A high CO2 carbonate gas field offshore Sarawak, Malaysia is scheduled for development. Fines migration is typically not a serious formation damage mechanism in typical carbonate formation due to the absence ofmigratory authigenic clays. However, these reservoirs (subdivided in S3, S2, S1 formation) have an average 8% clays, of which over 50% are migratory Illites and 15% migratory Kaolinite. Therefore, fines migration exacerbated by the low permeability of these rock (around 10 mD for the injection formation) become a potential production and injection problem.
A study involving rock mineralogy and dynamic flow to evaluate the potential fines migration damage within theproduction and injection interval was conducted. Critical flow ratesto the onset of fines migration for the various fluid including dry gas, wet gas and CO2 saturated brine were determined. This paper discusses the laboratory findings and factors contributing to fines migration of these carbonate rock types. Core flood tests were conducted to determine critical flow rate of high CO2 hydrocarbon gas (70:30 CO2:N2) and CO2 saturated brine within the production gas zone. Mineralogy analysis of S3, S2 and S1 formation rock were conducted to determine clay types, amount, location and morphology. Pore throat size distributions were determined through Mercury Injection Capillary Pressure (MICP) tests for pore level characterization of the rock. Total Suspended Solids (TSS) were measured during each incremental rate stage and Scanning Electron Microscopy, Energy Dispersive X-Ray (SEM-EDX) analysis was performed on the effluents collected. Similar methodology was applied to evaluate the injection interval. Core flood tests were performed with supercritical CO2.
Core floods performed with CO2 saturated brine on S3 and S2 cores showed damage at a critical flow rate of 10ml/min with permeability recovery of 65-75%. This shows that CO2 saturated brine tends to mobilize the fines by inducing movement of migratory clays such as Illite and Kaolinite which bridges the pore throats. Core floods performed with high CO2hydrocarbon gas showed permeability recovery of 93-96% at rates up to 10 ml/min. SEM analysis on effluents collected showedpresence of silicate and carbonate particle with size of 5-50μm. This indicates that migratory clays have been entrained and bridged the pore throats. S3 and S2 core plug pore throat size were measured with D(10): 5-15μm, D(50): 1-9μm and D(90), 0.02-3μm. These size distributions indicate susceptibility for bridging of clay minerals, mainlyIllite and Kaolinite having size < 4μm.
This study investigates the factors that contribute to fines migration incarbonate formation containing migratory clay minerals, which is novel. The effect of CO2 saturated brine and high CO2 dry hydrocarbon gas flow during production were studied. Results indicate that a high potential for fines migration in this type of carbonate system exist for CO2 saturated brine flow and less potential for dry CO2 flow.
