Characterization of Shale Gas Rocks Using Dielectric Dispersion and Nuclear Magnetic Resonance
Abstract: Shale gas has become a significant resource play in the USA over the past few years, and oil companies are now evaluating the shale gas potential of many sedimentary basins. The successful development of these shale gas systems has led to a strong exploration campaign in Saudi Arabia to investigate its several onshore basins. These "shales" have complex and varying mineralogies and require intensive petrophysical analysis to determine even basic characteristics. The renewed focus on rock sequences has necessit… Show more
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ADCO started its unconventional exploration campaign in 2012 targeting the tight carbonate sequences known as Wasia Group, onshore Abu Dhabi. A front-end loaded data gathering strategy was employed to acquire extensive latest generation logging data tailored for unconventional reservoirs. In a number of wells the entire reservoir section was cored, often up to 800 ft per well, leading to more than 3000 ft of core retrieved to date. ADCO applied unconventional core analysis technologies, such as retort analysis, to generate the optimal core results. Key parameters such as effective porosity, pore size distribution, TOC, source rock maturity, mineral compositions and fluid saturations were determined from logs and core data (where available). This paper will focus on the petrophysical challenges during the evaluation of the Wasia Group. We will demonstrate that conventional core analysis techniques have only limited applicability, whereas core analysis techniques designed specifically for unconventionals provide more relevant results. A log analysis methodology centered on the application and importance of NMR in unconventional liquid plays is presented. Porosity data measured through retort analysis provide an excellent fit to NMR log-based porosity measurements. Conventional core analysis results generated a poor fit to log porosity, and the resulting values exhibited scatter with a large standard deviation. T2 distribution from NMR log data suggests the presence of large pores with good fluid mobility, which requires confirmation through formation testing or production. Log data-derived rock typing was performed. It is based on principal component analysis of the reservoir section. Rock classification may help in selecting suitable zones for hydraulic fracture initiation. Lessons learned from the initial wells for core recovery and analysis techniques are summarized below and have been implemented in later wells: –Preserve part of the core for robust saturation measurements.–Stop acquisition of conventional poro-perm data–Focus on unconventional-specific retort-based techniques for core petrophysics–Focus on pulse decay permeabilities–Use scratch test to aid in core analysis sample selection process, especially for rock mechanics–Add core T1/T2 NMR and MICP to future core analysis programs The complete integration of core and log data has allowed for a thorough assessment of the unconventional hydrocarbon potential within the ADCO concession.
ADCO started its unconventional exploration campaign in 2012 targeting the tight carbonate sequences known as Wasia Group, onshore Abu Dhabi. A front-end loaded data gathering strategy was employed to acquire extensive latest generation logging data tailored for unconventional reservoirs. In a number of wells the entire reservoir section was cored, often up to 800 ft per well, leading to more than 3000 ft of core retrieved to date. ADCO applied unconventional core analysis technologies, such as retort analysis, to generate the optimal core results. Key parameters such as effective porosity, pore size distribution, TOC, source rock maturity, mineral compositions and fluid saturations were determined from logs and core data (where available). This paper will focus on the petrophysical challenges during the evaluation of the Wasia Group. We will demonstrate that conventional core analysis techniques have only limited applicability, whereas core analysis techniques designed specifically for unconventionals provide more relevant results. A log analysis methodology centered on the application and importance of NMR in unconventional liquid plays is presented. Porosity data measured through retort analysis provide an excellent fit to NMR log-based porosity measurements. Conventional core analysis results generated a poor fit to log porosity, and the resulting values exhibited scatter with a large standard deviation. T2 distribution from NMR log data suggests the presence of large pores with good fluid mobility, which requires confirmation through formation testing or production. Log data-derived rock typing was performed. It is based on principal component analysis of the reservoir section. Rock classification may help in selecting suitable zones for hydraulic fracture initiation. Lessons learned from the initial wells for core recovery and analysis techniques are summarized below and have been implemented in later wells: –Preserve part of the core for robust saturation measurements.–Stop acquisition of conventional poro-perm data–Focus on unconventional-specific retort-based techniques for core petrophysics–Focus on pulse decay permeabilities–Use scratch test to aid in core analysis sample selection process, especially for rock mechanics–Add core T1/T2 NMR and MICP to future core analysis programs The complete integration of core and log data has allowed for a thorough assessment of the unconventional hydrocarbon potential within the ADCO concession.
A New Laboratory SCAL Device and Processing Method for OBM Vug Density Quantification
Vugular porosity, per this work is defined as a set of secondary porosity development in carbonate rocks that could have developed through alteration of rock-fabric, commonly by proceses such as dolomization and dissolution (including mouldic porosity). It is one of the important secondary porosity types and its characterization can influence rock model petro-physical behavior. Traditionaly vugular porosity can be quantified on water-based mud (WBM) high resolution electrical imaging logs by counting the image pixels that has electrical resistivity lower than the rock background. With the popularity of non-conductive drilling fluid (OBM) and improvement of electrical image logging technology, it is possible nowadays to acquire similar high resolution electrical images in OBM environment. Vugs can be identified visually on these new types of electrical images. However open vugs filled with OBM can appear resistive on those images and invalidate the basic assumptions of secondary porosity quantification software that open vugs should have lower electrical resistivity. At the same time, the new generation of electrical imaging tools uses megahertz logging frequencies to decrease the capacitance of the OBM to acquire formation images. With this frequency range, both electrical conductivity and dielectric permittivity impact the image log acquired. This challenges our understanding of the tool response in terms of image color contrast and opens the question of how to integrate such OBM electrical images into current geologic interpretation software that commonly uses the image resistivity value thresholds. In this case, a new laboratory device that features the same logging frequency has been developed and presented in this paper. The device was used to investigate images of secondary porosity features acquired in OBM in controlled laboratory conditions. It is demonstrated that by integrating both resistivity image and dielectric permittivity image, it is possible to overcome the OBM filled resistive vug limitation and account for all fluid-filled effective vugs. A new post processing method is introduced by combining the effects of resistivity and dielectric permittivity and generating a new image called the rock Hayman factor image. On the Hayman factor image, it is possible to differentiate fluid-filled vugs from cemented vugs. Vug density quantification in WBM is based on resistivity threshold values to characterize conductive and resistive heterogeneity patches and can be highly user dependent. In OBM environment, mud filled features will appear resistive and may decrease the contrast with rock resistive matrix. This can make current vug quantification software miss parts of open vugs. Two popular vug separation and quantification methods (Gray-scale reconstruction transform plus watershed transform method and Sobel gradient edge detection plus Otsu's image thresholding method) are investigated first on a vuggy limestone outcrop and then on a downhole log as a case study to derive a vug density quantification workflow. The new Hayman factor image has proven on the laboratory data its ability to quantify both vugs beneath the rock surface and on the rock surface filled with OBM. Integrating the Hayman factor image, the procedure to set heterogeneity patches can be well-defined and not rely on user settings. The two image class separation methods can give very similar vug density quantified. Best practices of parameter adjustments on these two image classification methods are provided that can achieve the best results based on both laboratory data and downhole data. In this paper investigation results including both laboratory measurements and downhole logging data analysis of the OBM electrical image vug density quantification are presented. A new post-processing Hayman factor image is designed to quantify rock resistivity and the dielectric permittivity relationship and help vug density quantification. Application of this new image and best practices of how to set processing parameters using this image are investigated and summarized. Using the new vug characterization workflow involving Hayman image can improve OBM well vug quantification accuracy and repeatability.
Diyab has served as the source rock for various major Jurassic and Cretaceous oil & gas plays in Middle East, and a number of recent studies have indicated that the Diyab formation has potential to be explored as an unconventional source rock gas play. This assessment is mainly based on 1) the known facies distribution, 2) lithology and rock properties, 3) source rock TOC and maturity 4) mud gas, and 5) Stimulation results. The objective of this paper is to identify the resource potential of Diyab source rock as an unconventional gas play in UAE. Integration of valuable measurements is of upmost importance to reduce the uncertainty in reservoir quality assessment of any exploration setting. Quantifying TOC and maturity are one of the most important aspects in evaluating the source rock potential of an unconventional play. Due to the complexities seen in several source rock plays, assessing quality through interpreting mineralogy, porosity and water saturation can also be quite challenging. To achieve that, a comprehensive mud logging analysis program was carried out in the well complemented with a comprehensive and modern logging program combined with a detailed unconventional core analysis to identify rock properties, fluid type and saturations with the scope to assess unconventional resource potential of the Diyab formation in UAE. Initial petrophysical evaluation shows that Diyab source rocks have a good Shale Gas resource potential within three different zones of interest in the Diyab formation. These zones of interest are based on, 1) Total Organic Carbon, 2) Porosity and 3) Saturation. During the petrophysical evaluation advanced mud logging analysis was used as initial calibration for TOC and mineralogy, a multi-frequency dielectric tool was used for initial saturation estimation and nuclear magnetic resonance analysis was used for porosity estimation and characterization. In the final computation, unconventional core analysis was used to refine and further calibrate the results. The level of gas saturation exceeds common pay criteria which are known to produce economic volumes of gas in other basins around the world. Diyab as a source rock and as unconventional gas play in UAE in terms of reservoir quality has never been studied in detail. This is the first time that this kind of a comprehensive evaluation program was carried out using advanced log acquisition, advanced mud logging and specific unconventional core analysis and through this paper important aspects of reservoir quality for Diyab formation will be discussed and will be of great help for future exploration plans in the Diyab unconventional gas play.
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