Vapor desorption and electrical properties cwlsCentrifuge desaturation is not a recommended practice in electrical properties determinations mainly due to significant evaporation that occurs.With tight gas sands this evaporation error can exceed 20%.A secondary issue involves the potential uneven brine distribution within a sample due to residual effects The initial step in most electrical properties testing involves the determination of the formation factor, F.
This analysis is straightforward but basic protocols must be followed to avoid error and data artifacts.Each sample must be flushed with a sufficient volume of synthetic formation brine to establish rock / brine equilibrium and each sample must be 100% saturated with brine.Samples that are non-uniform and are of low porosity will exacerbate the problems associated with equilibrium and entrained gas.
In particular, the samples must be flushed with brine against back pressure, soak cycles employed and resistances monitored on a daily basis with the time base set against the permeability range of each sample.For example, high permeability high porosity sandstones may well equilibrate electrically within 4 to 6 days.With a tight gas sands, stability might not actually be reached until 4 to 6 weeks have elapsed.Independent assessment for any remaining gas must also be CoCw Clay Conductivity.
Clay conductivity determinations are useful in conventional reservoirs where the formation brine is relatively fresh (less than 50 g/L salt) and clay content is variable and generally above 5% of the grain structure by weight.CoCw analysis can also be of use where the formation brine is either variable or is not well Samples are flushed with a sequence of a minimum of three saline brines ending with the final formation brine.
The rock conductivity is monitored to stability for each brine using the techniques outlined in the preceding formation factor section.Typically, the saturation exponent is determined on initially clean and dry samples proceeding from 100% brine saturation to a final irreducible brine saturation, Swi.
(Issues of fresh/preserved state analysis, wettability and elevated temperature are outside of the scope of this study.)
The determination of the saturation exponent n (or of incremental n values) is dependant upon two main precepts: the control of an even desaturation process through use of a porous plate and the material balance the final brine saturation percent, Swi.During the desaturation process, the rock fabric controls the desaturation pressures needed and minimum time required.Many rock types are susceptible to desaturation that is too rapid, leading to non-uniform saturation profiles and anomalous resistivity response.
Therefore, incremental pressure steps should be employed to control the desaturation process
The determination of volumetric equilibrium at each pressure step is best approached with a conservative definition of stability.In practice, three days of no volumetric change is reasonable standard of equilibrium for most rock types.
The second critical element in determining laboratory based saturation exponents, is the ability to verify Swi values.Low porosity rotary and conventional plug samples are particularly susceptible to errors in Swi due to the relatively small pore volumes involved.
Specifically, production-based Swi values should be confirmed by the differences between pre and post-test dry weights and the Swi weight as well as final Dean-Stark extraction.Dean-Stark extraction must be carefully assessed with regard to the potential damage to rock structure as well as considerations to free and bound water issues.With 1 inch diameter samples uncertainties greater than 0.01 cc can introduce un-acceptable error.These errors are cumulative and are resultant from volumetric desaturation uncertainties, pore volume variability and most importantly dry weight variability.
1000 psi Plate / Membrane System.
The 1000 psi plate/membrane system was designed primarily to improve saturation exponent accuracy by lowering the final Swi saturation obtained in low porosity materials (3 to 8 % porosity).Few conventional reservoirs would require analysis with this high of a capillary pressure in order to model reservoir Uncertainty in saturation exponent values is usually unacceptable if conventional low porosity samples are desaturated to only 70 or 80 percent using an industry standard 15 bar plate with a maximum 200 psi air/brine desaturation pressure.Note again that cumulative errors greater than 0.01 cc often produce unacceptable results.
However, where pore structures exhibit varied micro and macro pore throat components, the higher desaturation pressure allows for a more inclusive investigation of the resultant variable saturation exponent n.If the resultant n values are basically linear over the full desaturation range, the gained confidence of Vapor Desorption.
Capillary Pressure and vapor pressure relationships have been investigated and presented in the literature by Calhoun (4), Collins (5) and Melrose (6).More recently, Newsham (7, 8), has expanded these earlier studies to define vapor desorption as a possible mechanism to describe the capillary pressure/rock fabric/brine salinity relationships within specific basin-centered tight gas sand reservoirs.Vapor desorption methodologies were developed within the laboratory to model these systems and achieved air / brine capillary pressures in excess of 12000 psi.
Pc = - ln (RH / 100 ) RT / Vm
7) The laboratory basics start with an initial desaturation of the samples to Swi using a maximum capillary pressure of 1000 psi.Both plate and centrifugation were used in the studies by Newsham (7, 8) to achieve the 1000 psi Swi step, but this investigation is limited to the use of plate capillary pressure as the appropriate methodology due to the salinity / saturation The 1000 psi step is foll