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MOUNT SOPRIS INSTRUMENT CO., INC.
Recognized World Leader for over 50 years manufacturing Borehole Geophysical Logging Systems
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Estimating Hydrogeologic Parameters with MGX II Loggers Utilizing portable logging systems in ground water studies has emerged as a cost-effective method of estimating important hydrogeological parameters. Water resistivity, storage coefficient, hydraulic conductivity, total dissolved solids, aquifer porosity & permeability, and several other salient properties can be estimated after a few logging runs with MSI portable loggers, and your understanding of local geology. There are several excellent technical papers on the subject, but a few fundamental relationships and summaries are valuable for getting quick results in the field. Once these essential parameters are known, inferences about complex ground water exploitation and development can be drawn. Ground Water Resistivity / Total Dissolved Solids Formation water quality is a fundamental parameter. Electric logs are commonly used to determine water quality, and Jorgensen (1996) described a ratio method to estimate these parameters essentially without using often unavailable porosity, cementation exponent, volume, and type of clay. The ratio method is a fairly simple method of estimating these parameters in full saturated aquifers; all you need is a normal resistivity output from the MSI Polyprobe string. Jorgensen's relationship says that resistivity of the formation water, at some temperature (Rx,t) is proportional to the ratio of the formation resistivity (Ro, the 64" normal from the polyprobe in water saturated formations) to the resistivity of the flushed zone (Rxo,t, usually the 8" or 16" normal from the polyprobe) multiplied by the resistivity of the mud filtrate, Rmf. Rw,t = (Ro,t/Rxo,t) * Rmf,t Since Specific Conductance, SC (measured in uS/cm) = 10000/Rw,t, the total dissolved solids (from Hem, 1989), TDS = (SC)*(P) where P is a conversion constant plus a dimensionless correlation factor in units of Siemens/cm. P typically ranges from about 0.5 for NaCl to 0.9 for some alkaline waters. Fisher and Friedman (1989) described P values of 0.67 for many fresh ground waters. With one pass in an open borehole, and environmentally corrected resistivity traces from the MSI Polyporbe, estimates of formation water quality and total dissolve solids can be made and plotted essentially in real-time. Contact MSI for more details. Porosity MGX II loggers equipped with density, neutron, or sonic probes can be used to estimate porosity. All of these methods measure some rock property related to porosity. Since both density and neutron methods require a downhole radioactive source, full waveform sonic may be your choice method. These logs , combined with MSI gamma and resistivity logs, and WellCAD interpretation software can be used to plot porosity profiles within aquifers or other hydrostratigraphic units. Sonic porosity can be calculated using the Wyllie Time-Average equation: Porosity (Sonic) = Tlog - Tma / Tf - Tma), where Tlog is the transit-time from the log, Tma is the matrix transit time, and Tf is the transit time of the fluid. Tf is typically about 189 - 208 uS/ft for fresh to saline waters. In uncompacted sand aquifers, sonic porosities generated by the Wyllie equation are too large, but ca be corrected using widely published charts, or by comparing porosity obtained from another source, such as core, and applying a correction factor for the log values. When combined with density data sonic data is used to draw inferences about a variety of litho-elastic properties including bulk modulus, Young's modulus, shear modulus, and Poisson's Ratio. Density porosity calculations are similar in form to the sonic derived porosity, but may be more quantitative in uncompacted (low clay-content) sands because the log records total bulk density of the matrix and fluid-filled poer space. Density Porosity = Dma - Dlog / Dma -Df, where Dma in the matrix density (usually 2.65 gm/cc for sand), Dlog is the log density, and Df is the fluid density (1.0 for water). Estimating porosity with MSI neutron probes requires the user to determine an empirical relationship for each probe. A typical relationship has the form Neutron Porosity = 10 ^(((0.00125*D-0.00130)*CPS + 2.825D-0.2947)), where D is the borehole diameter (inches) , and CPS is the log response in counts per second. This emperically-derived relationship was generated using an MSI LLP-2676 with a 3 Curie Am241Be source. The formula will change slightly with each probe / source package. Effective Porosity Cross-plotting porosities derived from density and neutron logs has been used in the ground water industry to estimate effective porosity. This technique corrects for clay content and yields values close to effective porosity derived from core analysis. The equation, EffPor = ((Dpor^2 + NPor^2)^1/2) + ((Dpor^2 + NPor^2)^1/2) / 2, can be solved with a simple spreadsheet filled with DPor (log derived density porosity) and NPor (log derived neutron porosity values. Permeability This important parameter is difficult to derive directly from logs, but it is possible when there is a relationship between porosity and permeability, as in formations with higher permeability values. Temples (1996) developed a technique to estimate permeability from log derived porosity values in sands with known permeabilities greater that 100 millidarcies (mD). For more information about the relationship, y = 30.713 * 10^0.061797P, contact MSI. Temples compared permeability from cored sections of a well to log-deerived permeability with a correlation coefficient of 0.872. Hydraulic Conductivity This fundamental hydrogeological parameter can be calculated once formation permeability is known. to calculate intrinsic permeability from logs, use the well known equation, K = k*(pg/u), where u is the dynamic viscosity of the fluid at formation temperature, p id the density of the fluid at formation temperature, and k is the log derived permeability. |
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