Evaluating hydrologic reservoir constraint in coal seams and shale formations
Methods and apparatus suitable for quickly and accurately measuring 13 C levels and supporting data in an aqueous fluid reservoir. Interpreting the resulting data to indicate key factors regarding a reservoir and completion methods, including reservoir constraint, gas producibility, and completion success. A sensor and to a sensing method that evaluates the level of hydrologic constraint in aquifers occurring in unconventional reservoirs, such as shales and coals is disclosed. Specifically, Raman spectroscopy is disclosed as a sensor and a sensing method that measures the level of naturally-occurring 13 C in an aqueous reservoir and compares the level of 13 C to the levels typical for highly constrained and highly unconstrained reservoirs. The disclosed sensor and sensing method also monitors the level of naturally-occurring 13 C in a reservoir. Also disclosed is a method of using δ 13 C Dic to evaluate geographic areas of coal bed reservoir water having biologic methanogenic activity.
1. A method for determining hydrologic constraints in reservoirs, comprising:
a. determining an expected level of naturally-occurring 12 C and 13 C isotopes in a reservoir using a Raman spectroscopy instrument placed downhole and in communication with a computer for data analysis;
b. determining a measured level of naturally-occurring 12 C and 13 C isotopes in a target reservoir using the Raman spectroscopy instrument placed downhole and in communication with the computer for data analysis;
c. comparing the expected and measured levels of 12 C and 13 C;
d. the computer calculating concentrations of 12 C and 13 C isotopes at each measured depth; and
e. comparing the levels determined in steps a. and b. to determine the relative hydrologic constraint of the target reservoir;
wherein step e. comprises determining δ 13 C DIC , where δ 13 C DIC =[[( 13 C/ 12 C) measured /( 13 C/ 12 C) expected ]−1]×1000; and
wherein the Raman spectroscopy instrument comprises an epi-illuminated Raman spectrometer comprising a high pressure steel tube with a diameter less than 5 inches, a sapphire window at a bottom end of a housing to irradiate samples, a laser excitation source, and a charge-coupled device detector.
2. The method of claim 1 , wherein an analyte is subjected to Raman spectroscopy, the analyte comprising dissolved inorganic carbon (DIC) containing both 12 C and 13 C isotopes.
3. The method of claim 1 , wherein the Raman spectroscopy instrument is continuously collecting Raman spectra.
4. The method of claim 1 , wherein the Raman spectroscopy instrument collects Raman spectra at least once every 60 seconds.
5. The method of claim 1 wherein the levels of 13 C in the target reservoir are compared to the levels of 12 C in the target reservoir to produce a 13 C enrichment level of the target reservoir.
6. The method of claim 1 , wherein the reservoir and the target reservoir are coal bed reservoirs or shale fluid reservoirs.
7. The method of claim 1 , further comprising using a difference in respective levels of 12 C and 13 C isotopes between the reservoir and the target reservoir to predict gas production levels of the target reservoir.
8. The method of claim 1 , further comprising performing an operation in the target reservoir, wherein the operation in the target reservoir comprises at least one of cementing of casing in a wellbore intersecting the target reservoir, hydraulic fracturing of the target reservoir, and horizontal drilling in the target reservoir.
9. The method of claim 8 , wherein the levels of 13 C are compared to the levels of 12 C in order to produce a 13 C enrichment level.