Underground sources of drinking water (USDW) is the current term used to cover fresh and brackish water resources that could be exploited by drilled wells, in contrast to water from surface sources such as lakes and rivers. The base of fresh water (BFW) is the true vertical depth of the deepest aquifer that can produce water of a specified TDS. BFW can be contoured to provide insight into the disposition of USDW. Porosity-thickness and permeability-thickness maps can be generated from petrophysical analysis results. These give volumetric and productivity information that will aid water source development. Governments are taking more interest in USDWs. The US EPA defines any aquifer with less than 10,000 mg/liter TDS as potentially useful water for humans. Many aquifers in the USA are protected by the EPA, which means that these aquifers cannot be used for disposal of oilfield or industrial waste water. Other restrictions on use may also be in force in specific cases. Some aquifers are exempt from protection rules due to existing licenses that permit injection. Water salinity usually increases with depth so shallower aquifers are more likely to fall into the fresh and brackish category. There are many exceptions. Meteoric water can enter porous rock at its outcrop edge, bringing fresh or brackish water to considerable depths. Examples are the Black hills of South Dakota feeding meteoric water into the Cretaceous reservoirs in northern tier States and southern Alberta and parts of Saskatchewan. Another is the western slopes of the Sierras feeding the adjacent deeper rocks in California. Examples of interspersed brackish and saline waters are not hard to find during oilfield evaluations. Shallow water wells are logged by observation of the drill cuttings and potential porous and permeable intervals are noted. Copies of the report are given to the well owner and to appropriate government agencies who assess and map aquifer quality and thickness. A pump-down test is used to determine flow capacity in gallons or liters per minute.
3:
Vshxnd = (PHIN - PHID) / (PHINSH - PHIDSH)
For
coarse to medium grained sands, KBUCKL = 0.0300 to 0.0500,
higher for fine grain, lower for carbonates. Default =
0.0400.
See
Initial Productivity Estimates to convert Kh to
a flow rate.
META/PERM Compare
Permeability Calculated from Various Methods
STEP 5: Calculate apparent water resistivity at formation temperature. In relatively clean rocks, the Archie model using appropriate electrical properties is sufficient:
11: Rwa@FT = (PHIt ^ M) * RESD / A
It is useful to also calculate Rwa at 75F or 25C using Arp's equation, to allow us to compare log derived values to lab water analysis reports or water catalogs: 12: Rwa@75F = Rwa@fT * (FT+
6.8) / (75 +
6.8) with temperatures in
FahrenheitOR 13: Rwa@25C = Rwa@fT * (FT+ 21.5) / 275 + 21.5) with temperatures in Celsius R ECOMMENDED
PARAMETERS:
for carbonates A = 1.00 M = 2.00 (Archie Equation as first published) for sandstone A = 0.62 M = 2.15 (Humble Equation) A = 0.81 M = 2.00 (Tixier Equation - simplified version of Humble Equation) Asquith (1980 page 67) quoted other authors, giving values for A and M, with N = 2.0, showing the wide range of possible values: Average sands A = 1.45 M = 1.54 Shaly sands A = 1.65 M = 1.33 Calcareous sands A = 1.45 M = 1.70 Carbonates A = 0.85 M = 2.14 Pliocene sands S.Cal. A = 2.45 M = 1.08 Miocene LA/TX A = 1.97 M = 1.29 Clean granular A = 1.00 M = 2.05 - PHIe Equation 11 is not shale corrected. If prospective water sands are quite shaly (Vsh > 0.25) or RSH is very low (< 2.5 ohm-m) the Simandoux equation can be inverted to solve for RWa: 14: 2 / RESD = (PHIe ^ M) / (A * Rwa@FT * (1 - Vsh) + Vsh / (2 * RSH) 15: Rwa@FT = xxxx If you get this solved before I do, let me know the result. META/RW Calculate RW
at formation temperature - 5 methods.
Metric and English Units
STEP 6: Convert Rwa@FT to NaCl equivalent (ppm) and TDS (ng/l) Calculate formation temperature:
16: FT = SUFT + (BHT - SUFT) / BHTDEP * DEPTHIF FT is Celsius, convert to Fahrenheit 17: THEN FT1 = 9 / 5 * FT + 32 18: OTHERWISE FT1 = FT Using Crain's Equation inverted for water salinity WSa in ppm NaCl equivalent:
19: WSa = 400000 / FT1 / ((RWa@FT) ^ 1.14)
An alternate method Baker Atlas (2002) 19A: WSa = 10 ^ ((3.562 - (Log (RW@75 - 0.0123))) / 0.955) Convert WSa (ppm) to TDSa (mg/l) using the density of the water plus its so;ute: 20: DENSw = 1.00 + (WSa * 2.16 / 1000000) 21: TDSa = WSa * DENSw Note that 2.16 is the real density of halite – log bulk density is 2.03 g/cc. CAUTION: If hydrocarbons are present, Rwa will be higher and TDSa will be lower than the truth. Always investigate the well history file, especially the sample log, for indications of oil or gas in the interval to be studied. The Bateman and Konen equation, and the Kennedy equation, need Excel Solver to solve for WSa. These equations use RW@75F, so Rwa#FT would have to be converted to 75F as in equation 11. Crain's equation matches other methods closely, as shown in the graphs below.
Graph 2:
Cw Models - Red
line = Crain, Black line = Bateman and Konen, Blue line =
Kennedy.The differences above 150,000 ppm NaCl have little impact on water saturation.
LOG ANALYSIS EXAMPLE IN AQUIFER EVALUATIONThis example shows
how conventional petrophysical analysis can assist in
evaluation of potential water wells. The salinity curve,
derived from the porosity and resistivity log data, can be
used to determine the base depth to any given water quality.
Track 1 contains gamma ray and caliper, Track 2 is deep
resistivity, Track 3 is density and neutron porosity. This
raw data is used to calculate shale corrected porosity
(Track 4), apparent water resistivity (Rwa in Track 5), and
salinity in Track 6. The right hand track shows the
lithology with shale volume shaded black. The salinity curve
is shaded between the curve and 10,000 ppm total dissolved
solids (TDS) to help identify useable water sources. Note
that TDS values in shaly zones seldom indicate useful water
zones.
Image courtesy Aptian Technical. |
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