Abstract

In the Mobile Bay area of the Gulf of Mexico, occurrence of unique mineral scales in producing wells and flow lines has been a challenge to maintaining normal oil and gas production; and it has a high impact on operation cost. In addition to common scales (calcium carbonate and barium sulfate), the produced water often has a tendency of forming "exotic" scales, such as calcium fluoride, zinc sulfide and lead sulfide, which can cause plugging of subsea safety valves, and restrictions in the production flow lines, production headers and surface vessels. Furthermore, the problem has been compounded with the harsh system conditions (high temperatures, and high TDS and high calcium brines). This paper describes a successful scale inhibition program that has been developed from a careful analysis of the system and nature of the problem, a comprehensive laboratory product development and evaluation, and the implementation of the chemical treatment in the field.

Introduction

The concerned fields in Mobile Bay produce gas and gas condensate from the Norphlet trend1, a complex and mineral rich sandstone formation. The gas and condensate are produced from very hot wells (>400°F @ bottomhole). The formation brines typically contain a very high amount of total dissolved solids (TDS), often in excess of 250 grams per liter. Scaling ions such as barium (Ba2+), calcium (Ca2+), sulfate (SO42-) and hydrogen carbonate (bicarbonate, HCO3-) naturally occur in the formation and are produced in the brines. The scaling ions then deposit scale throughout the system, as a result of either temperature decrease (barium sulfate) or pressure decrease (calcium carbonate) from the well bottomhole to the wellhead, and finally to the arrival on the surface. Unique to these fields, the formation also produces naturally occurring fluoride ions (F-), and as fluid temperature decreases, fluoride and calcium ions together form calcium fluoride (CaF2) scale, an extremely insoluble deposit (less soluble than barium sulfate). The deposition of calcium fluoride along with barium sulfate (BaSO4) and calcium carbonate (CaCO3) has created a major production problem in these fields. The scale deposition has resulted in jamming of the surface controlled subsea surface safety valve (SCSSV) in the producing wells, restrictions in the production flow lines and production headers, and malfunction in the surface equipment.

The previous scale inhibition program, including continuous injection of a scale inhibitor into the producing wells at the SCSSVs, at the wellheads, and at the production headers, did not appear to achieve complete inhibition of scale deposition in the system. Worse, in the recent years, four capillary tubings used for scale inhibitor injection into the wellbore at the SCSSVs were plugged, and two scale inhibitor injection lines to the wellheads were plugged at the check valve entering the wells.

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