Mud logging, in essence, is a wellsite operation that investigates, records, and analyzes measurements obtained from the circulating drilling fluid that results in the measurement of cuttings and gas. It plays a vital role in the identification of downhole geological conditions, such as hydrocarbon presence and stratigraphy along with monitoring drilling conditions, to ensure safety of operations and improve efficiency. The objective of this paper is to utilize advanced mud logging analysis characteristics to establish a workflow to potentially identify fractures along an interval. Helium is used as the correlation parameter to potentially indicate fractures along the intended formation. Advanced mud logging provides a quantitative hydrocarbon measurement from the drilling mud rather than the qualitative measurements that regular mudlogging provides. Helium is one of the major components that advanced mudlogging provides and acts as an indicator of permeability in a formation. The process starts with a novel method to utilize helium to be a correlator of fractures within a formation in terms of identification and measurements along with tracking the fracture with time. The combination between current formation imaging procedures and helium readings from mud logs is proven to be a key potential indicator to establish a fracture identification pattern. The utility of the correlated helium readings from mud logging and fractures from formation image logs is a major breakthrough into identifying formation fracture features. Tracking changes of such features in which mud logging readings are used directly as potential indicators of fractures in an anticipated well. Loss of circulation prediction and LCM designs are enhanced directly by this extra knowledge.
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.
This paper presents an approach by integrating advanced cutting analysis, such as x-ray fluorescence (XRF), and open-hole logs for enhanced formation evaluation of complex clastic formations in near real-time. To verify the methodology, results of surface cuttings analyses are compared to and validated with downhole elemental spectroscopy measurements. In general, when the formation contains clays, the minimum logging requirement to evaluate clastic formations is a triple combo (density, neutron and resistivity) with spectral gamma ray (SGR) logs. In addition to correcting the impact of the drilling fluid additives and properties such as the presence of k-formate in mud, SGR logs become very crucial to differentiate clay types present in the formation. In the absence of SGR, advanced cuttings measurements can be utilized to provide elemental data of major elements including SGR components from the cuttings in near real-time. A comparison was made to evaluate the cuttings analysis as a replacement for SGR. As a part of this work and to validate the petrophysical evaluation results, downhole wireline SGR and elemental spectroscopy data were acquired and compared to the analysis using advanced cutting measurements. This work was conducted in a siliciclastic formation containing abrasive sandstones of mixed clean quartz and clay minerals. The analysis of cuttings XRF was integrated with basic downhole logs to quantify the clay typing required for representative formation evaluation and well geosteering. Limitations of this approach are identified in drilling complex clastic formations including cutting sampling frequency and effects of drilling including drilling fluid contamination, mud additives, drilling parameters and drilling driving mechanism. Controlling these factors has led to good results from cuttings measurements. The advanced cuttings XRF analysis was benchmarked with wireline SGR and elemental spectroscopy logs. This approach of using cuttings XRF analysis and basic open-hole logs is a valid option for geosteering in a complex clastic mineralogy formation and providing a near real-time formation evaluation in the absence of spectral gamma ray or elemental spectroscopy. XRF has been proven to provide near real-time analysis with improved reliability across bad hole, wider spectrum of elements and eliminate critical operations risk. Recommendations to optimize the parameters for reliable measurements will be discussed in this paper.
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