Browsing by Subject "fractures"
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Item Experimental and Simulation Studies to Evaluate the Improvement of Oil Recovery by Different Modes of CO2 Injection in Carbonate Reservoirs(2011-02-22) Aleidan, Ahmed Abdulaziz S.Experimental and numerical simulation studies were conducted to investigate the improvement of light oil recovery in carbonate cores during CO2 injection. The main steps in the study are as follows. First, the minimum miscibility pressure of 31?API west Texas oil and CO2 was measured using the slimtube (miscibility) apparatus. Second, miscible CO2 coreflood experiments were carried out on different modes of injection such as CGI, WF, WAG, and SWAG. Each injection mode was conducted on unfractured and fractured cores. Fractured cores included two types of fracture systems creating two shape models on the core. Also, runs were made with different salinity levels for the injected water, 0 ppm, 60,000 ppm, and 200,000 ppm. Finally, based on the experimental results, a 2-D numerical simulation model was constructed and validated. The simulation model was then extended to conduct sensitivity studies on different parameters such as permeability variations in the core, WAG ratio and slug size, and SWAG ratio. The results of this study indicate that injecting water with CO2 either simultaneously or in alternating cycles increases the oil recovery by at least 10 percent and reduces the CO2 requirements by 50 percent. The salinity of the injected water has shown a detrimental effect on oil recovery only during WAG and SWAG injections. Lowering injected water salinity, which increases the CO2 solubility in water, increases oil recovery by up to 18 percent. Unfractured cores resulted in higher recovery than all fractured ones. CGI in fractured cores resulted in very poor recovery but WAG and SWAG injections improved the oil recovery by at least 25 percent over CGI. This is because of the better conformance provided by the injected water, which decreased CO2 cycling through the fracture. CO2 injection in layered permeability arrangements showed significant decrease in oil recovery (up to 40 percent) compared to the homogenous case. For all injection modes during the layered permeability arrangements, the best oil recovery was obtained when the flow barrier is in the middle of the core. When the permeability was arranged in sequence, each injection mode showed different preference to the permeability arrangements. The WAG ratio study in the homogenous case showed that a 1:2 ratio had the highest oil recovery, but the optimum ratio was 1:1 based on the amount of injected CO2. In contrast, layered permeability arrangements showed different WAG ratio preference depending on the location of the flow barrier.Item GPR Method for the Detection and Characterization of Fractures and Karst Features: Polarimetry, Attribute Extraction, Inverse Modeling and Data Mining Techniques(2011-02-22) Sassen, Douglas SpencerThe presence of fractures, joints and karst features within rock strongly influence the hydraulic and mechanical behavior of a rock mass, and there is a strong desire to characterize these features in a noninvasive manner, such as by using ground penetrating radar (GPR). These features can alter the incident waveform and polarization of the GPR signal depending on the aperture, fill and orientation of the features. The GPR methods developed here focus on changes in waveform, polarization or texture that can improve the detection and discrimination of these features within rock bodies. These new methods are utilized to better understand the interaction of an invasive shrub, Juniperus ashei, with subsurface flow conduits at an ecohydrologic experimentation plot situated on the limestone of the Edwards Aquifer, central Texas. First, a coherency algorithm is developed for polarimetric GPR that uses the largest eigenvalue of a scattering matrix in the calculation of coherence. This coherency is sensitive to waveshape and unbiased by the polarization of the GPR antennas, and it shows improvement over scalar coherency in detection of possible conduits in the plot data. Second, a method is described for full-waveform inversion of transmission data to quantitatively determine fracture aperture and electromagnetic properties of the fill, based on a thin-layer model. This inversion method is validated on synthetic data, and the results from field data at the experimentation plot show consistency with the reflection data. Finally, growing hierarchical self-organizing maps (GHSOM) are applied to the GPR data to discover new patterns indicative of subsurface features, without representative examples. The GHSOMs are able to distinguish patterns indicating soil filled cavities within the limestone. Using these methods, locations of soil filled cavities and the dominant flow conduits were indentified. This information helps to reconcile previous hydrologic experiments conducted at the site. Additionally, the GPR and hydrologic experiments suggests that Juniperus ashei significantly impacts infiltration by redirecting flow towards its roots occupying conduits and soil bodies within the rock. This research demonstrates that GPR provides a noninvasive tool that can improve future subsurface experimentation.Item Numerical modeling of time-lapse seismic data from fractured reservoirs including fluid flow and geochemical processes(2009-05-15) Shekhar, RaviFractured reservoirs, especially in low permeable carbonate rocks, are important target for hydrocarbon exploration and production because fractures can control fluid flow inside the reservoir. Hence, quantitative knowledge of fracture attributes is important for optimal hydrocarbon production. However, in some cases fractures can cause leakage of injected CO2 during enhanced oil recovery (EOR) or CO2 sequestration. Furthermore, CO2 can geochemically interact with reservoir fluids and host rock. Hence, time-lapse monitoring of the progress of CO2 in fractured reservoirs is also very important. In order to address these challenges, I have developed an integrated approach for studying fluid flow and seismic wave propagation in fractured media using Discrete Fracture Network (DFN) models. My seismic simulation study suggests that CO2 saturated reservoir shows approximately ten times more attenuation than brine saturated reservoir. Similarly, large P-wave velocity variation in CO2 saturated reservoir and amplitude variation with offset (AVO) results for our example model predicts that CO2 is easier to detect than brine in the fractured reservoirs. The effects of geochemical processes on seismics are simulated by time-lapse modeling for t = 1000 years. My modeling study suggests that intra-aqueous reactions are more significant during injection of CO2 for t = 6 years, while slower mineral reactions dominate after pressure equilibrium is achieved that is from t = 6 to 1000 years. Overall both types of geochemical reactions cause change in reflection coefficient of 2 to 5%, which may be difficult to detect in some cases. However, the significant change in the seismic properties at the boundary of the CO2 front can be used to detect the flow path of CO2 inside the reservoirs. Finally, a method for generating stochastic fracture models was extended and improved to more realistic field model for seismic and fluid modeling. My detail analysis suggests that fractures generated by isotropic stress field favor orthogonal sets of fractures in most subsurface rocks that can be converted to seismic model, similar to DFN study. The quality and validity of the models is assessed by comparisons to DFN models, including calculations of fractal dimension measures that can help to characterize fractured reservoirs.Item The imbibition process of waterflooding in naturally fractured reservoirs(Texas A&M University, 2005-02-17) Huapaya Lopez, Christian A.This thesis presents procedures to properly simulate naturally fractured reservoirs using dual-porosity models. The main objectives of this work are to: (1) determine if the spontaneous imbibition can be simulated using a two phase CMG simulator and validate it with laboratory experiments in the literature; (2) study the effect of countercurrent imbibition in field scale applications; and (3) develop procedures for using the dual-porosity to simulate fluid displacement in a naturally fractured reservoir. Reservoir simulation techniques, analytical solutions and numerical simulation for a two phase single and dual-porosity are used to achieve our objectives. Analysis of a single matrix block with an injector and a producer well connected by a single fracture is analyzed and compared with both two phase single and dual-porosity models. Procedures for obtaining reliable results when modeling a naturally fractured reservoir with a two phase dual-porosity model are presented and analyzed.