Effective matrix acidizing of horizontal and multilateral wells can be a highly challenging task. Unlike vertical wells, horizontal wells can extend several thousand feet into the formation. Reservoir heterogeneity and the length of the horizontal leg can make acid placement and diversion very difficult. In addition, the low drawdown encountered in horizontal wells results in longer times to lift the spent acid from the well, especially in tight formations. To achieve better acid diversion in horizontal wells drilled in carbonate reservoirs, a viscoelastic-surfactant-based system was used. The components of this new system are HCl and a visco-elastic surfactant. The acid dissolves calcite and dolomite minerals and produces calcium and magnesium chlorides. The increase in pH forces the surfactant molecules to form rod-shaped micelles. The produced chloride salts further stabilize these structures, especially at high temperatures. The rod-shaped micelles will significantly increase the viscosity of the acid, diverting the acid into tight, unstimulated, or severely damaged zones. More than 100 wells with openhole (OH) completions were successfully stimulated in two offshore oil fields in Saudi Arabia by use of the new acid system. With a water zone 30 ft away from these OH sections in one of the fields, 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 BOPD more than conventionally treated wells, with no indication of water production. Field results [pre- and post-oil and -water production rates and flowing wellhead pressure (FWHP)] demonstrate the effectiveness of the new acid system to matrix acidize long horizontal wells with OH completions. The simplicity of the system makes it the fluid of choice, especially in offshore and sour environments. The absence of metallic crosslinkers in this system eliminates problems associated with sulfide precipitation in sour wells.
