Centrifuge Measurements of Capillary Pressure: Part 1-Outflow Boundary Condition
- D.J. O'Meara Jr. (Shell Development Co.) | G.J. Hirasaki (Shell Development Co.) | J.A. Rohan (Shell Development Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- February 1992
- Document Type
- Journal Paper
- 133 - 142
- 1992. Society of Petroleum Engineers
- 2.4.3 Sand/Solids Control, 5.1 Reservoir Characterisation, 1.6.9 Coring, Fishing, 2.2.2 Perforating, 4.3.4 Scale, 5.2.1 Phase Behavior and PVT Measurements, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 1.2.3 Rock properties, 5.5 Reservoir Simulation, 5.4.1 Waterflooding
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The capillary pressure curve is estimated from centrifuge measurements withthe assumption that Pc=0 at the outflow end of the core. With the proper endpiece to support the core sample, this boundary condition is valid inpractically all circumstances. If the end piece is wetted by the producedliquid, however, experiments show that the capillary pressure curve is alteredsignificantly. Theoretical analysis demonstrates that film drainage candisplace the zero-capillary-pressure boundary condition to the bottom of theend piece or farther. Accordingly, we recommend that the displaced phase notwet the end piece; in the case of water, a teflon end piece is recommended overthe commonly used rubber one. When the end piece is not a problem, thecondition for 100% liquid saturation at the outflow face can be expressed as acritical Bond number, which is rarely exceeded. Saturation profiles for anexperiment just below the critical Bond number were in excellent agreement withpredictions. Pendant drops at the outflow face can result in a nonzerocapillary pressure at the outflow face, but this effect is insignificant. Goodagreement was obtained between mercury injection and centrifuge-measuredcapillary pressure curves. Attention is drawn to the importance of corejacketing, core cleaning, and equilibration times in providing reliablecentrifuge-measured capillary pressure curves. pressure curves. Purpose andScope Purpose and Scope Although the centrifuge method for measuring capillarypressure has been used throughout the industry for more than 40 years,questions about its validity continue. The two most important questions concerncavitation and the outflow boundary condition of zero capillary pressure. Theboundary condition is investigated here; cavitation is discussed in Ref. 1.
Before discussing the boundary condition, we raise several otherconsiderations. We begin with a brief description of the centrifuge method andcompare it with other methods for measuring capillary pressure curves. Then wepresent our modifications to the traditional pressure curves. Then we presentour modifications to the traditional centrifuge method. The next sectionexamines the topic of comparing capillary pressure curves, whether fromdifferent methods or from the centrifuge method applied to slightly differentcores. We then discuss important sources of error in centrifuge measurements.Having identified sources of error and methods of crosschecking results, weconsider the boundary condition. The first part of the boundary-conditionsection ignores the effects of the end piece supporting the core; the finalsections show under what circumstances the end piece is likely to beimportant.
The centrifuge method entails increasing the centrifuge speed in steps andmeasuring at each step the amount of liquid produced from the core atequilibrium when all flow has ceased. An important assumption in analyzing thisexperiment is that Pc=0 at the outflow end.
There are a variety of possible centrifuge capillary pressure experiments,depending on the drainage mode and fluids involved. Measurements usually aremade in the primary drainage mode where oil or air replace water in a core pluginitially 100% saturated with water. There are other possibilities however:e.g., secondary drainage from a core initially saturated at waterflood residualoil and imbibition into a core at irreducible water saturation. The conclusionsof the present study are relevant to all these experiments. For simplicity, thecore is assumed to be initially 100% saturated with the displaced phase. Thephase entering the core is called the invading phase.
Centrifuge Measurement of Capillary Pressure Compared With Mercury,Porous-Plate, and Water-Vapor-Desorption Methods
The two major alternatives to the centrifuge method are the porousplate andthe mercury-injection methods. Mercury injection has porousplate and themercury-injection methods. Mercury injection has the advantages that it isquick (results in a few hours) and can attain high pressures. Its disadvantagesare that wettability and clay effects are not modeled suitably. By comparison,the porous-plate method can use reservoir conditions and fluids relativelyeasily. However, weeks or months may be required to complete a measurement. Thecentrifuge method falls somewhere between these two. It is slower than mercurymethod (days vs. hours) but uses more representative fluids. It is much fasterthan the porous-plate method but does not provide reservoir conditions aseasily. The recently developed water-vapor-desorption method is not really analternative to the above methods but rather a means to extend them intohigh-capillary-pressure regimes.
Several recent papers compare the equivalency of the various methods.Omoregie provided examples for which differences between mercury/air andcentrifuge curves result from clay and wettability effects. In addition, hedemonstrates how porous-plate measurements are affected by the time allottedfor equilibration. Porous-plate and centrifuge results differ when 4 days areallotted Porous-plate and centrifuge results differ when 4 days are allottedfor equilibration for each pressure in the porous-plate method but agree when10 days are allotted. In a comparison of the centrifuge, porous-plate, andwater-vapor-desorption methods, Melrose porous-plate, andwater-vapor-desorption methods, Melrose showed excellent agreement in a studyof matched Berea cores covering capillary pressures from 0.7 to 9.5 MPa. Thisagreement is particularly relevant to our cavitation investigation because thecentrifuge and water-vapor-desorption methods entail negative absolutepressures (i.e., tension) in the displaced phase, whereas the porous-platemethod has positive phase pressures. Ward and porous-plate method has positivephase pressures. Ward and Morrow obtained good agreement between the centrifugeand water-vapor-desorption methods when they used the latter method to extendcentrifuge curves of low-permeability sandstones to higher capillarypressures.
Centrifuge Capillary Pressure Curves by Parameter Estimation
The present method for calculating capillary pressure curves fromequilibrium data (measured or extrapolated) is a modified version of Hasslerand Brunner's traditional method. For the following derivation, we assume thatcavitation is not present and that Pc=0 at the outflow.
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