Browsing by Subject "carbonate"
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Item Acid Fracture and Fracture Conductivity Study of Field Rock Samples(2013-11-15) Underwood, JarrodAcid fracturing is a well stimulation strategy designed to increase the productivity of a producing well. The parameters of acid fracturing and the effects of acid interaction on specific rock samples can be studied experimentally. Acid injection data and fracture conductivity measurements obtained in the research presented in this thesis yielded results that qualified and quantified the impact of a specific acid system on rock samples of varying acid solubility. Six rock samples from a carbonate reservoir were labeled A through F to protect proprietary information included in this research. A 2% potassium chloride solution was used for the acid system and fracture conductivity measurements to prevent clay swelling. Injection temperature, contact time, and injection rate were designed to simulate field treatment conditions. The effects of a chelating agent on fracture conductivity were also studied. Before and after images of the rock samples indicated that the effect of 15% hydrochloric acid on the samples was limited but correlated with the rock acid solubility. Samples E and F had a greater value of acid solubility and showed noticeable surface etching. Samples A, B, and C had lower values of acid solubility and did not show signs of surface etching. Sample D was of moderate acid solubility and showed minimal signs of surface etching. Fracture conductivity did not correlate directly with acid solubility, but likely was a function of inherent matrix permeability based on leak-off measurements and fracture conductivity measurements. Finally, the fracture conductivity of Sample D increased after exposure to a chelating agent. Commonly, acid fracture experimental studies are carried out with outcrop rock samples. The samples have more homogenous properties and without hydrocarbon content. In this study, cores from downhole formation were used. The original condition was preserved as much as possible to simulate real field situations. However, using field rock samples does present challenges not generally associated with outcrop rock samples. Based on the information gathered from the work presented in this thesis, conclusions were drawn concerning the effectiveness of a 15% hydrochloric acid treatment in this formation and the challenges of using field rock samples.Item Applications of artificial neural networks in the identification of flow units, Happy Spraberry Field, Garza County, Texas(Texas A&M University, 2005-02-17) Gentry, Matthew DavidThe use of neural networks in the field of development geology is in its infancy. In this study, a neural network will be used to identify flow units in Happy Spraberry Field, Garza County, Texas. A flow unit is the mappable portion of the total reservoir within which geological and petrophysical properties that affect the flow of fluids are consistent and predictably different from the properties of other reservoir rock volumes (Ebanks, 1987). Ahr and Hammel (1999) further state a highly "ranked" flow unit (i.e. a good flow unit) would have the highest combined values of porosity and permeability with the least resistance to fluid flow. A flow unit may also include nonreservoir features such as shales and cemented layers where combined porosity-permeability values are lower and resistance to fluid flow much higher (i.e. a poor flow unit) (Ebanks, 1987). Production from Happy Spraberry Field primarily comes from a 100 foot interval of grainstones and packstones, Leonardian in age, at an average depth of 4,900 feet. Happy Spraberry Field is unlike most fields in that the majority of the wells have been cored in the zone of interest. This fact more easily lends the Happy Spraberry Field to a study involving neural networks. A neural network model was developed using a data set of 409 points where X and Y location, depth, gamma ray, deep resistivity, density porosity, neutron porosity, lab porosity, lab permeability and electrofacies were known throughout Happy Spraberry Field. The model contained a training data set of 205 cases, a verification data set of 102 cases and a testing data set of 102 cases. Ultimately two neural network models were created to identify electrofacies and reservoir quality (i.e. flow units). The neural networks were able to outperform linear methods and have a correct classification rate of 0.87 for electrofacies identification and 0.75 for reservoir quality identification.Item Characterization of Small Scale Heterogeneity for Prediction of Acid Fracture Performance(2010-10-12) Beatty, Cassandra VonneRecently developed models of the acid fracturing process have shown that the differential etching necessary to create lasting fracture conductivity is caused by the heterogeneous distributions of permeability and mineralogy along the fracture faces. To predict the conductivity that can be created by acid in a particular formation, the models require information about these formation properties. This research aims to quantify correlation lengths using a geostatistical description of small scale heterogeneity to ascertain the distribution of permeability and mineralogy in a carbonate formation. The correlation length parameters are a first step in being able to couple acid transport and rock dissolution models at reservoir scale with a model of fracture conductivity based on channels and roughness features caused by small scale heterogeneity. Geostatistical parameters of small scale heterogeneity affecting wells in the Hugoton Field are developed. Data leading to their derivation are obtained from a combination of well logs and cores. The permeability of slabbed core is measured to yield vertical correlation length. Well logs are used to estimate permeability via an empirical relationship between core plug permeability and well log data for calculation of horizontal correlation length. A fracture simulator computes the acid etched fracture width for known treatment conditions. The resulting geostatistical parameters and acid etched width are used to predict acid fracture performance for a well in the Hugoton Field. Application of new model conductivity correlations results in a unique prediction for the acid fracture case study that differs from the industry standard. Improvements in low cost stimulation treatments such as acid fracturing are the key to revitalizing production in mature carbonate reservoirs like the Hugoton Field. Planning and development of new wells in any carbonate formation necessarily must consider acid fracturing as a production stimulation technique. Reliable models that accurately predict acid fracture conductivity can be used to make an informed investment decision.Item Distinguishing carbonate reservoir pore facies with nuclear magnetic resonance as an aid to identify candidates for acid stimulation(Texas A&M University, 2006-10-30) Genty, CoralieThe determination of reservoir quality and its spatial distribution is a key objective in reservoir characterization. This is especially challenging for carbonates because, due to the effects of diagenesis, quality rarely follows depositional patterns. This study integrates data from thin sections and core analyses with measurements of Nuclear Magnetic Resonance (NMR) T2 relaxation times. It exposes a novel approach to the use of NMR by applying geological and statistical analysis to define relationships between pore characteristics and the T2 data, from which a method to identify pore origin from NMR only is developed. One hundred and three samples taken from eleven wells located in fields of the Middle East, Alabama and Texas were used in the study. Modeling of the T2 spectra, as the sum of three normal components, resulted in the definition of 9 parameters representing the average, the variability and the percentage of total porosity of the specific pore sizes present in the sample. Each specific pore size corresponds to one of the following genetic pore types: intergranular, matrix, dissolution-enhanced, intercrystalline, vuggy and cement-reduced. Among the 9 parameters, two variables were identified as having the highest degree of geological significance that could be used to discriminate between pore categories: ????max which represents the largest average pore size of all pore types identified in the sample, and ????main which represents the size variability of the most abundant pore type. Based on the joint distribution of ????max and ????main computed for each pore category, the probability that an unclassified sample belongs to each of the pore categories, is calculated and the sample is assigned to the category with the highest probability. The accuracy of the method was investigated by comparing NMR predicted pore origin and genetic pore type described from thin section. A result of 89 successful predictions out of 103 samples was obtained. These promising results indicate that T2 time can be a useful identifier of carbonate pore types. Success in this work takes us closer to identifying genetic pore types from NMR logs with minimal calibration against borehole cores and will help predict the spatial distribution of poroperm facies in complex carbonate reservoirs with much improved accuracy.Item Early Miocene Carbonate Dissolution in the Eastern Equatorial Pacific(2014-11-19) Wilson, Julia KeeganAs the world?s largest ocean, the Pacific Ocean is intrinsically linked to the major changes in current and past climate via several mechanisms, in particular, through the carbon cycle?s influence on atmospheric CO2 concentrations. One way to understand the ocean carbon cycle is to study fluctuations in the CaCO3 content of ocean sediments and determine the nature, timing, and drivers of those events. As part of a larger effort to reconstruct Cenozoic paleoclimate in the eastern equatorial Pacific, XRF records are used here to define an early Miocene low CaCO3 event in the equatorial Pacific that corresponds to the seismic horizon termed ?Lavender?. The low CaCO3 interval is correlated at submeter scale in 4 drill sites from IODP Expedition 320/321 and has been identified by seismic reflection throughout the region, indicating a large perturbation in the global carbon cycle at the time of its deposition. Using multi-element XRF data, we have determined that this event was caused by CaCO3 dissolution, with peak dissolution occurring at 16.9 Ma. The event begins prior to the Miocene Climate Optimum (MCO) with peak dissolution coinciding with the initial warming step of the MCO. Three potential disrupters of CaCO3 deposition are investigated: a change in organic carbon burial to the deep ocean, reducing deep dissolved inorganic carbon (DIC); a change in mantle CO2 outgassing associated with the formation and emplacement of the Columbia River basalt province; and basin-basin fractionation, where development of a new CaCO3 depocenter in the Atlantic reduces CaCO3 deposition in the Pacific. Of these 3 possibilities, the timing of the Lavender event favors basin-basin fractionation.Item Experimental High Velocity Acid Jetting in Limestone Carbonates(2014-04-30) Holland, ChristopherAcid jetting is a well stimulation technique that is used in carbonate reservoirs. It typically involves injecting acid down hole at high flow rates through small orifices which cause high velocities of acid to strike the borehole wall. The combination of high kinetic energy and chemical reaction of the acid removes drilling mud filter cake from the borehole wall and produces long conductive channels, called wormholes, into the formation, therefore improving well performance. Studies have shown that injecting fluid down hole at high velocities can mitigate damage to wellbore caused by drilling mud filter cake. Both water and acid have shown positive results in such cases. However, there are no laboratory results on how high velocity acid impacts the borehole wall and the formation of wormholes. The purpose of this study is to investigate how the high velocity acid affects the acidizing treatments. The experiments are conducted on 4? diameter by 16? length Indiana limestone cores with acid injected at the velocity of 106 ft/s, 150 ft/s, and 200 ft/s. The experiments are conducted with a constant pressure differential across the core. 15%wt Hydrochloric acid is injected at room temperature at various flux rates. The results show that the higher the velocity of jetted acid, the further it penetrates into the formation. The 200 ft/s acid penetrates furthest into the core, thus potentially lowering the skin factor the greatest. A large cavity is formed into the core from the high velocity acid. This large cavity creates a pathway for acid to divert into the core to create wormholes. Acid jetting cannot be directly compared with matrix acidizing because of the formation of these large cavities. The Buijse-Glasbergen model that is used to predict the formation of wormholes does not accurately match the acid jetting data due to the formation of these large cavities, so the optimum flux and pore volume to breakthrough cannot be accurately determined.Item Identification of pore type and origin in a Lower Cretaceous carbonate reservoir using NMR T2 relaxation times(Texas A&M University, 2004-09-30) Lodola, Domenico DomenicoDetermining the distribution of porosity and permeability is one of the main challenges in carbonate petroleum reservoir characterization and requires a thorough understanding of pore type and origin, as well as their spatial distributions. Conventional studies of carbonate reservoirs require interpretation and analysis of cores to understand porosity. This study investigates the use of NMR logs in the determination of pore type and origin. This study is based on the analysis of both thin section petrographic and NMR data from a single well that cored the Lower Cretaceous (Aptian) shelf carbonates belonging to the Shuaiba Formation of the Middle East. Photographs of thin sections were used to determine pore type and origin according to Ahr's genetic classification of carbonate porosity. Descriptive statistics and modeling were used to analyze the NMR T2relaxation time distributions. Descriptive statistical analyses included estimating arithmetic average, standard deviation, skewness, median, mode and 90th percentile. T2modeling was performed by fitting multiple log-normal distributions to the measured T2distribution. Data from thin section petrography and from NMR measurements were then compared using conditional probabilities. As expected, thin section analysis revealed the predominance of mud-supported fabrics and micropores between matrix grains Vugs and dissolved rudistid fragments account for most of the macro porosity. Descriptive statistics showed that the mode and th percentile of the T2distribution had the greatest power to discriminate pores by origin. The first principal component (PC1) of the mode-90th percentile system was then used to compute the probabilities of having each pore origin, knowing that PC1 belongs to a given interval. Results were good, with each origin being predictable within a certain range of PC1. Decomposition of the T2distributions was performed using up to 3 log-normal component distributions. Samples of different pore origin behaved distinctively. Depositional porosity showed no increase in fit quality with increasing number of distributions whereas facies selective and diagenetic porosity did, with diagenetic porosity showing the greatest increase.Item Implications of Carbonate Petrology and Geochemistry for the Origin of Coal Balls from the Kalo Formation (Moscovian, Pennsylvanian) of Iowa(2012-10-19) Jones, CourtneyCoal balls are carbonate concretions formed in peat during the Pennsylvanian and early Permian. Microprobe and microscope analysis reveal that polycrystals of high-Mg calcite (HMC), which are also high in Sr, are the earliest calcium carbonate to form in the Williamson No. 3 coal balls from the Kalo formation in Iowa. This HMC has early diagenetic rims of ferroan and non-ferroan low-Mg calcite (LMC) suggesting diagenesis in meteoric water. The combination of HMC followed by LMC suggests the earliest coal ball carbonate formed in a hydrologically dynamic environment, where saltwater influx into the mire was followed by a return to meteoric pore water. Subsequent generations of carbonate are ferroan and non-ferroan LMC and appear to result from diagenesis of the original HMC fabric with LMC rims. HMC polycrystals from coal balls are among the first abiotic HMC to be reported from the mid-Pennsylvanian; coal balls may be a good source of Pennsylvanian HMC. Coal balls that formed in porous peat (i.e. wood and surficial leaf mats) commonly have abundant radiating arrays of HMC polycrystals. Coal balls that formed in matrix-rich, low porosity peats consist primarily of permineralizing anhedral calcite, which is ferroan LMC. The link between the HMC and porous permeable peat is supported by the distribution of HMC and ferroan LMC in plant cells. Wood cells, which have porous walls, are filled with HMC; fiber cells, which have impermeable walls, are filled with ferroan LMC. This study demonstrates a link between pore volume, porosity, plant cell type, and carbonate fabric.Item Mineralogy and Geochemistry of Pb, Zn and Ag Mine Tailings Originating From Carbonate-Rich Deposits(2012-11-29) McClure, Roberta 1981-Mining for silver, lead, zinc, and copper in Zimapan, Hidalgo State, Mexico has been ongoing since 1576. Unsecured tailings heaps and associated acid mine drainage have presented problems related to soil quality, water quality, and dust emission control in the Zimapan area. Objectives of the study of the mine tailings are (1) to determine mineralogy of the tailings in order to identify acid-producing minerals and heavy metals at risk for release in acidic conditions, and (2) to quantify carbonate minerals and (3) to determine heavy metal content that may be released by the products of sulfide mineral weathering. Representative mine tailings have been sampled from a site located north of Zimapan. Mineralogical characterization has been conducted with X-ray diffraction (XRD), and scanning and transmission electron microscopes (SEM and TEM). Total carbonates have been determined the Chittick procedure. X-Ray Fluorescence (XRF) has been utilized to determine total elemental composition. XRD and SEM analyses have confirmed the presence of pyrite and arsenopyrite indicating a potential for acid mine drainage. Calcite has been confirmed to have a significant presence in the unweathered samples by XRD and the Chittick procedure, with some samples containing an average of 19.4% calcite. NAA and XRF have revealed significant concentrations of toxic elements such as As, Pb and Zn in both the oxidized and unoxidized samples.Item Modeling of Seismic Signatures of Carbonate Rock Types(2011-02-22) Jan, Badr H.Carbonate reservoirs of different rock types have wide ranges of porosity and permeability, creating zones with different reservoir quality and flow properties. This research addresses how seismic technology can be used to identify different carbonate rock types for characterization of reservoir heterogeneity. I also investigated which seismic methods can help delineate thin high-permeability (super-k) layers that cause early water breakthroughs that severely reduce hydrocarbon recovery. Based on data from a Middle East producing field, a typical geologic model is defined including seal, a thin fractured layer, grainstone and wackestone. Convolutional, finite difference, and fluid substitution modeling methods are used to understand the seismic signatures of carbonate rock types. Results show that the seismic reflections from the seal/fractured-layer interface and the fractured-layer/grainstone interface cannot be resolved with conventional seismic data. However, seismic reflection amplitudes from interfaces between different carbonate rock types within the reservoir are strong enough to be identified on seismic data, compared with reflections from both the top and bottom interfaces of the reservoir. The seismic reflection amplitudes from the fractured-layer/grainstone and the grainstone/wackestone interfaces are 17% and 23% of those from the seal/fracturedlayer interface, respectively. By using AVO analysis, it may be possible to predict the presence of the fractured layer. It is observed that seismic reflection amplitude resulting from the interference between the reflections from overburden/seal and seal/fractured-layer does not change with offset. The thin super-k layer can also be identified using fluid substitution method and time-lapse seismic analysis. It shows that this layer has 5% increase in acoustic impedance after oil is fully replaced by injecting water in the layer. This causes 11% decrease and 87% increase in seismic reflection amplitudes from the seal/fractured-layer interface and the fractured-layer/grainstone interface after fluid substitution, respectively. These results show that it is possible to predict carbonate rock types, including thin super-k layers, using their seismic signatures, when different seismic techniques are used together, such as synthetic wave modeling, AVO, and time-lapse analysis. In future work, the convolutional model, AVO analysis, and fluid substitution could be applied to real seismic data for field verification and production monitoring.Item Paleoenvrironmental Controls on Diagenesis of Organich-Rich Shales in the Eagle Ford Group(2014-08-27) Kruse, KendraCarbonate precipitation can be either promoted or inhibited by microbial processes in different redox zones. It is therefore possible for basin redox evolution to indirectly control early carbonate diagenesis and modify reservoir properties of corresponding shale units. The goals of this study were to analyze geochemical characteristics of the Late Cretaceous Eagle Ford Group in McMullen County, Texas to test the hypotheses that (1) the redox state of the water column controlled carbonate cement abundance and (2) carbonate cement lowered organic matter content by volumetric dilution. An x-ray analytical microscope was used to map elemental compositions of fresh core samples within the Eagle Ford Group. Resultant maps were used to characterize carbonate cements and to estimate the redox state of the overlying water column during deposition, as indicated by the relative abundances of the trace metals Mo, V, and Cr. Results indicate that cementation occurred early relative to compaction. Ti Kal normalized Mo Kal and CaKal fluorescence intensities are positively correlated throughout the unit, suggesting that carbonate cementation was related to the redox state. Total organic carbon is negatively correlated in the upper Lower Eagle Ford with (Ca Kal)/(Ti Kal) fluorescence ratio, consistent with volumetric dilution of organic matter by diagenetic cementation prior to compaction. In contrast, there is no significant correlation between total organic carbon and carbonate content in the more organic-rich Lower Eagle Ford.