Abstract

An alternative scale inhibition method using encapsulated scale inhibitor has been successfully used to control calcium carbonate scaling in the Ghawar Field oil wells of Saudi Aramco, Saudi Arabia. Field tests indicate that the Ghawar Field requires a low threshold scale inhibitor concentration for scale mitigation. The encapsulated phosphonate scale inhibitor treatment procedure is thus ideally suitable for scale control in such fields. Since 1994, a total of 89 wells have been treated with this procedure. After more than five years of treatment, there has been no reappearance of scale deposit on the treated wells except for two instances of scaling. Low treatment cost and the resulting long treatment life have provided an excellent incentive to treat more wet producers using the encapsulated scale inhibitor treatment method instead of treatment by conventional scale inhibitor squeezes. An additional benefit is realized from this encapsulated inhibitor method when treating marginal high water cut wells and wells located in low pressure areas by ensuring that the wells sustain flow after the treatments without the need for extensive livening work.

This paper presents the results of a field case study where calcium carbonate scaling was controlled using the encapsulated scale inhibitor treatment procedure. Treatment design, job procedure, inhibitor return monitoring, economic analysis and comparison with conventional squeeze treatments as well as future applications are presented.

Introduction

Calcium carbonate is the most commonly encountered scale in the Ghawar Field of Saudi Arabia. Due to the presence of high reservoir pressure (>2700 psi) with reservoir fluid containing 5–20% dissolved acidic gases (CO2 and H2S), there is no CaCO3 scaling problem in the reservoir and at the bottom-hole of the tubing. However, as the fluid moves up the production tubing, the dissolved gases flash out of the brine as pressure is reduced. The loss of CO2 results in an increase in fluid pH. This shifts the thermodynamic balance, and results in the brine being supersaturated with respect to CaCO3. By the time the nucleation and kinetics of crystal growth favor the scale formation, the brine has reached the top 1000 feet and higher of the production tubing. The CaCO3 scale formation is represented by the equation:

Ca2++2HCO3=CaCO3+H2O+CO2

Calcium carbonate scale formation has resulted in production loss and severe field operational problems in the Ghawar Field oil producers. Since 1987, conventional scale inhibitor squeeze treatments have been used to control this scaling problem. All wet producers in the Ghawar Field require scale inhibitor treatments to avoid production loss as well as expensive scale removal from the producing tubing string, wellhead, manifold and flow-lines. Initial squeeze treatment costs were very high due to high pumping cost, particularly since a large number of wells had to be treated. An alternative scale inhibition method was therefore sought to reduce the treatment costs. The first trial of an encapsulated scale inhibitor treatment was performed in 1994 with encouraging results. Consequently, more wells were treated using the encapsulated scale inhibitor method in 1997 and subsequent years.

Low MIC

After monitoring residual inhibitor levels of the squeezed wells, it was recognized that a low threshold scale inhibitor concentration or MIC (minimum inhibitory concentration) requirement existed for the Ghawar Field.

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