Novel Approach for Sandstone Acidizing Using in Situ-Generated Hydrofluoric Acid with the Aid of Thermochemicals.

Abstract

In this study, an in situ-generated hydrofluoric acid (HF) was used for sandstone acidizing, where an acid precursor (ammonium fluoride NH4F) reacted with a suitable oxidizer (sodium bromates NaBrO3) in an exothermic reaction. First, the new chemical mixture was prepared to react with pure quartz samples and the reaction effluent was analyzed to identify the presence of Si+ ions using the inductively coupled plasma (ICP) technique. Core flooding experiments were performed using Gray Berea sandstone cores (6 in. length and 1.5 in. diameter). A preflush stage of 5 PV of 7 wt % HCl was injected to remove any calcite content in the core. The main chemicals were then flushed for 3 successive cycles of 1 PV each. To assure core integrity, scratch tests and NMR scans were run on the core sample before and after the treatment. The new chemical mixture could dissolve the quartz sample and reduce its weight by 80 mg. The concentration of the dissolved Si+ ions was more than 90 ppm. This proves the capability of the chemical mixture to generate HF. The initial core permeability was measured at a stabilized flow rate of 2 cm3/min to be 33 mD. After the acid preflush stage, the core permeability reduced to 31 mD. Core permeability increased immediately after the first treatment cycle and reached 41 mD. At the end, the core flooding results showed a permeability improvement for Gray Berea sandstone cores by almost 40%. The ICP analysis of the effluent showed a total amount of chelated Si+ ions of about 10.5 mg. In addition to the high temperature generated in the near-wellbore area, the pressure increased because of the produced nitrogen gas from the exothermic reaction and reached about 600 psi. The scratch test showed an increase in the sample uniaxial compressive strength from 7432 to 9235 psi. The dynamic Poisson’s ratio and the dynamic Young’s modulus increased as well from 0.17 to 0.19 and from 2159 to 3585 ksi, respectively. The enhancement in the mechanical properties of the core can be attributed to the presence of the potassium element in Berea cores and its solidification reaction with the HF generated. The NMR measurements of the core sample before and after the acidizing process show an increase in the core porosity; however, the core preserved its original pore system. Upon application of this new stimulation technology, the true production potential of sandstone reservoirs can be achieved, well tubular corrosion will be minimized, and handling hazardous chemicals such as HF will be avoided. Most importantly, controlling the reaction rate, by controlling the amount of exothermic chemicals, can ensure deep acid penetration as well.