The Role of Reservoir Lithology in Design of an Acidization Program: Kuparuk River Formation, North Slope, Alaska

Abstract

ABSTRACT Sandstone lithologies in the Lower Cretaceous Kuparuk River formation were tested to study formation damage during acid treatments. Two lithologies were analyzed before and after acidization: 1) a glauconitic sandstone cemented by siderite, and 2) a sandstone containing clay minerals with minor carbonate cements. These samples were treated with hydrochloric acid containing iron-chelating and silt-suspending agents, with hydrofluoric acid, and with fluoroboric acid, all at reservoir pressure and temperature. Formation damage was evaluated by scanning electron microscopy (SEM), X-ray diffraction, petrographic analyses, and acid effluent analyses. Carbonate cement and clay mineral content were critical factors that governed the extent of formation damage. Hydrofluoric acid is commonly used to stimulate sandstone reservoirs. However, results showed that this acid caused formation damage in both Kuparuk River lithologies. SEM examination of siderite-cemented sandstones after treatment showed abundant iron-rich and poorly crystalline fines dispersed in pores. These mobile fines included partially dissolved siderite, iron precipitates, and colloidal silica. In sandstones with clays, hydrofluoric acid partially dissolved and released clay particles which could plug pore throats during production. Hydrochloric acid with iron-chelating and silt-suspending agents gave the best results in siderite-cemented sandstones. This acid prevented iron released from siderite and glauconite from forming precipitates and cleaned up the wellbore. Fluoroboric acid was the best treatment in sandstones containing clay minerals. SEM examination showed that this acid cemented undissolved clay particles to framework grains. Thus, pore throats remained open. Selection of the acid treatment compatible with reservoir lithology will minimize formation damage and lead to increases in production.