Matrix acidizing is a remedial well stimulation that done to overcome formation damage near wellbore or improve the permeability. Although acidizing treatments are proven and abundant there is still inherit from formation damage when pumped. Acid-induced asphaltene sludging is becoming an increasing cause of oil well stimulation Failure.
The objective of the paper is to evaluate the performance of coconut oil as a bio-oil dispersant against commercial dispersants in preventing asphaltene sludge while acidizing carbonate cores with 15 wt.% HCl and a chelating agent. A Kuwaiti crude oil was used in this study has an API of 38° and 2% asphaltene content. The crude oil was characterized by a variety of analytical techniques including total acid and base numbers (TAN, TBN), saturates, aromatics, resins and asphaltene analysis (SARA), density, viscosity and elemental analysis. Indiana limestone cores were used with average porosity of 16% and permeability ranges (9-13) md. X-ray diffraction (XRD) was used to analyze the mineral and clay content in the cores. Sludge tests were used to examine the acid and oil compatibility using anaging cell under 500 psi and 160°F with oil to an acid ratio of 1:1. Coreflooding experiments under reservoir condition were done with the selected two acid systems, 15 wt. % HCl and achelating agent. Indiana limestone cores with a permeability of 7-12 md were initially saturated with the crude oil then acid was injected until breakthrough. The injected acid volume was recorded and CT-scan imaging of the cores after the acid treatment was used to evaluate the structure and the propagation of the wormhole. The effluent fluids were analyzed by inductively coupled plasma (ICP) and pH measurements.
The results for a Kuwaiti crude oil showed the formation of 13 wt% sludge with 15 wt% HCl and it increased to 19 and 30 wt% with increasing acid concentrations to 20 and 28 wt%, respectively. The presence of iron(III) in the system increased the sludge precipitation to 17.8 wt% at 15 wt% HCl and 3,000 ppm iron concentration. The sludging decreased to 7.5 wt% by adding 300 ppm coconut oil to the system. The formation of asphaltene sludge in the carbonate acidizing retards the wormhole propagation. Hence, the injected acid volume to the breakthrough decreased from 1 to 0.4 by adding 300 ppm coconut oil to the acid system. A conical wormhole was formed with the injection of 15 wt% HCl, comparing to a uniform wormhole in the presence of coconut on the acid system. In the case of stimulating the cores with achelating agent (20 wt% GLDA), the coconut oil exceeds the expectations with the minimum pore volume needed to breakthrough compared to the GLDA alone or with the chemical dispersant B.
This study concluded that the use of dispersant can help reduce the asphaltene sludge and create better acid propagation through the core. The results can be employed to design the optimum acid formulation and create the desired wormhole in carbonate formations.