Browsing by Subject "model"
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Item A Global Model for Fracture Falloff Analysis(2014-10-29) Marongiu-Porcu, MatteoThe reservoir permeability is an essential input for the optimum design of modern hydraulic fracture treatments, which are undeniably the crucial technology involved in the development of tight and/or unconventional gas reservoirs. The fracture geometry and pumping execution, as well as the well architecture, can be designed to maximize the well productivity, provided the reservoir permeability is known, but in tight formations estimation of permeability and pressure can be impractical or even impossible to determine by conventional pressure buildup transient tests because no flow will occur without hydraulic fracture stimulation. Various authors have shown how fracture calibration tests, intended for the final fracture treatment calibration (i.e. estimation of closure stress, leakoff coefficient and fracture fluid efficiency) can be used to estimate reservoir permeability as well. However, all the proposed techniques depend on specialized plots that are designed to show a straight line for a portion of the data, from which parameters are determined either from the slope of the line or from its endpoints, and there is a risk that apparent straight lines may lead to erroneous results, particularly when the absence of late time pseudo-radial flow data is ignored or not recognized. This dissertation introduces a new global model for the before-closure and after-closure analysis of the pressure falloff following a step-rate or constant rate fracture calibration test, using a single log-log diagnostic plot, as common practice within the pressure transient analysis literature. This model provides a complete assessment tool that allows quantification of all fracture parameters (closure stress, closure time, fracture fluid efficiency, leakoff coefficient and estimate of the induced fracture geometry) as well as reservoir permeability and formation pressure, provided that enough time is allowed for the falloff to reach pseudo-radial flow regime. Both oil and gas reservoirs can be effectively studied. Another major advantage provided by this approach is that this model can be used to optimize a priori the design of the fracture calibration test that would allow determination of all the involved parameters, including reservoir permeability. Field data will be used to validate the model and demonstrate its added value over current interpretation methods.Item A model for adaptive livestock management on semi-arid rangelands in Texas(Texas A&M University, 2006-08-16) Dube, SikhalazoA stochastic, compartmental Model for Adaptive Livestock Management (MALM) was developed for cow-calf enterprise for Rolling Plains of Texas from an existing model, Simple Ecological Sustainability Simulator (SESS). The model simulates forage and animal production. It runs on a monthly time step. Two stocking strategies, flexible and fixed, were evaluated at seven stocking levels for effects on forage and animal production, range condition, and net ranch income. Evaluation data were obtained from published and unpublished data from Texas A&M Agricultural Experimental Station at Vernon for Throckmorton. The model adequately simulated forage and animal production. Light fixed stocking rates and flexible stocking strategies resulted in cows of median body condition score (BCS) 5, compared to low BCS of 4 under moderate fixed stocking rate, and BCS of 3 under heavy fixed stocking. BCS declined from autumn to early spring and peaked in summer. Cows under light fixed stocking rates and under flexible stocking were heavier (460 kg) compared to those under heavy fixed stocking (439 kg). Replacement rates were lower under light stocking (22 %), compared to flexible (37 %) and heavy stocking (56 %). Calf crops were all above the reported 90 % expected for bred heifers because of the replacement policy. Flexible stocking strategy resulted in higher net income ($19.62 ha-1), compared to fixed light ($5.93 ha-1) or fixed heavy ($-17.35 ha-1) stocking strategies. Coefficient of variation (CV) in net income was highest under heavy stocking (90%) compared to light stocking (60%) and flexible stocking (50%). Maximum net income was obtained between 0.05 AUM??ha-1 and 0.13 AUM??ha-1 when fixed stocking strategy was used but when flexible stocking strategy was used maximum net income was obtained between 0.1 AUM??ha-1 and 0.17 AUM??ha-1. Range condition rapidly declined under fixed heavy stocking, increased under fixed and light flexible stocking, and remained constant under moderate flexible stocking. Heavy fixed stocking decreased range condition rapidly over a 20-year period. MALM was an effective tool to demonstrate effects of different management strategies. The model can function as a strategic or a tactical decision aid. It is concluded that there is potential for this model to assist managers in improving the sustainability of agriculture.Item A model for matrix acidizing of long horizontal well in carbonate reservoirs(2009-06-02) Mishra, VarunHorizontal wells are drilled to achieve improved reservoir coverage, high production rates, and to overcome water coning problems, etc. Many of these wells often produce at rates much below the expected production rates. Low productivity of horizontal wells is attributed to various factors such as drilling induced formation damage, high completion skins, and variable formation properties along the length of the wellbore as in the case of heterogeneous carbonate reservoirs. Matrix acidizing is used to overcome the formation damage by injecting the acid into the carbonate rock to improve well performance. Designing the matrix acidizing treatments for horizontal wells is a challenging task because of the complex process. The estimation of acid distribution along wellbore is required to analyze that the zones needing stimulation are receiving enough acid. It is even more important in cases where the reservoir properties are varying along the length of the wellbore. A model is developed in this study to simulate the placement of injected acid in a long horizontal well and to predict the subsequent effect of the acid in creating wormholes, overcoming damage effects, and stimulating productivity. The model tracks the interface between the acid and the completion fluid in the wellbore, models transient flow in the reservoir during acid injection, considers frictional effects in the tubulars, and predicts the depth of penetration of acid as a function of the acid volume and injection rate at all locations along the completion. A computer program is developed implementing the developed model. The program is used to simulate hypothetical examples of acid placement in a long horizontal section. A real field example of using the model to history match actual treatment data from a North Sea chalk well is demonstrated. The model will help to optimize acid stimulation in horizontal wells.Item Brittle Fracture Modeling with a Surface Tension Excess Property(2012-10-30) Ferguson, LaurenThe classical theory of linear elastic fracture mechanics for a quasi-static crack in an infinite linear elastic body has two significant mathematical inconsistencies: it predicts unbounded crack-tip stresses and an elliptical crack opening profile. A new theory of fracture developed by Sendova and Walton, based on extending continuum mechanics to the nanoscale, corrects these erroneous effects. The fundamental attribute of this theory is the use of a dividing surface to describe the material interface. The dividing surface is endowed with an excess property, namely surface tension, which accounts for atomistic effects in the interfacial region. When the surface tension is taken to be a constant, Sendova and Walton show that the theory reduces the crack-tip stress from a square root to a logarithmic singularity and yields a finite angle opening profile. In addition, they show that if the surface tension depends on curvature, the theory completely removes the stress singularity at the crack-tip, for all but countably many values of the two surface tension parameters, and yields a cusp-like opening profile. In this work, we develop a numerical model using the finite element method for the Sendova-Walton fracture theory applied to the classical Griffith crack problem in the case of constant surface tension. We show that the numerical model behaves as predicted by the theory, yielding a reduced crack-tip singularity and a finite opening angle for all nonzero values of the constant surface tension. We also lay the groundwork for the numerical implementation of the curvature-dependent model by constructing an algorithm to determine the appropriate threshold values for the surface tension parameters that guarantee bounded crack-tip stresses. These values can then be directly applied to the forthcoming numerical model.Item Evaluation of an approximate method for incorporating floating docks in harbor wave prediction models(Texas A&M University, 2005-11-01) Tang, ZhaoxiangComputer models are nowadays routinely used in harbor engineering applications. Models based on the two-dimensional elliptic mild-slope equation can simultaneously simulate refraction, diffraction, reflection, and dissipation in completely arbitrary coastal domains. However, floating structures such as floating breakwaters and docks are often encountered in the modeling domain. This makes the problem locally 3- dimensional. Hence it is problematic to incorporate a floating structure into the 2-d model. Tsay and Liu (1983) proposed a highly simplified but approximate approach to handle this problem practically. The validity of their approach is examined in detail and it is found that the actual solutions deviate considerably from the theoretical solutions, although their approximation provides results with the correct trend. Therefore, correction factors have been developed and may be used to produce more reliable results using the framework of Tsay and Liu (1983). The resulting method is applied to Douglas harbor in Alaska. The result shows that docks in the harbor distort the wave field considerably and create a reflective pattern that can affect navigation safety in some areas. Also plots are developed for the transmission coefficients for waves propagating past rectangular and cylindrical floating objects of infinite extent for a wide range of conditions encountered in practice.Item Financial Implications of Intergenerational Farm Transfers(2013-11-25) Peterson, Devin RichardThis study seeks to address the challenge of family farm succession. A recursive, stochastic, simulation model is employed to estimate the financial impacts and accompanying risk incurred through the intergenerational transfer of farm assets and management. The model assists in creating a before and after comparative analysis of succession for a large, medium, and small sized representative farm in Texas. Eight methods of farm transfer are analyzed: a will, trust, buy-sell and lease-to-buy agreements, the formation of business entities, life insurance, gifting, and selling farmland to outside investors. These methods are employed to help minimize estate taxes, create retirement income for the owner, or decrease general transfer costs such as probate fees. The simulation model utilizes stochastic and control variables to create pro -forma financial statements that aid in determining net income, debt requirements, and debt outstanding each year for a ten year time period. Key output variables such as combined net present value (NPV) of the owner and successor and the debt to asset ratio are used to analyze financial performance and position. Combined NPV is also employed to rank risky alternatives from most to least preferred using the method of stochastic efficiency with respect to a function. Output variables of estate and gift taxes and debt capital volume are also examined to compare across methods of transfer and to view their effects upon NPV, debt levels, and cash flows. The study finds that the most preferred method varies by farm size, net worth, and the underlying goals of the farmer.Item Modeling Nutrient Dynamics in Coastal LagoonsTurner, Evan L.Item Population Dynamics of Northern Bobwhites in Southern Texas(2010-01-16) Demaso, StephenNorthern bobwhites (Colinus virginianus) are an important cultural, ecological, and economical part of the southern Texas landscape. I used radio-telemetry data from 2000?2005, part of a long-term, bobwhite study in southern Texas, to test the nest-concealment hypothesis, develop a stochastic simulation model for bobwhite populations, and evaluate the influence of brush canopy coverage (BCC) on short- and long-term demographic performance of bobwhites. Bobwhite nests tend to be situated in taller and denser vegetation than would be expected if nest-site location was a random process. I compared 4 microhabitat variables between successful (n = 135) and depredated nests (n = 118). I documented similar microhabitat attributes between successful and depredated nests. The discriminant function correctly classified only 48?59% of nest fates into the correct group, but only 18% of the variation in nest fate. Thus, my results did not support the nest-concealment hypothesis. My stochastic simulation model for bobwhite populations is based on difference equations (?t = 3 months) and simulations run for 100 years using STELLA? 9.0.2. The probability of persistence for 100 years for the spring population was 74.2% and 72.5% for the fall population. Simulated population parameters were similar to those observed in the field for 5 of 6 population parameters. Only simulated male adult annual survival differed by 275.0% from field estimates. Despite this difference, my model appears to be a good predictor of bobwhite populations in the Rio Grande Plains of Texas. I estimated bobwhite density, survival, and production (proportion of hens nesting, nesting attempts per hen, and clutch size) in 3 study areas with ~10%, ~25%, and >30% BBC. All demographic parameters were similar among the 3 BCC classes. However, simulation modeling indicated that long-term demographic performance was greater on the ~25% and >30% BCC classes. The probability of fall population persistence was greater in the ~25% (90.8%) and >30% (100.0%) BCC classes than in the ~10% BCC class (54.2%). My study highlights the shortcoming of considering only short-term effects when comparing habitat given that short- and long-term effects of habitat on demographic performance can differ.Item Texas white-tailed deer Internet harvest model(2009-05-15) Garrett, Jennifer NicoleWhite-tailed deer (Odocoileus virginianus) is an intensively managed game species throughout Texas and the United States. Modeling is a tool that has been used to evaluate various management practices and their potential impacts on wildlife populations; however, many existing models are complicated for the average land manager, require a significant amount of demographic data that may not be readily available, or require expensive software. I developed a white-tailed deer (WTD) harvest model, Texas Deer Manager (TDM), using STELLA? (High Performance Systems, Inc., Version 7.0.3) and NetSim Creator? (High Performance Systems, Inc. Version 2.0), that incorporates the field data that are currently collected and/or can be easily collected, and can be used by interested landowners and state biologists. Unlike other population models specific for WTD, my TDM simulation model has an easy-to-use interface and is available on-line via the Internet. Age classes in the model are represented as fawns (<12 months), yearlings (1-2 years), and adults (>2 years) for each sex. Harvest is controlled by the model user. Currently the parameters such as survival and fecundity rates are set for an average year in the Edwards Plateau Ecological Region of Texas; however, model users can adjust survival and fecundity based on the specifics of their land management area. The website also provides general information about WTD biology and management. I conducted an on-line survey (n = 29) to evaluate the perceived usefulness and satisfaction of the TDM and website. I found that overall participants found the TDM and website were useful with 59% responding they would ?probably? or ?definitely? use it as a tool to help them determine their harvest plans. There seems to be a strong interest and need for a tool like the TDM and website. It would be beneficial to continue to develop the website and make it applicable for all the regions in Texas. Also, since the TDM model is easily accessible via the Internet and simple to use, it could be incorporated as a tool to teach population dynamics in the classroom.Item White-tailed deer population dynamics and management on the Lyndon B. Johnson Space Center(Texas A&M University, 2004-11-15) Whisenant, Shane WestonWhite-tailed deer (Odocoileus virginianus) numbers on the National Aeronautics and Space Administration's (NASA) Johnson Space Center (JSC) in Houston, Texas have increased in recent years and are a cause of urban-related accidents (e.g., deer-vehicle collisions, negative interation with humans). Safety personnel for the JSC are interested in reducing human-deer interaction by a reduction in overall population numbers. My overall study objectives were to (1) estimate population parameters for JSC deer, (2) develop a computer simulation model for the JSC deer, and (3) evaluate 2 management strategies to control JSC deer numbers a priori using the JSC deer model. The 2 management strategies I evaluated were the efficacy of SpayVac immunocontraceptive vaccine (sterilization) and trap and translocation (deer removal) efforts in managing white-tailed deer on JSC. In general, single treatments of removals or sterilization (less than 75 percent of female deer treated) were not effective in reducing population growth (R greater than 1). Approximately 50% of female deer needed to be removed annually to reduce population growth whereas approximately 25% of female deer needed to be treated annually with SpayVac for the same effects. A combination of trap and removals and sterilizations was effective in reducing population growth when applied to approximately 25% of the female population annually. I recommend the use of sterilization annually (25%) or a combination of sterilization and removal (25%) to achieve the goals of JSC in maintaining current deer numbers. Removing or sterilizing > 50% of the female deer annually caused the JSC deer population to decrease to a level near eradication.