Comparative Study of Fracture Conductivity in Various Carbonate Rocks Treated with GLDA Chelating Agent and HCl Acid

Acid fracturing is applied to increase the productivity of carbonate formations. The acid creates rough fracture surfaces and channels that keep the fractures open after closure. This study investigated a chelating agent and HCl acid as acid fracturing fluids in three different carbonates: Austin chalk, Indiana limestone, and Silurian dolomite. The impact of rock hardness and surface roughness on conductivity was thoroughly studied. We collected six core samples, two from each type, with 1.5-in. × 6-in. dimensions. A saw was used to cut the samples, creating smooth initial fracture surfaces. A Brinell hardness tester (FH-9 model) was used to measure the rock strength, while a Kruss high-resolution surface analyzer (SRA) was used to scan the rock surfaces and measure the roughness. The acid fracturing treatment was carried out using a coreflooding system at 100 °C and 1000 psi confining pressure. The fracture conductivity was measured before and after treatment utilizing different flow rates (2.5-10 cc/min) and overburden pressures (1000-2500 psi). The reactivity of acid with a rock was quantified by tracing the calcium ions in the effluents collected from coreflooding outlet at a fixed time interval. The analysis was conducted with inductively coupled plasma (ICP). It is observed that rock hardness and roughness have a significant impact on fracture conductivity. Also, the type of treating fluid and rock determines the generated rock roughness, where higher reactivity results in higher roughness and hence conductivity. Glutamic diacetic acid (GLDA) chelating agent generated sufficient fracture conductivity in chalk and limestone rocks, while HCl acid generated high conductivity in the three rock types. This study sheds new insights on the selection of the acid fracturing fluid for different carbonates, and less reactive rocks such as dolomites are not good candidates to be treated with chelating agents because of its low reactivity. Calcite rocks (i.e., limestone and chalk) can be treated with chelating agents or strong acids.