Investigating the Effect of Oil Saturation on Acid Propagation during Matrix Acidization of Carbonate Rocks
Abstract
The existence of an optimum injection rate for wormhole propagation, and face
dissolution at low injection rates during matrix acidizing are well established. However,
little has been documented that describes how the presence of residual oil affects
carbonate acidizing. This study demonstrates the impact of oil saturation on wormholing
characteristics while acidizing field and outcrop cores under reservoir conditions
(200°F). Knowledge of the effect of different saturation conditions on acid performance
will contribute towards designing more effective acid treatments.
Coreflood experiments at flow rates ranging from 0.5 to 20 cm3/min were performed to
determine the optimum injection rate for wormhole propagation when acidizing
homogeneous calcite and dolomite reservoir cores, and low permeability Indiana
limestone cores of dimensions 3 and 6 in. length and 1.5 in. diameter. Absolute
permeability of the cores ranged from 1 to 78 md. The study involved acidizing cores
saturated with water, oil, and waterflood residual oil using 15 wt% HCl. The viscosity of
the crude oil used was 3.8 cP at 200°F. CAT scans were used to characterize wormholes
through the cores. The concentrations of the dissolved calcium and magnesium ions were
measured using Inductively Coupled Plasma–Optical Emission Spectroscopy (ICP-OES)
and the effluent samples were titrated to determine the concentration of the acid.
HCl was effective in creating wormholes with minimal branches for cores with residual
oil (Sor=0.4-0.5) at injection rates 0.5 to 20 cm3/min. Compared to brine and oil
saturated cores, waterflood residual oil cores took less acid volume to cause
breakthrough. Additionally, the wormholing efficiency of regular acid improved with increases in acid injection rates in the presence of residual oil. A decrease in acid pore
volumes to breakthrough for oil saturated cores was noted at high acid injection rates,
which could be attributed to viscous fingering of acid through oil. Unlike brine saturated
and oil saturated cores, waterflood residual oil cores showed no face dissolution at low
acid injection rates. Conclusions from this work aid in the designing of better acid jobs
by highlighting the impact of oil saturation on wormholing characteristics of acid while
acidizing carbonate rocks.
Citation
Kumar, Rahul Pradeep (2014). Investigating the Effect of Oil Saturation on Acid Propagation during Matrix Acidization of Carbonate Rocks. Master's thesis, Texas A & M University. Available electronically from https : / /hdl .handle .net /1969 .1 /152452.