Wireline tools and log analysis methods were not designed for unconventional reservoirs. As a result, hydrocarbon assessment for shale source rock plays have significant uncertainties. This study focuses on petrophysical interpretation uncertainty from a single historic Barnett Shale well TP Sims #2 of Wise County, TX. The Barnett Shale is one of the major source rock plays in the United States. The large body of research and information from the well-drilled Barnett Shale provides a good opportunity to understand and adjust OGIP modeling approaches from volumetric analysis to well performance data. Several factors unique to shale source rocks such as TOC and pyrite have been incorporated into wireline log interpretation using core-derived correlations. Key petrophysical parameters that are estimated from well logs calibrated to core data include: mineral volumes, porosity, net pay, and water saturation. Volumetric OGIP calculations from a range of well log analysis results that have been calibrated to core for TP Sims #2 are compared with EUR data. The resulting recovery factors are larger than expected which may mean that volumetric OGIP remains deficient for resource assessment of shale plays.
In 2007, the Barnett Shale in the Fort Worth basin of Texas produced 1.1 trillion cubic feet (Tcf) gas and ranked second in U.S gas production. Despite its importance, controls on Barnett Shale gas well performance are poorly understood. Regional and vertical variations of reservoir properties and their effects on well performances have not been assessed. Therefore, we conducted a study of Barnett Shale stratigraphy, petrophysics, and production, and we integrated these results to clarify the controls on well performance. Barnett Shale ranges from 50 to 1,100 ft thick; we divided the formation into 4 reservoir units that are significant to engineering decisions. All but Reservoir Unit 1 (the lower reservoir unit) are commonly perforated in gas wells. Reservoir Unit 1 appears to be clay-rich shale and ranges from 10 to 80 ft thick. Reservoir Unit 2 is laminated, siliceous mudstone and marly carbonate zone, 20 to 300 ft thick. Reservoir Unit 3 is composed of multiple, stacked, thin (~15-30 ft thick), upward coarsening sequences of brittle carbonate and siliceous units interbedded with ductile shales; thickness ranges from 0 to 500 ft. Reservoir Unit 4, the upper Barnett Shale is composed dominantly of shale interbedded with upward coarsening, laterally persistent, brittle/ductile sequences ranging from 0 to 100 ft thick. Gas production rates vary directly with Barnett Shale thermal maturity and structural setting. For the following five production regions that encompass most of the producing wells, Peak Monthly gas production from horizontal wells decreases as follows: Tier 1 (median production 60 MMcf) to Core Area to Parker County to Tier 2 West to Oil Zone-Montague County (median production 10 MMcf). The Peak Monthly oil production from horizontal wells is in the inverse order of gas production; median Peak Monthly oil production is 3,000 bbl in the Oil Zone-Montague County and zero in Tier 1. Generally, horizontal wells produce approximately twice as much oil and gas as vertical wells. This research clarifies regional variations of reservoir and geologic properties of the Barnett Shale. Result of these studies should assist operators with optimization of development strategies and gas recovery from the Barnett Shale.
A comprehensive textbook presenting techniques for the analysis and characterization of shale plays Significant reserves of hydrocarbons cannot be extracted using conventional methods. Improvements in techniques such as horizontal drilling and hydraulic fracturing have increased access to unconventional hydrocarbon resources, ushering in the “shale boom” and disrupting the energy sector. Unconventional Hydrocarbon Resources: Techniques for Reservoir Engineering Analysis covers the geochemistry, petrophysics, geomechanics, and economics of unconventional shale oil plays. The text uses a step-by-step approach to demonstrate industry-standard workflows for calculating resource volume and optimizing the extraction process. Volume highlights include: Methods for rock and fluid characterization of unconventional shale plays A workflow for analyzing wells with stimulated reservoir volume regions An unconventional approach to understanding of fluid flow through porous media A comprehensive summary of discoveries of massive shale resources worldwide Data from Eagle Ford, Woodford, Wolfcamp, and The Bakken shale plays Examples, homework assignments, projects, and access to supplementary online resources Hands-on teaching materials for use in petroleum engineering software applications The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals.
A comprehensive overview of the key geologic, geomechanical and engineering principles that govern the development of unconventional oil and gas reservoirs. Covering hydrocarbon-bearing formations, horizontal drilling, reservoir seismology and environmental impacts, this is an invaluable resource for geologists, geophysicists and reservoir engineers.
The Barnett Shale, Fort Worth Basin, remains one of the largest shale plays in the world. Whereas gas production in most of the play is considered economical, a limiting factor is the tendency of wells to produce large volumes of water, particularly outside the Newark East Field, in the expansion area. This thesis investigates water production trends in the expansion area, using production and well data from 3,720 wells, by testing the hypothesis that faults and karst features (collapse chimneys) act as water conduits when fracked into, causing communication between the Ellenburger and Barnett and increased water production. To rule out an explanation of water production trends in terms of completion parameters, well architecture and fracability, these variables were correlated with water production using univariate and multivariate regression analyses. The only statistically significant relationship was between injected water and returned water, yielding an R2 = 98%. Next, using seismic data to model faults and chimneys, water production was compared in wells that were drilled into either feature with the nearest neighboring wells that were not. This was accomplished by integrating seismic and well log data to display wellbores in a 3D interpretation space so that horizontal wellbores intersecting (and thus fracking) into faults or chimneys could be determined. The results of comparing water production in wells that penetrate a fault or chimney showed that, on average, wells that penetrate a fault or chimney produce insignificantly more water than the nearest neighboring wells that do not. Thus, this study found the hypothesis that faults and chimneys act as water conduits when fracked into, resulting in higher water production, to be false.
This thesis presents five studies of a gas shale reservoir using diverse methodologies to investigate geomechanical and transport properties that are important across the full reservoir lifecycle. Using the Barnett shale as a case study, we investigated adsorption, permeability, geomechanics, microseismicity, and stress evolution in two different study areas. The main goals of this thesis can be divided into two parts: first, to investigate how flow properties evolve with changes in stress and gas species, and second, to understand how the interactions between stress, fractures, and microseismicity control the creation of a permeable reservoir volume during hydraulic fracturing. In Chapter 2, we present results from adsorption and permeability experiments conducted on Barnett shale rock samples. We found Langmuir-type adsorption of CH4 and N2 at magnitudes consistent with previous studies of the Barnett shale. Three of our samples demonstrated BET-type adsorption of CO2, in contrast to all previous studies on CO2 adsorption in gas shales, which found Langmuir-adsorption. At low pressures (600 psi), we found preferential adsorption of CO2 over CH4 ranging from 3.6x to 5.5x. While our measurements were conducted at low pressures (up to 1500 psi), when our model fits are extrapolated to reservoir pressures they reach similar adsorption magnitudes as have been found in previous studies. At these high reservoir pressures, the very large preferential adsorption of CO2 over CH4 (up to 5-10x) suggests a significant potential for CO2 storage in gas shales like the Barnett if practical problems of injectivity and matrix transport can be overcome. We successfully measured permeability versus effective stress on two intact Barnett shale samples. We measured permeability effective stress coefficients less than 1 on both samples, invalidating our hypothesis that there might be throughgoing flow paths within the soft, porous organic kerogen that would lead the permeability effective stress coefficient to be greater than 1. The results suggest that microcracks are likely the dominant flow paths at these scales. In Chapter 3, we present integrated geological, geophysical, and geomechanical data in order to characterize the rock properties in our Barnett shale study area and to model the stress state in the reservoir before hydraulic fracturing occurred. Five parallel, horizontal wells were drilled in the study area and then fractured using three different techniques. We used the well logs from a vertical pilot well and a horizontal well to constrain the stress state in the reservoir. While there was some variation along the length of the well, we were able to determine a best fit stress state of Pp = 0.48 psi/ft, Sv = 1.1 psi/ft, SHmax = 0.73 psi/ft, and Shmin = 0.68 psi/ft. Applying this stress state to the mapped natural fractures indicates that there is significant potential for induced shear slip on natural fracture planes in this region of the Barnett, particularly close to the main hydraulic fracture where the pore pressure increase during hydraulic fracturing is likely to be very high. In Chapter 4, we present new techniques to quantify the robustness of hydraulic fracturing in gas shale reservoirs. The case study we analyzed involves five parallel horizontal wells in the Barnett shale with 51 frac stages. To investigate the numbers, sizes, and types of microearthquakes initiated during each frac stage, we created Gutenberg-Richter-type magnitude distribution plots to see if the size of events follows the characteristic scaling relationship found in natural earthquakes. We found that slickwater fracturing does generate a log-linear distribution of microearthquakes, but that it creates proportionally more small events than natural earthquake sources. Finding considerable variability in the generation of microearthquakes, we used the magnitude analysis as a proxy for the "robustness" of the stimulation of a given stage. We found that the conventionally fractured well and the two alternately fractured wells ("zipperfracs") were more effective than the simultaneously fractured wells ("simulfracs") in generating microearthquakes. We also found that the later stages of fracturing a given well were more successful in generating microearthquakes than the early stages. In Chapter 5, we present estimates of stress evolution in our study reservoir through analysis of the instantaneous shut-in pressure (ISIP) at the end of each stage. The ISIP increased stage by stage for all wells, but the simulfrac wells showed the greatest increase and the zipperfrac wells the least. We modeled the stress increase in the reservoir with a simple sequence of 2-D cracks along the length of the well. When using a spacing of one crack per stage, the modeled stress increase was nearly identical to the measured stress increase in the zipperfrac wells. When using three cracks per stage, the modeled final stage stress magnitude matched the measured final stage stress magnitude from the simulfrac wells, but the rate of stress increase in the simulfrac wells was much more gradual than the model predicted. To further investigate the causes of these ISIP trends, we began numerical flow and stress analysis to more realistically model the processes in the reservoir. One of our hypotheses was that the shorter total time needed to complete all the stages of the simulfrac wells was the cause of the greater ISIP increase compared to the zipperfrac wells. The microseismic activity level measured in Chapter 4 also correlates with total length of injection, suggesting leak off into the reservoir encouraged shear failure. Numerical modeling using the coupled FEM and flow software GEOSIM was able to model some cumulative stress increase the reservoir, but the full trend was not replicated. Further work to model field observations of hydraulic fracturing will enhance our understanding of the impact that hydraulic fracturing and stress change have on fracture creation and permeability enhancement in gas shales.
Reservoir characterization as a discipline grew out of the recognition that more oil and gas could be extracted from reservoirs if the geology of the reservoir was understood. Prior to that awakening, reservoir development and production were the realm of the petroleum engineer. In fact, geologists of that time would have felt slighted if asked by corporate management to move from an exciting exploration assignment to a more mundane assignment working with an engineer to improve a reservoir’s performance. Slowly, reservoir characterization came into its own as a quantitative, multidisciplinary endeavor requiring a vast array of skills and knowledge sets. Perhaps the biggest attractor to becoming a reservoir geologist was the advent of fast computing, followed by visualization programs and theaters, all of which allow young geoscientists to practice their computing skills in a highly technical work environment. Also, the discipline grew in parallel with the evolution of data integration and the advent of asset teams in the petroleum industry. Finally, reservoir characterization flourished with the quantum improvements that have occurred in geophysical acquisition and processing techniques and that allow geophysicists to image internal reservoir complexities. Practical resource describing different types of sandstone and shale reservoirs Case histories of reservoir studies for easy comparison Applications of standard, new, and emerging technologies
Presents an unstinting exploration of controversial fracking technologies to consider the arguments of its supporters and detractors, profiling key contributors while explaining how the practice is changing the way energy is used.
