Experimental Study of Acid Fracture Conductivity on Soft Carbonates

Acid fracturing is a well stimulation technique used in carbonate formations. Its success relies upon the creation of conductive pathways that remain open after acid pumping stops. Through acid fracture conductivity experiments, it is possible to optimize treatment variables such as acid type, acid concentration, and flowrate. In this study, eleven Kansas Chalk outcrops and four North Sea cores were tested for acid fracture conductivity. Both carbonate rocks are relatively soft. The feasibility of acid stimulation is investigated. The reaction of these rocks to straight 15% HCl, and to 15% HCl with corrosion inhibitor, surfactant, non-emulsifying agent, scale inhibitor and gelling agent, were analyzed. The variables in the experimental study include: temperature, acid type, acid concentration and contact time. These parameters were selected to mimic the field conditions. The acid tests were performed in a lab setup that reproduces these field settings. In order to quantify the volume of rock dissolved, the rock’s surfaces were scanned before and after acidizing with a profilometer device. The created conductivity was measured for most samples at different closure stresses. In addition, porosity, Brinell Hardness, solubility tests, X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF) and Scanning Electron Microscope (SEM) were performed on selected samples. The results of this study show that including the aforementioned additives in the acid decreased the differential etching of soft carbonates. The effect was seen for both formations. The North Sea cores developed a film on the fracture surface during the acid treatment. After analyzing portions of rock and residue, it was found that this insoluble material was mostly composed of quartz and some clay. Differential dissolution of the North Sea cores was not observed. Therefore, poor conductivity from these cores was obtained under the experimented conditions. In addition, it was also noticed that increasing the contact time from 15 minutes to 25 minutes did not enhanced the resultant conductivity. Furthermore, for both formations, higher volume of rock dissolved did not result in higher conductivity. The Kansas Chalk outcrop produced high conductivity at low closure stress when using straight acid, but it rapidly declined with increasing closure stress. Based on the observed results, acid fracturing stimulation with the tested fluids is not recommended for the North Sea cores. The feasibility of using other methods to increase productivity should be investigated