Summary
Fines migration is a recognized source of permeability reduction andproductivity decline in consolidated and unconsolidated cores. Previousinvestigations have shown that salinity, flow rate, pH, and temperaturesignificantly affect the process. The presence of residual oil saturation(ROS), fractional flow of oil and water, polarity of oil, and core wettabilityare other factors whose effects have not been studied systematically. Thisexperimental study investigates the effect of these factors. Both oilsaturation and the wettability condition of the core are shown to affect theextent and rate of permeability impairment significantly. Wettabilitydeterminations made from end point relative permeabilities clearly show thatoil-wet cores sustained slower and less damage than water-wet cores.
Introduction
Formation fines may be defined as unconfined solid particles made up of clayminerals or non-clay species deposited over geologic time or introduced duringcompletion or drilling operations. Fines are always present in sandstonereservoirs. and their migration with flowing fluids can cause permeabilityimpairment. More detailed knowledge of fines mobilization, migration, andplugging processes is necessary for a complete understanding of the problem. Toinvestigate the potential for fines migration, "water-shock"experiments are carried out. These involve a sudden change from salt to freshwater in flow through a sandstone core sample. The major factors affecting thefines-migration process are salinity, flow rate, pH. temperature, rate ofsalinity change, and adsorption of organic material from oil. In this study, additional factors that come into play when two fluid phases are present in acore are investigated.
Coreflooding Apparatus
Core samples (about 2.54 cm [1 in.] in diameter and 8.0 cm [3.1 in.] long)were cut along the bedding plane from blocks of Berea sandstone. The cores wereplaced in a Hassler core holder. The difference between core fluid pressure andthe confining pressure was kept at a minimum of 2.1 MPa [300 psi]. Multiplepressure taps along the length of the core were used for measuring pressuredrops across the inlet section (Section 1, 1.5 cm [0.6 in.] long), middlesection (Section 2, 5.1 cm [2 in.] long), and outlet section (Section 3, 1.7 cm[0.7 in.] long). For thin-core studies, the cores were carefully cut in slicesperpendicular to the bedding plane. Experiments on these cores were conductedby "spotting" them in epoxy. Two accumulators with internal pistonswere used for on-line storage of mineral-oil/crude-oil and brine. In this way, it was possible to avoid flowing brine or crude through the pumps. Three setsof transducers (nine transducers in all) of increasing pressure range (0 to 69,0 to 1380, and 0 to 6900 kPa [0 to 10, 0 to 200, and 0 to 1,000 psi]) were usedto monitor different sections of the core to record differential pressure withbetter accuracy. The different fluids used in the experiments were brine (3%NaCl), mineral oil (Sontex 55TM, viscosity 10.0 mPa-s at 24C [10 cp at 75F]), Crude Oil 1 (mixture of 65.0% crude oil and 35.0% toluene, viscosity 6.85 mPa-sat 24C [6.85 cp at 75F, asphaltene content 3.5%), Crude Oil 2 (viscosity 22.9mPa-s at 24C [22.9 cp at 75F], asphaltene content 6.5%). Toluene Extract 1(3.0% asphaltene). Toluene Extract 2 (6.5% asphaltene). and pentane extract. Polar pentane extracts were prepared by mixing crude oil in excess pentane, separating the lighter pentane-soluble part from the heavier pentane-insolublepart by centrifuging (at 10,000 rev/min for 20 minutes), and collecting the topfraction by decanting. The bottom fraction was recovered with toluene and isdescribed as the toluene extract.
Experimental Results
The experiments can be divided into the following categories:single-phase experiments,two-phase experiments with nonpolar oil,two-phase experiments with polar crude oil,two-phase experiments withpolar compound extracts,steady-state two-phase experiments, andscanning electron microscope (SEM) study.
For the purpose of this discussion.the single-phase permeability ratio (PR) is the ratio of freshwaterpermeability to brine permeability. The two-phase PR is the ratio of freshwatereffective permeability to brine effective permeability. The two-phase PR(absolute) is the ratio of freshwater effective permeability to brine absolutepermeability. A damage time (DT) is defined as the PV of fresh water injectedafter which the PR remains unchanged. A particle discharge time (PDT) isexpressed as the PV's injected after which no particles were observed in theeffluent. Porosity values for the cores ranged from 19.0 to 21.0% and PV'sranged from 8.0 to 9.0 mL. except in the thin-core studies. DT ranged from 1.35to 13.0 PV. Sor, Swr, and oil recovery were also calculated.
JPT
P. 646⁁
