Seawater injection has been used to maintain the reservoir pressure in a carbonate reservoir in Saudi Arabia. However, the injectivity of some vertical wells in this field is limited due to low reservoir permeability (less than 20 md). Conventional matrix acid treatments did improve the injectivity to varying degree. However, to achieve the required injection rate, some of water injectors need to be stimulated by acid fracturing.
Various acid systems have been evaluated to acid fracture injection wells. A thorough literature survey and lab testing eliminated acid systems that utilize acid-soluble polymers (both linear and cross-linked). This is mainly due to the formation damage associated with polymers, and also these systems required back-flow of the treated wells for cleanup.
To eliminate formation damage due to polymers, flowback for cleanup, and to enhance well performance, a polymer-free viscoelastic surfactant fluid was utilized in acid fracturing treatments. The fluid system includes the non-damaging viscous pad and polymer-free leak-off controlled acids. The treatment was successfully applied in the field. Cleanup by flow back was eliminated with resulting in significant increases in well injectivity comparable to horizontal wells drilled in the same area.
Many improvements have been made since the first commercial acid treatment in 1932, and acid fracturing is one among them. In acid fracturing, the fracturing principle is applied to increase the live acid penetration into the rock.
It is an accepted practice to use a non-reactive fluid for fracture initiation and to condition the formation for maximum acid penetration.1 This will also help to
provide effective fracture extension due to controlled leakoff
cool down the formation to ext end the reaction time for acid
induce greater fracture width to reduce acid contact area and extend penetration
saturate natural fractures and vugs to minimize acid leakoff.
Generally, surface treating pressure is limited by wellhead equipment and tubing string. There are several reports on optimizing fracture designs by selection of fluids and injection rate2,3 though these are based on several assumptions. The effectiveness of any fracturing treatment depends on the fracture length and conductivity. The conductivity, in turn, will depend on the fluidloss,4,5 rock dissolution,6 and the damage associated with the treatment fluid.7,8 To control leakoff, industry has been using emulsions and acids gelled with polymers.4,9 Of them, polymers are the most widely used, though it is known for its stability limitations at high bottom hole temperature, especially in the presence of acid due to hydrolysis.4,10
Highly retarded acid-in-diesel emulsion that is stable up to 350°F has been developed11 and was found to be very effective in both matrix and fracturing treatments.12,13
Many carbonate formations contain micro-fractures (~ 5×10−4 inch wide) and during acid fracturing leak-off occurs through these micro-fractures as well as through the matrix. Acid can increase the permeability of these fractures several thousand-fold and the leak-off control is always a challenge. The fluid-loss could be controlled to some extent by the use of alternating stages of pad fluids and acids.5,14 Fine sand particles are also used in conjunction with polymers to help buildup filter-cake on the fracture surface and to clog the micro -fractures and interconnected vugs.15,16