Matrix Acidizing Innovation Surpasses Competing Methods in Saudi Carbonate

Abstract

Technology Update Matrix acidizing is a method of stimulating carbonate reservoirs for oil and gas production. Unlike acid fracturing, matrix acidizing creates conductive flow channels, also known as wormholes, which possess much higher conductivity than the reservoir rock. These conductive channels transport reservoir fluids from within the formation matrix directly into the wellbore overcoming both low permeability and near wellbore damage. In matrix acidizing of moderate to prolific carbonate reservoirs, efficient acid placement is a major challenge, as acid tends to flow preferentially toward intervals with the highest permeability. It can result in overstimulation of these areas, leaving the low-permeability intervals untreated. A significant percentage of matrix acidizing treatments do not meet expectations because of an improper job design and acid placement. In some cases, an increase in water production occurs after a stimulation job as a result of preferential stimulation of high-permeability sections associated with water. Case Study: Khuff Carbonate Khuff is a carbonate formation deposited on a shallow continental shelf in the Ghawar structure of eastern Saudi Arabia. It is divided into four intervals, A to D, with production mainly coming from the two gas-bearing layers: Khuff-B, a tight dolomite, and Khuff-C, a more prolific calcite. Since its initial appraisal in the late 1970s, the majority of Khuff’s development activities have focused on the Permian Khuff-C formation. Extensive heterogeneity in stress, reservoir quality, and reservoir fluids throughout the field, combined with the deep and hot nature of the reservoir, makes effective stimulation of all layers a challenging task. All Khuff gas-producing wells require acid stimulation either by acid fracturing or matrix acidizing to obtain high production rates and sufficient flowing wellhead pressure to be tied into production facilities. Until mid-2011, the majority of Khuff horizontal gas wells were drilled toward the maximum horizontal in-situ stress (σmax) to enhance wellbore stability and achieve the best penetration rates. However, when multistage fracture stimulation is desired and the well is drilled in the max direction, the fractures will grow longitudinally (parallel to the wellbore) and cause potential risk of overlapping with subsequent fractures. Therefore, initiation of the second and third fractures becomes a challenge because of possible pressure communication across the first induced fracture. To avoid fracture overlapping, a well slated for multistage fracturing should be drilled toward the minimum horizontal in-situ stress (emin), which allows transverse fracture growth (perpendicular to the wellbore). Drilling wells toward min often poses challenges, such as wellbore instability and differential sticking, but the improved long-term productivity justifies the strategy.