New Methodology to Effectively Stimulate Water Disposal Wells Drilled in Acid Sensitive Sandstone formations in Saudi Arabia

Abstract

Acid treatments for sandstones differ significantly from those for carbonate rocks. Unlike carbonate acidizing where HCl reacts with carbonates to generate reaction products soluble in water, sandstone acidizing using mud acids produces reaction products that may precipitate and reduce rock permeability. The challenges of sandstone acidizing are to optimize damage removal while minimizing formation-damaging precipitates. Understanding formation mineralogy and the nature of damage is critical for designing a proper acid treatment. Sandstone acidizing has been always a challenging task, especially if the formation is acid-sensitive. Traditionally, sandstone reservoirs in the Northern Area of Saudi Arabia are not stimulated due to their negative reaction to acidizing. Wells A and B were drilled and completed as vertical cased hole wastewater disposal wells for a refinery in the northern area of Saudi Arabia. They have been used to dispose the refinery wastewater since 1998. The injectivity of Well A was maintained since it was placed on injection, whereas well B showed low injectivity with a skin damage of + 20. The major minerals found in the sandstone reservoir are quarts, dolomite, and calcite with kaolite and smectite. The latest analysis of the refinery injection wastewater showed that the wastewater contained suspended solids (iron sulfide) and oil. After several negative clean-up trials by flowing the well back in order to restore injectivity and maintain adequate wastewater disposal capacity of the refinery, it was decided to stimulate or sidetrack the well. A new design methodology with extensive lab testing was applied to design a tailored treatment to address these challenges. It included a pre-flush of aromatic solvents, then stages of 10 wt% HCl, 9:1 HCl:HF acids. To achieve better acid diversion for the long perforated intervals (200 ft) with high permeability contrast, foamed diverter stages (quality 70%) were incorporated in the treatment. A carefully designed train of treatment fluids was applied to remove formation damage induced by drilling and injection fluids. Injectivity tests before and after each step of the treatment was recorded and evaluated. Proper design and execution of the stimulation treatment almost doubled the well injectivity index from 47 to 86 b/d/psi. The maximum injection rate increased from 50 to 92 BPM, at 1,500 psi injection pressure. Challenges, fluid selection, design criteria, field treatment, lessons learned, and results of the acid treatments will be discussed in this paper. Introduction Matrix acidizing reestablishes productivity or injectivity in many damaged wells in a cost effective way. A matrix acid treatment forces the injected acid into the formation at a pressure below its fracture pressure. The treatment often involves several stages that may be repeated. The damage can be natural, caused by reservoir fluids moving through the formation, or induced by fluids used in the operations, such as drilling, completions and workovers, or stimulation. Typical examples of formation damage include: fines migration, scale formation, deposition of paraffins, asphaltenes, or mixed organic and inorganic deposition.[1–4] It can also result from plugging by foreign particles in injected fluids, wettability changes, clay swelling, emulsions, precipitates or sludges caused by acid reactions, bacterial activity and water block.[3–7] Wellbore cleanup, matrix stimulation treatments or acid fracturing can be used to remove or bypass the damage. Acid treatments conducted in sandstones reservoirs differ significantly from those performed in carbonates rocks. Carbonate rocks dissolve readily in hydrochloric acid, and the reaction products are water-soluble. A matrix acid job is usually designed to bypass near wellbore damage by dissolving carbonates and creating high permeability channels, or wormholes, in the rock, thus providing a flow path past the near wellbore damage.