Matrix Acidizing
Authors: McLeod Harry O. 
Published: December 2007

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleum engineering.

Introduction
Many excellent and useful papers have been written on the subject of matrixacidizing. Included in this article is an extensive bibliography that should beuseful to the engineer in the design and execution of a matrix acidizing treatment in limestone or sandstone formations. The first matrix acidizing jobswere very successful in stimulating oil production in carbonates. However, mostof the recent attention to matrix acidizing concerns sandstones and the use ofvarious hydrofluoric acid systems. Matrix acidizing in carbonate formations still is beneficial in high-permeability, damaged formations (50 md or more). Damage can occur during drilling, completion or production of a well. Incarbonates with permeabilities less than 10 md, acid fracturing generally is used because much greater stimulation is obtained with long, acid-etched fractures in low-permeability reservoirs. Although the acid systems used insandstones and carbonates differ, the same practices apply to both.

Well Performance (Need for Acidizing)
Successful acidizing depends on the presence of damage and its location and intensity. The closer the damage is to the perforations, the more easily acidcan get to it. Compacted or crushed zone damage from perforating overbalancedcan be removed easily by acid, since only about 1/2 in. [1.3 cm] of damage mustbe removed directly around the perforation. Precipitates from previous acid treatments more than 1 ft [0.3 m] from the wellbore in sandstone or 5 ft [1.52m] in carbonate will be either impossible to reach with matrix acidizing or too expensive to treat. Deep solid plugging, will be corrected more effectively bycreating a conductive fracture through the damage either by sand fracturing oracid fracturing. Nonplugging damage (e.g., oil wetting) may be several feet deep around the wellbore, but reverse wetting surfactants can penetrate andreverse the formation to a water-wet condition at reasonable cost. Oil wetting damage usually is less severe than solid plugging damage, so correctivechemicals can reach the affected area easily.
High-permeability formations (those with 100 md or more) seem to be dominated by either formation damage or tubing size flow restrictions. This is particularly true of gravel-packed offshore wells. When well flow is markedlyless than similar wells in the same reservoir, most of the drawdown probably isoccurring at the wellbore through a small zone of reduced permeability. Mostrecent gravel-pack-damage research has focused on gravel-packed tunnels andquality of the gravel in the tunnel. Current techniques have improved so muchin recent years that gravel-packed tunnels usually offer little flow resistance when perforating density is adequate. Nevertheless. reduced flow throughgravel-packed wells still occurs. Current research focuses onincompletely packed tunnels andformation-sand damage near the entrance to the tunnels.
Torrest and Stein described gravel shifting in tunnels when the gravel pack is not packed tightly during placement. Damage to formation sand before gravelplacement will cause premature pressure outs resulting from viscous fluidsentering damaged or reduced permeability near the perforations. Because of highpressures, pumping may be halted before the gravel has concentrated adequately in the perforation tunnels. If the pumping stops too soon, the tunnels will befilled only partially with quality gravel. When the well is produced, formations and will enter the tunnels, bridging on the gravel inside the tunnel andpacking the partially void tunnel with formation sand, which is much lower inpermeability than the gravel. As the formation sand fills the tunnels. the pressure drop through the completion increases and the flow rate declines.
JPT
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