Effective matrix acidizing of horizontal and multi-lateral wells is a very difficult task. Unlike vertical wells, horizontal wells can extend several thousands feet in the formation. Reservoir heterogeneity and the length of the horizontal leg make acid placement and diversion a real challenge. In addition, the low drawdown encountered in horizontal wells results in longer times to lift the spent acid from the well.

To achieve better acid diversion in horizontal wells drilled in carbonate reservoirs, a viscoelastic surfactant-based self-diverting acid system was utilized. The components of this new system are HCl and a viscoelastic surfactant. The acid dissolves calcite and dolomite minerals and produces calcium and magnesium chlorides. The increase in pH forces the surfactant monomers to form rod-shape micelles. The produced chloride salts further stabilize these structures, especially at high temperatures. The rod shape micelles will significantly increase the viscosity of the acid, diverting the acid into tight un-stimulated zones.

More than forty-five horizontal wells with openhole completions were successfully stimulated in two offshore oil fields in Saudi Arabia using the new treatment system. There is a water cap 30 feet away from these openhole (OH) sections in one of the fields, and growth of any dominant wormhole into these sections could increase water production. The wells that used the new treating fluid produced an average of 1,600 barrels of oil per day more than conventionally treated wells, with no indication of water production. Field results (pre and post oil and water production rates and FWHP) demonstrate the effectiveness of the new system to matrix acidize long horizontal wells with openhole completions. The simplicity of the system makes it the fluid of choice, especially in offshore environments. The absence of metals in this system helps in minimizing sulfide precipitation in sour wells.

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