Deep clastic gas reservoirs, characterized by heterogenetic, low porosity and permeability rocks, and varying quantities of clay, are often drilled using oil-based mud (OBM) systems. It is well known, that presence of OBM affects surface mud gas data due to OBM contamination and drill bit metamorphism (DBM) phenomenon. The scope of this work is to identify and attempt to eliminate OBM contamination and DBM effects, therefore extracting only true formation gas response in order to identify zones of hydrocarbon potential from advanced mud gas (AMG) logging data. Enhancements in AMG analysis enabled identification of DBM while drilling by measuring artificially generated hydrocarbons, such as ethene (C2H4), the so-called cracked gas, which is not found in formation hydrocarbons. The use of the DBM detection from AMG analysis allows for the differentiation of formation hydrocarbons from artifacts induced by drilling processes. The provided workflow also aids in identifying tighter reservoir intervals, which can be particularly useful in cases of long horizontal sections. This study also presents cases, where a DBM workflow and mud gas fluid compositional data assists in estimating zones of higher porosity with hydrocarbon potential, as well differentiating between prolific and tight sections. Utilization of the workflow in exploration wells is useful in many instances to optimize formation evaluation during downhole formation testing, and subsequent drill stem test (DST) design. In addition, in development wells, the workflow can aid multi-stage fracking design and net-to-gross estimation. Harsh drilling conditions in tight abrasive rocks drilled using OBM muds introduces artificial hydrocarbons not indigenous to the formation and introduces false positive gas peaks on mud logs. Recognition and removal of such artifacts from analyses is important in an integrated formation evaluation. To-date, mud logging companies have been able to identify the effect of DBM on mud gas and often correlate the phenomena to bit degradation, which is an important indicator for drilling optimization. The authors attempt to summarize such findings into a formation evaluation process to assist in the identification and delineation of sampling, production, and stimulation of reservoir intervals.
Accurate quantification of fluid fractions in transition zones and low resistivity pay reservoirs often poses a challenge to the interpreter. Dry oil production from low resistivity intervals, mixed flow production, or disparity between formation evaluation analyses and surface tests can lead to lower confidence in the downhole measurements or interpretation and increase the uncertainty of the calculated hydrocarbon volume in place. A variety of measurements may be exploited to mitigate these concerns and, in this paper, the authors present an assessment of the relationship between normalized hydrocarbon levels while drilling, and computed oil fraction from wireline logging of logging-while-drilling (LWD) measurements based on a comprehensive dataset of various surface and downhole measurements. The study investigated the validity of advanced mud logging data for oil fraction quantification for particular borehole and reservoir settings and found a consistent trend across the range of bulk oil proportions and normalized gas values. The results of the obtained regression, and alternative cluster-based models are presented. The sources of uncertainties and possible correction approaches are discussed. The findings suggest that in consistent environmental and reservoir settings, advanced mud logging data can be used for hydrocarbon fraction estimation while drilling to assist in pay zones delineation and identification of fluid contacts. Potential was also seen to identify intervals of fractional flow production in transition zone reservoirs, low permeability formations, and dry oil production in low resistivity pay cases. In addition, developing such relationships and models is useful for correct identification of mud log artifacts, such as produced hydrocarbons while drilling, gases from circulation, and zones of overpressure. Another application is to assist in defining petrophysical model parameters, aiding or replacing other supporting information.
Spectral gamma-ray (SGR) data were acquired from a new slim logging-while-drilling (LWD) tool and from surface cuttings in a near vertical well and in a horizontal well across clastic deposits. Comparison of the data from both measurements indicates that there are advantages from both methods. X-ray diffraction (XRD) and X-ray fluorescence (XRF) data from cuttings also support the findings. The formation evaluation objective is to quantify the volumes of each mineral and fluid present in the formation. SGR data brings the required additional information to reduce the mineral volume uncertainty, especially for the clays in the formation with complex mineral assemblages. In the studied clastic deposits, several clay types are present (with the dominant contribution from illite and kaolinite) together with feldspars and trace elements like zircon and other heavy minerals. The presence of gas introduces another unknown, since it affects the porosity measurements and fluid volume calculation through bulk density and neutron porosity. The comparison of SGR data from LWD logs and from cuttings brings robustness to our conclusions. Comparison of the thorium, potassium, and uranium concentrations from LWD logs and from cuttings shows good agreement in the measurements for the low-angle well. The high-angle well data also shows good agreement between the two measurements except for the cleaner sand section. The results from the cuttings are affected by the accuracy of sample depth control due to the poor borehole conditions and inefficiency in evacuating cuttings in high-angle wells compared to low-angle wells. The trend of the SGR is maintained. The LWD SGR elemental concentrations are then used to solve the formation mineral fractions, which are compared with the same fractions from the XRD on cuttings. Similar conclusions are drawn for the elemental concentrations. The potassium concentration enables the quantification of illite and potassium feldspar. Uranium brings a significant contribution to the total GR measurement, which could lead to a clay volume overestimation if the uranium contributions weren’t excluded. In conclusion, LWD provides superior quality SGR data compared with SGR from cuttings because of the better depth control and vertical resolution. SGR on cuttings can be an alternative when combined with other LWD measurements and accepting a higher uncertainty, in case LWD SGR cannot be run due to certain borehole conditions. This paper compares the results of a slim tool LWD and cuttings SGR data for the first time and concludes on the applicability of each technique.
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