In-Situ Saturation Measurements Improve Analysis and Interpretation of Laboratory Miscible and Immiscible Displacement Processes
- J.F. Berry (BP Research Centre) | A.J.H. Little (BP Research Centre) | H.J. Salt (BP Research Centre)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1991
- Document Type
- Journal Paper
- 429 - 436
- 1991. Society of Petroleum Engineers
- 4.6 Natural Gas, 4.1.2 Separation and Treating, 1.6.9 Coring, Fishing, 5.3.4 Reduction of Residual Oil Saturation, 1.8 Formation Damage, 5.4.2 Gas Injection Methods, 5.4 Enhanced Recovery, 5.4.9 Miscible Methods, 5.3.1 Flow in Porous Media, 4.3.4 Scale, 5.7.5 Economic Evaluations, 5.8.9 HP/HT reservoirs, 5.5.2 Core Analysis, 5.4.1 Waterflooding, 5.2.1 Phase Behavior and PVT Measurements, 5.2 Reservoir Fluid Dynamics
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A laboratory evaluation of miscible and immiscible oil recovery processeswas performed on a sandstone core composite containing viscous processes wasperformed on a sandstone core composite containing viscous crude oil. Becauseconventional effluent data cannot clearly identify the evolution of transientsaturation distributions, in-situ saturation monitoring was used to providedistributed saturation histories. The data obtained suggest that phase behavioris a major factor determining the amount and distribution of residual oil.
Estimates of miscible and immiscible hydrocarbon recovery efficiencyobtained from laboratory-scale displacement tests on reservoir core materialare used in economic evaluation of reservoir development options. Livereservoir fluids are often used at reservoir conditions in such tests toincrease the applicability of results to modeling reservoir displacementprocesses.
Past work has described problems associated with obtaining representativemicroscopic displacement efficiencies from coreflood data. The capillarypressure discontinuity at the core outlet face is known to influence theresults of immiscible drainage displacement tests significantly. Viscousinstabilities influence both miscible and immiscible laboratory-scaledisplacements, and doubts about the applicability of results to the reservoirscale remain. Fine-grid numerical simulation may provide a method for scalinglaboratory-derived data to reservoir-scale processes. However, gravitysegregation and nonequilibrium mass transfer also may have large effects oncore displacement tests. These factors lead to fluid distributions that areunrepresentative of the microscopic efficiency of the displacement beingstudied.
Methods of monitoring in-situ saturation or concentrations are being usedincreasingly to study the evolution of fluid distributions during variousrecovery processes. For water/oil displacements, the additional data have beenshown to improve interpretation by determining the degree to whichlaboratory-scale artifacts influence pressure and production data. The primarygoal of much of the pressure and production data. The primary goal of much ofthe flow-visualization work reported has been to improve our understanding ofthe fundamentals of fluid-displacement mechanisms. These studies often useoutcrop rock and/or refined fluids for convenience.
Alternatively, laboratory flow tests conducted to evaluate recoveryprocesses for specific reservoirs often are limited to conventional processesfor specific reservoirs often are limited to conventional effluent data becauseexperimental conditions preclude the use of in-situ saturation measurementtechniques.
The methodology applied in this study was to integrate tests comparingvarious recovery mechanisms for reservoir evaluation with tests designed tocharacterize both stable and unstable miscible displacements in the same core.The characterization tests, which use refined oils, gave baseline recoveryresponses to improve interpretation of miscible tests for reservoir evaluation.The use of gamma attenuation saturation monitoring (GASM) throughout the studyenabled the identification of capillary end effects and viscous instabilities.GASM also provided a direct comparison of saturation histories during stablefirst-contact-miscible (FCM) and unstable FCM and multiple-contact-miscible(MCM) displacements.
Procedure. All floods were conducted on preserved composite core Procedure.All floods were conducted on preserved composite core (three 6.35cm [2.5-in.]-long core plugs) of 33% porosity. All relative permeability data are based onthe effective oil permeability (at S) presented in Table 1.
Reservoir-condition experiments were performed with live reservoir crude andlive simulated formation brine. Characterization experiments used FCM refinedoils, with and without dopants to enable concentration profiles to be generatedby GASM. Live fluids were not doped because of the possible alteration of phasebehavior. Table 2 gives fluid properties.
Fig. 1 is a schematic of the coreflood apparatus used. The experiments wereperformed at reservoir conditions of 18 degrees C [64 degrees F] and a coreoutlet pressure of 11 MPa [1,600 psig]. A special low-density core holderallowed in-situ saturation monitoring. The core holder used was a 0.3048-m[1-ft] -long, high-pressure carbon composite vessel that was transparent togamma rays. All floods were performed vertically downward to eliminate gravityoverride. A performed vertically downward to eliminate gravity override. A flowrate of 10 cm/h was used for all reservoir-condition corefloods. Table 3 givesthe test sequence. Because the miscible injectant (MI)was dry, it is reasonableto assume that vaporizing-gas drive is taking place in these experiments.
In-Situ Saturation Measurements. In-situ saturation data were obtained byuse of the GASM technique. The average saturation within a 4.064cm (1.6-in.]-wide cross section of the core at five positions was measured. The dataobtained were used to aid interpretation of both the reservoir-conditionexperiments and core-characterization measurements. GASM saturation dataobtained during the reservoir-condition displacements show flood frontpropagation and breakthrough time, increasing confidence in mass-balancecalculations.
Saturation data are presented in two ways. Saturation history describes thechange in saturation at a detector location during the displacement. Saturationprofiles are derived from the saturation at each detector at specific timesduring the displacement plotted against their fractional position along thecore.
To quantify the initial oil saturation before each of thereservoir-condition experiments, displacements were performed with doped andundoped refined oil. During these displacements, brine was immobile; thereforenormalized concentration profiles are presented for each detector.
The HCPV was determined from material balance by use of incremental effluentconcentrations. Saturation monitoring provided the initial saturationdistribution along the core. The mean saturation was determined from an averageof these distributed data. Saturations were calculated from calibration valuesgenerated at 100% fluid saturation at the end of the test sequence.Quantitative GASM data are available for the refined-oil displacements only. Anexperimental error of + 1 % is associated with the saturation measurements.
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