Face Dissolution
Face Dissolution Conical Channels Ramified Wormholes Wormholes 1.0
0.2 Flow rate
> Carbonate dissolution patterns. Wormhole structure is related to the efficiency of the acidizing operation and can be viewed by plotting the number of pore volumes to core breakthrough (PVBT) versus the flow rate. Porosity patterns obtained from a software model calibrated with experimental data illustrate how dissolution proceeds with increasing flow rate. The least efficient acidizing operation is face dissolution—the entire matrix must dissolve in order to advance the reaction front. Slightly more efficient at higher flow rates is the creation of large, conical channels. The most efficient operation occurs at the curve minimum, with creation of highly dispersed wormhole channels. At even higher flow rates, the curve turns upward and large channels, called ramified wormholes, form. Increasing to higher flow rates leads again to uniform face dissolution.
to substantial lengths, resulting in efficient use of acid to bypass damage. In conditions that are less favorable, the acid creates short channels that do little to increase production. For any formation being treated, there is an optimal set of treatment parameters that creates wormholes with the most efficient use of acid (above).3
In contrast to carbonate formations, the quartz and other minerals that make up most sandstone reservoirs are largely acid insoluble. Acid treatment for sandstone—HF usually combined with HCl—seeks to dissolve the damaging particulates that block the pores and reduce permeability (below).4
Acidizing in
A
B C D E
> Sandstone matrix. The framework of sandstone reservoirs is typically made up of grains of quartz cemented by overgrowth of carbonates (A), quartz (B) and feldspar (C). Porosity reduction occurs from pore-filling clays such as kaolinite (D) and pore-lining clays such as illite (E).
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sandstone targets damage in the first 0.9 to 1.5 m [3 to 5 ft] radially from the wellbore—the area that experiences the largest pressure drop during production and is critical for flow. This area is typically damaged from migrating fines, swelling clays and scale deposition. Sandstone acidizing reactions occur in areas where acid meets minerals that can be dissolved. The primary dissolution reactions of the clays and feldspar with a typical HF-HCl mix form aluminosilicate products. Sandstone acidizing chemistry is complex, and the initial reaction products can react further and possibly cause precipitation. These secondary reactions are slow compared with the primary dissolution reactions and rarely present problems with mineral acids except at higher temperatures (next page, top). Extension of matrix acidizing to tempera - tures above 93°C presents the operator with both possibilities and concerns. The possibilities are obvious—acidizing at higher temperatures allows stimulation of hot wells using familiar field procedures. However, at higher tempera - tures, use of HCl causes a host of problems. In carbonates, the rapid HCl reaction rate at elevated temperature may lead to face attack instead of wormhole creation and may create acid-induced sludge with high-viscosity crudes. High-temperature problems in sandstones are different. Clay dissolution may be too rapid, decreasing penetration by the acid, and secondary reactions may cause precipitation. Finally, rapid reaction rates can deconsolidate the sandstone matrix, creating mobile sand. Of particular concern in high-temperature sandstone and carbonate reservoirs is accelerated corrosion of tubulars and other wellbore equip - ment. Although increased injection of inhibitors may adequately control corrosion rates, the greater inhibitor loading at higher tempera tures may itself cause formation damage.5 The challenges of extending matrix acidizing to higher temperatures have led to development of new treating fluids and techniques. Treating fluids include acid-internal emulsions to retard reaction rates in carbonate reservoirs and mild, slightly acidic chemical agents for treating both carbonates and sandstones. New techniques include a simplified sandstone-treating system that uses laboratory data and predictive software—in combination with new chemical treating agents—to arrive at a simplified procedure. These new treatments and tech - niques can be easily understood by examining some of the laboratory data that were instrumental in their development.
Oilfield Review
Pore volumes to core breakthrough
Porosity
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