Novel Viscoelastic Surfactant-Based Self-Diverting Acid Systems for Carbonate Acidizing

Viscosity-Reducing Agents (Breakers) for Viscoelastic Surfactant Gels for Well Stimulation

“…Polymers and viscoelastic surfactants (VES) are used to improve well productivity in carbonate and sandstone reservoirs. − These compounds are used in matrix acidizing and hydraulic fracturing. ,− In matrix acidizing, polymers and VES are used as chemical diverters and are known as self-diverting agents. ,− Using diverters ensures that the low viscosity acid pumped into the well goes into the low-permeability zones and not in the high-permeability zones, thus increasing the effectiveness of matrix acidizing. ,− , Diversion by gels replaces mechanical diversion techniques because mechanical diversion methods are not always recommended, especially for long horizontal or extended-reach wells. , In hydraulic fracturing, polymers and VES act as a fracturing fluid and a proppant transporter as a result of their high viscosity. ,,,, They are pumped at or above the formation fracturing pressure to create fractures in the wellbore, thus improving well productivity, especially in low-permeability zones . However, the usefulness of gels is not only based on their diversion or fracturing abilities.…”

“…First, polymer-based fluids are not easily cleaned up by reservoir oils after being fully viscosified when compared to VES. ,− This is because viscosity of VES fluids can be reduced by reservoir oils, leading to less formation damage. Moreover, polymer-based fluids need more resources to prepare at the surface when compared to VES. ,, VES solutions only need salt to viscosify the solution and a co-surfactant to promote or inhibit gelling if necessary. ,, The calcium ions (Ca 2+ ) released during the acid reaction with carbonate in carbonate reservoirs help viscosify the low-viscosity VES/acid mixture pumped during acidizing diversion. ,, Meanwhile, polymers usually require cross-linkers to build their high viscosity at reservoir conditions . However, the efficiency of these cross-linkers is also dependent upon reservoir conditions, such as the temperature and hydrogen sulfide. , Moreover, VES-based gels have lower losses in pressure in pipes compared to polymer gels. , With these advantages, VES-based stimulation fluids have been applied in the field, as discussed in the reviews of Hull et al and Kang et al …”

“…5,13−20 Using diverters ensures that the low viscosity acid pumped into the well goes into the lowpermeability zones and not in the high-permeability zones, thus increasing the effectiveness of matrix acidizing. 5,[14][15][16]21 Diversion by gels replaces mechanical diversion techniques because mechanical diversion methods are not always recommended, especially for long horizontal or extendedreach wells. 5,22 In hydraulic fracturing, polymers and VES act as a fracturing fluid and a proppant transporter as a result of their high viscosity.…”

“…31,33,34 VES solutions only need salt to viscosify the solution and a co-surfactant to promote or inhibit gelling if necessary. 13,16,35 The calcium ions (Ca 2+ ) released during the acid reaction with carbonate in carbonate reservoirs help viscosify the low-viscosity VES/acid mixture pumped during acidizing diversion. 13,16,35 Meanwhile, polymers usually require cross-linkers to build their high viscosity at reservoir conditions.…”

“…13,16,35 The calcium ions (Ca 2+ ) released during the acid reaction with carbonate in carbonate reservoirs help viscosify the low-viscosity VES/acid mixture pumped during acidizing diversion. 13,16,35 Meanwhile, polymers usually require cross-linkers to build their high viscosity at reservoir conditions. 25 However, the efficiency of these cross-linkers is also dependent upon reservoir conditions, such as the temperature 36 and hydrogen sulfide.…”