Unconventional reservoirs have low porosity and complex mineral composition containing quartz, feldspar, calcite, dolomite, pyrite and kerogen, which may seriously reduce the accuracy of the porosity measurement. The multi-detector pulsed neutron logging technique was already used for determining porosity through the combination of inelastic and capture gamma ray information in different spacing. In this paper, the new parameter, which is characterized by thermal neutron count ratio and lithology factor based on element content, is proposed to determine porosity from the three-detector pulsed neutron element logging in unconventional reservoir. To evaluate mineral composition, lithology, and gas/oil/water saturation in unconventional reservoir, a new multi-detector pulsed neutron logging tool was put out. The instrument consists of two He-3 thermal neutron detectors and a LaBr3 gamma detector. Therefore, the combination of thermal neutron count ratio between near detector and long detector with lithology factor of element content can measure neutron porosity and eliminate the influence of complex lithology. Based on some calibration pit data measured in laboratory, as well as the numerical simulation method, the influences of different lithological characters and mineral types on the neutron count ratio were studied. Meanwhile, large numbers of stratigraphic models with different lithological characters and different mineral compositions were established using Monte Carlo simulation method, and the content of silicon, calcium, hydrogen, oxygen, magnesium, aluminum and iron under different stratigraphic conditions was determined by the spectral element solution. A regression analysis was conducted to establish the relationship between the content of elements and the lithologic factor. The count ratio difference stemming from different lithological and mineral compositions was eliminated through a combination of lithological correction factor and thermal neutron count ratio. Different mineral compositions of stratigraphic simulation models were set up for verification. The absolute error of porosity measurement was less than 1.0p.u. in the formations with porosity less than 15p.u., which verified the accuracy of this method for neutron porosity evaluation in complex lithological characters formations. Two field examples were processed by this new parameter which in combination of thermal neutron count ratio and formation elements content information from the three-detector pulsed neutron instrument, which indicated a good accuracy for unconventional oil and gas reservoir evaluation.
Unconventional oil and gas resources, such as tight oil and gas, have become indispensably succeeding energy sources in nowadays. At the stage of exploration, gas saturation is essential for the evaluation of tight formation, which can provide the key parameters for reserves calculation and development plans making. Conventional logging technologies including acoustic logging and resistivity logging have played a role in gas formation identification and evaluation. Besides, inelastic and capture gamma energy spectrum or time spectrum from pulsed neutron logging tools with NaI, BGO, LaCl3, or LaBr3 detectors are used to realize the quantitative evaluation of gas saturation. With the development of nuclear technology, the new detector, called CLYC (Cs2LiYCl6:Ce), can simultaneously measure the signals of gamma ray and thermal neutron, providing a new mean for gas saturation evaluation use pulsed neutron logging technique. The CLYC scintillation crystal with a density of 3.31g/cm3 has an energy resolution in the order of 4%-5% (0.662MeV), and its light output efficiency of gamma ray and neutron are 20000 photons/MeV and 70500 photons/MeV. Meanwhile, its excellent temperature characteristics in the range from -30℃ to 180℃ can fit the downhole environment. Consisting of the D-T neutron source and CLYC detector, the pulsed neutron logging system is designed in this paper, in which the burst gate is 0 to 40 microseconds and the capture gate is 50 to 100 microseconds. To evaluate gas saturation, this system combines the inelastic gamma ray and thermal neutron recorded from the burst gate and the capture gate. The new pulsed neutron logging tool consists of two LaBr3 detectors and a CLYC detector, and the spacing of the CLYC detector is 75cm. In addition to the conventional C/O and Sigma measurement functions, the new instrument can also realize the quantitative evaluation of gas saturation by the CLYC detector. The inelastic gamma, capture gamma, and thermal neutron distribution in long-detector are simulated by the Monte Carlo method under the condition of tight gas saturated formation with porosity from 3% to 20%. Based on the spatial flux distribution characteristic of inelastic gamma and thermal neutron, the new parameter (RGTH) is defined as the ratio of inelastic gamma counts to thermal neutron counts from the CLYC detector to calculate gas saturation. The results imply that RGTH is positively correlated with porosity and negatively correlated with gas saturation, and the gas and water dynamic range is about 36% under the condition of a sandstone formation with 10% porosity. Different lithology has different RGTH benchmark values. RGTH is not affected by the yield of the neutron source and water salinity, and the subtract coefficient can be accurately determined by the time spectrum of the thermal neutron to acquire the pure inelastic gamma. A tight lime-bearing sandstone formation with 5% porosity has been set by MCNP to check validity, the absolute error of gas saturation calculated by RGTH is less than 5%.
As the critical parameter of hydraulic fracture evaluation, fracture density is significant for fracturing model optimization and production prediction. To compensate for the low response sensitivity of fracture density and the saturation of fracture evaluation in nonradioactive technology (NRT), we have developed a method based on modifying the neutron self-shielding effect to quantitatively calculate fracture density. The gamma counts of elements with content that did not change before and after fracturing were used to calculate the self-shielding correction factor. We established the mathematical relationship between the corrected gamma counts of boron and fracture density to achieve a quantitative calculation of fracture density. Compared to NRT, the new method used measured energy spectral information to eliminate the effect of neutron self-shielding on fracture density and improve calculation accuracy. Meanwhile, the scope or range of accurate fracture density estimation extended from the original 4% to more than 10%. Moreover, we further examined fracture response under different formations and borehole conditions using the Monte Carlo N-particle transport code. The salinity of formation water and borehole water had the most significant impact on fracture density calculation, shale content and porosity, lithology, and fluid type of formation; in contrast, borehole parameters were the least significant. Finally, in this paper, we illustrated the feasibility of the method through a continuous-depth numerical calculation.
The low porosity and permeability characteristics of tight oil reservoirs have brought challenges to monitoring oil saturation recently. Although carbon/oxygen logging is effective for oil saturation evaluation, the statistical fluctuations of the measured energy spectrum in tight reservoirs make it impossible to distinguish the different signals between oil and water. Thus, Noise Adjusted Singular Value Decomposition (NASVD) is applied to denoise the raw energy spectrum and evaluate the oil saturation quantitatively. The energy spectrum matrix, which is composed of the energy spectrum of the measurement point and its adjacent depth points, is decomposed and reconstructed to remove non-informative signals and improve the signal-to-noise ratio (SNR) of the raw energy spectrum. The parameter K evaluates the smoothness of the logging curves, reflecting the influence of the number of energy spectra and singular values on NASVD. Meanwhile, the NASVD, Savitzky-Golay (S-G) filtering and depth averaging methods are compared for calculating the accuracy of C/O, Si/Ca and oil saturation with the Monte Carlo method, indicating that NASVD is better than the other two methods for eliminating the statistical fluctuations of the raw energy spectrum. A simulation example indicates that NASVD can control the calculation errors of tight reservoir oil saturation to within 15%, which significantly improves the accuracy of the estimated oil saturation. An oil field example shows that the oil saturation interpretation result for tight reservoirs is in good agreement with the oil saturation from open hole log analysis, signifying that the NASVD energy spectrum denoising method can provide a quantitative estimate of oil saturation in tight oil reservoirs.
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