Deviation From Standard Sandstone Acidizing Practices: Chronicle of a Forgiving Acid System

Abstract

The process of sandstone acidizing is fraught with the danger of further damaging the reservoir. The odds of increasing formation damage is raised if standard acidizing practices are not followed primarily due to non-technically related issues like logistics. This paper reviews the stimulation of two low-pressure oil producers and another oil producer, which was previously producing below potential. Standard practice indicates that a HF based system be flowed back or lifted with nitrogen to reduce the risk of additional formation damage due to factors including emulsion / sludge / secondary precipitate formation. For these case histories, the residence time of the acid in the reservoir was between 18 hrs to 96 hrs due to the non-availability of nitrogen and poor well inflow. The remarkable aspect of these operations was that there was no evidence of further impairment in all the wells. In fact, two of the wells demonstrated superior performance after production rate stabilized. These successes are partially attributed to the unique chemistry of the retarded acid system, favorable rock mineralogy, exhaustive lab testing / field QC, the presence of an acidic over flush and the use of additives that prevented formation of emulsion and sludge. This acid system can be used in depleted / low GOR reservoirs where over-displacing is a poor option in the absence of flow-back aid and also for single stage acidizing in low carbonate sandstone reservoirs. This paper attempts to follow through the process of candidate selection with a focus on nodal analysis. It also defines the factors affecting success rates in the stimulation of depleted reservoirs, offers a superior acidizing system for mitigating the effect of deviations from standard acidizing practices and reinforces the possibility of single step sandstone acidizing (with a brine cushion ahead) especially in low carbonate content sandstone.