Laboratory evaluation of acid effectiveness for well stimulation in the Niger Delta, Nigeria

This paper presents data that attest to the need for acid effectiveness evaluation for determining formation specific optimum acid recipes prior to acid work in the Niger Delta reservoirs. The results of a complete acid effectiveness evaluation study for two oil reservoirs (A and B) and one aquifer system (C) show that the regular mud acid (RMA) commonly used for stimulation in the Niger Delta may be damaging under many circumstances. The extent of damage by the RMA and the improvement through properly designed recipes are shown by laboratory analytical tests that are scaled to field and well condition. The analytical work includes characterization of the reservoirs. draulic unit (HU) concept that combines the geological (texture and mineralogy) and petrophysical (porosity, permeability and capillary pressure) controls of the rock quality was used to delineate the reservoirs into units with similar fluid flow characteristics. This zonation enabled the determination of the acid recipes by HUs and thus facilitated the extrapolation to uncored intervals or wells. timum acid sequences for each HU were determined on the basis of pre- and post-stimulation indices, compressive strengths and critical velocities. Visual observations of core material at the end of the acid flush and iron sequestrant analysis were also used as interpretation tools.