite, siderite, and dolomite are relatively insoluble in water, and are often found in the rock matrix of hydrocarbon reservoirs. Unlike these conventional minerals, trona is extremely water soluble at reservoir conditions. Under normal circumstances, if left untreated, the production of the mineral saturated brine leads to significant deposition of scale in the production tubing and surface facilities.

Two major challenges are presented with trona scale deposition.

The first challenge is in the prevention of the mineral deposits. Like conventional carbonate scales, inhibition is possible with chemical inhibitor treatment. There are also non-typical treatment options, both chemical and thermodynamic, that were investigated and evaluated for effectiveness.

The second challenge is in the removal of deposited scale. It is common to remove carbonate scales by the application of an acid, but the trona scale is an unusual exception. The addition of an acid actually encourages deposition since the composition and solubility of this mineral scale are very sensitive to changes in pH.

This paper discusses the laboratory work performed that led to an understanding of the problem and to the proposal of rather unconventional procedures necessary to prevent the scale and to eliminate the existing deposits.

Introduction

Oilfield scales are the hard, adherent deposits formed when inorganic minerals dissolved in water precipitate during the production of oil and gas. If left untreated, scale can reduce the productivity of the well by blocking the flow paths of the oil and gas into the wellbore. Downhole and surface hardware can be damaged by the accumulation of scale, further reducing the ability to produce and transport hydrocarbons. The potential economic impact of scale formation is large, ranging from losses incurred by remedial treatments to lost production from permanent formation damage.

Scale typically forms when a change in the physical conditions or composition of the brine occurs. Common disturbances include, for example, a change in temperature, pressure, or composition of the water. An example of a temperature change causing precipitation can be found in the example of barium sulfate scale. The decrease in brine temperature as it travels from the (usually) hotter reservoir to the cooler surface can result in a supersaturation of barium and sulfate ions dissolved in the brine. The lower temperature causes a lower solubility of barium sulfate in the brine.

The change in equilibrium conditions causes the brine to establish a new equilibrium as it is being produced. In many instances, this new equilibrium is achieved by the loss of dissolved solids. For every change in conditions, the capacity of the water to hold dissolved solids may be reduced and any mineral solids deposited are described as scale.

The necessary components for scale formation are present in oilfield waters and their associated production systems. Soluble metals, sodium, calcium, and magnesium to name a few, are found in natural waters and often in significant concentrations in formation brines. Natural waters also contain alkalinity, usually in the form of the bicarbonate ion, HCO3, or, less frequently, the carbonate ion, CO3−2. The concentrations of CO2, the bicarbonate and carbonate species are governed by the pH of the system.

The gas fields found near Green River, Wyoming, in Sweetwater County, are subject to an unusual, and perhaps unique, carbonate scale, locally known as trona.

Trona, Sodium Sesquicarbonate

Trona, or sodium sesquicarbonate, Na3H(CO3)2*2H2O, is a naturally occurring carbonate mineral. Rather rare, the mineral is found in economically significant accumulations in only a few areas of the world.

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