The primary objective of acid fracturing is to generate a conductive flow path between wellbore and reservoir. The factors that governed the effectiveness of acid fracturing treatments are etched fracture penetration and its conductivity. The effective fracture length and its conductivity usually depend on (a) acid convection to the rock surface, b) acid reaction rate and (c) acid leak-off into the reservoir. The etched fracture length is limited by the distance traveled by acid along the fracture. This limit is a greater problem especially in high temperature reservoirs, as the reaction rate is very high with conventional acid systems. Excessive fluid leak-off is another factor that limits fracture extension and etched penetration.
After treatment, the etched non-smooth fracture surfaces leave open pathways, in addition to the wormholes and channels created from the fracture to the formation. The induced fracture tends to close due the effect of in-situ stresses and decline the conductivity. Factors that contribute to the reduction in fracture conductivity upon completion of an acid fracturing treatment are (a) elastic response, (b) compressive failure of the asperities and (c) creeping effect. Therefore, in most of the cases, the initial response of acid fracturing treatment is highly encouraging. However, the sustenance of production may not be adequate because of either insufficient extension of fracture or closure of fractures under the effect of in-situ stresses. Type of acid and treatment methodologies viz. injection rate, leak-off control measures etc. are having significant impact on success of treatments.
To address the issues, different combinations of acid systems were applied in various low producer wells in a tight carbonate reservoir of an offshore field. This paper will present the treatment design, methodology and impact of various acid systems on effectiveness of acid fracturing treatments from field application results.