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.