Evaluation of the Effectiveness of Matrix Stimulation Recipes for High CO2 Carbonate Reservoirs for Bypassing Formation Damage
Authors: Sazali Y. , Misra S. , Sazali W. L. , Ibrahim J. M. , Graham G. M. , Kidd S. L. , Ohen H. A. , Gödeke S. 
Published: January 2018

Abstract
This paper presents the evaluation of the effectiveness of two novel organic acid stimulation fluids for matrix acidizing of a high temperature, high CO2 gas carbonate reservoir offshore Sarawak, Malaysia. During high CO2 gas production, formation damage arising from fines migration and calcite scales may plug the near wellbore pore throats resulting in productivity impairment.
Therefore, a study was conducted to investigate the effectiveness of two different stimulation recipes for matrix acidizing of a high CO2 carbonate reservoir. Core-flood tests were performed to determine the pore volume to breakthrough (PVBT) and time to breakthrough (TBT) at the various selected injection rate. Characterization and morphology studies of wormhole were done using micro CT imaging.
Initial selection of stimulation fluids for matrix acidizing was based on the mineralogical composition of the main producing formation. Bottle tests were conducted whereby concentration of ion Ca2+ and weight of rock sample versus time was measured. Two stimulation fluids, products A and B were selected and core-flood test were performed at 275°F on the reservoir cores. Product A is a proprietry chemical available in the market whilst product B is an in-house developed chemical.
Four different stimulation fluid injection rates (1.5, 3, 5 and 7 ml/min) were selected. Pore volume breakthrough (PVBT) and time to breakthrough (TBT) versus injection rate were evaluated and optimum injection rate for each product was determined. Wormhole length and morphology created by the stimulation fluids were visualized and determined through micro CT imaging.
Mineralogy analysis of the production zone cores showed content of 70-80% calcite, 6-7% authigenic clays, 2-5% dolomite 2% plagioclase, 2% K-feldspar and 1-2% pyrite. Quartz and siderite are less than 1%. Selection of stimulation fluids was based on the mineral compositions and its dissolution rate in the bottle test. Products A and B both recorded the same optimum injection rate. Observation also showed increase in injection rate from 3ml/min to 5 and 7ml/min lead to higher consumption of stimulation fluids with no significant changes to time to breakthrough. On the contrary, reducing injection rate from 3ml/min to 1.5 ml/min lead to formation face dissolution (Product B). However, product B showed temperature instability at 275°F whereby white emulsion effluent were collected during core-flood test.
Therefore, optimization of product B is required to improve its stability. Dissolution pattern for each stimulation fluid were evaluated through micro CT imaging. At 3 ml/min, ideal wormhole morphology was created for both Product A and B. This paper presents the study of two novel organic acid stimulation recipes in matrix acidizing the high CO2 carbonate reservoir. Performance and stability of each stimulation fluids at 275°F were evaluated by determining the pore volume and time to break through. The paper also present recommendations for optimum injection rates for matrix acidizing a high CO2 carbonate reservoir.