- Harry O. McLeod (Conoco Inc.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- December 1984
- Document Type
- Journal Paper
- 2,055 - 2,069
- 1984. Society of Petroleum Engineers
- 1.11 Drilling Fluids and Materials, 5.1.1 Exploration, Development, Structural Geology, 1.14 Casing and Cementing, 1.8 Formation Damage, 2.4.5 Gravel pack design & evaluation, 5.2.1 Phase Behavior and PVT Measurements, 2 Well completion, 3 Production and Well Operations, 2.7.1 Completion Fluids, 4.1.2 Separation and Treating, 2.2.2 Perforating, 4.2.3 Materials and Corrosion, 5.3.4 Integration of geomechanics in models, 2.2.3 Fluid Loss Control, 3.2.4 Acidising, 1.10 Drilling Equipment, 5.6.4 Drillstem/Well Testing, 1.4.3 Fines Migration, 4.3.1 Hydrates, 6.5.2 Water use, produced water discharge and disposal, 6.5.5 Oil and Chemical Spills, 5.2 Reservoir Fluid Dynamics, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.2.3 Rock properties, 4.1.5 Processing Equipment, 2.4.3 Sand/Solids Control, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.5.2 Core Analysis, 4.3.4 Scale, 4.3.3 Aspaltenes, 5.8.7 Carbonate Reservoir, 1.6 Drilling Operations
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Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area,these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.
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 acidizingtreatment 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 formationsstill 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 isused because much greater stimulation is obtained with long, acid-etchedfractures 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 andintensity. 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 acidtreatments 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 tooexpensive 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 feetdeep around the wellbore, but reverse wetting surfactants can penetrate andreverse the formation to a water-wet condition at reasonable cost. Oil wettingdamage 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 bedominated by either formation damage or tubing size flow restrictions. This isparticularly 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 resistancewhen perforating density is adequate. Nevertheless. reduced flow throughgravel-packed wells still occurs. Current research focuses on (1) incompletelypacked tunnels and (2) formation-sand damage near the entrance to the tunnels.Torrest and Stein described gravel shifting in tunnels when the gravel pack isnot 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 adequatelyin the perforation tunnels. If the pumping stops too soon, the tunnels will befilled only partially with quality gravel. When the well is produced, formationsand 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. thepressure drop through the completion increases and the flow rate declines.
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