Virtual Testing: The Key to a Stimulating Process
Matrix acid stimulation in a sandstone reservoir involves complex chemical reactions that are strongly dependent on mineralogy . A new process includes a model for simu- lating acid reactions to help operators choose an optimal treatment for a formation.
Sy ed Ali Chev ronTex aco Hou ston, Tex as, USA
Way ne W. Frenier Bruno Lecerf Murtaz a Ziauddin Su gar Land, Tex as
Hans Kristian Kotlar Statoil Trondheim, Norw ay
Hisham A. Nasr-El-Din Sau di Aramco Research and Dev elopment Dhahran, Sau di Arabia
Olav Vikane Statoil Stav anger, Norw ay
For help in preparation of this article, thanks to Ernie Brown, Steve Davies and Vincent Dury, Sugar Land, Texas; Keng Seng Chan and Ray Tibbles, Kuala Lumpur, Malaysia; Matt Gillard and Richard Warren, Aberdeen, Scotland; Abigail Matteson, Ridge eld, Connecticut, USA; and Carlos Torres, Maturin, Venezuela.
ClayACID, ELANPlus (Elemental Log Analysis), NODAL, PLT (Production Logging Tool) and Virtual Lab are marks of Schlumberger.
Matrix acidizing reestablishes productivity in many damaged formations in a cost-effective way. The damage can be natural, an artifact of produced reservoir fluids moving through a formation, or induced by fluids used in well operations, such as drilling, completions and workovers, or stimulation.
Formation damage can be caused by fines migration, scale formation, deposition of paraf- fins, asphaltenes or other organic material, or mixed organic and inorganic deposition. It can also result from plugging by foreign particles in injected fl uids, wettability changes, clay swelling, emulsions, precipitates or sludges caused by acid reactions, bacterial activity and water blocks.1 These damage mechanisms can be either natural or induced. Wellbore cleanup, matrix stimulation treatments or acid fracturing may be used to remove or bypass near-well damage. This article focuses on sandstone matrix
acidizing.2
A matrix acid treatment forces acid into a formation at a pressure below its fractur- ing pressure. The treatment often involves several stages that may be repeated. A new matrix-treatment design process includes use of the Virtual Lab software, a state- of-the-art geochemical simulator that properly accounts for secondary and tertiary reaction mechanisms. Field examples from the North Sea and from the Gulf of Thailand demonstrate the usefulness of this new process for evaluating treatment designs.
Reacting to Damage
Acid treatments for sandstones differ signifi- cantly
from those for carbonate rocks.
Carbonate rocks dissolve rapidly in hydrochloric acid [HCl], and the reaction products are solu- ble in water. In carbonate rocks, a matrix acid job is usually designed to bypass near-well damage by dissolving minerals and creating channels, or wormholes, in the rock, providing a flow path past the near-well damage. Acid- fracturing techniques in carbonates create a hydraulic fracture that has a differentially etched surface, so that the fracture maintains its conductivity during production.3
HF dissolves silica and silicates, and HCl or organic acids are included to keep reaction products in solution.
In contrast to acidizing reactions in carbon- ate rocks, the reaction chemistry for silicate rocks is quite complex. Sandstones comprise quartz grains, clays of various types, feldspars, chert, micas, and carbonate materials as cement or overgrowths on grains, along with other minerals (next page). HCl is not effective in dis- solving most constituents of silicate rocks. Sandstone acid jobs typically use hydrofluoric acid [HF] in combination with HCl, formic or acetic acid.4
Sandstone matrix acidizing primarily targets damage from migrating fines, swelling clay, carbonate or hydroxide scales, and plugging par- ticles from drilling and completion operations. Understanding formation mineralogy and the
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Oilfi eld Review
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