The goal of this project was to develop and advance technologies that could result in the discovery and development of new gas reserves within unconventional, low-permeability natural gas reservoirs.
University of Wyoming, Institute for Energy Research (IER) – Project management and all research products
Santa Fe Snyder Corporation – Wells logs, seismic data and geological/geophysical information
Laramie, WY 82071
The Wind River basin, specifically the Riverton Dome and Emigrant areas, was selected for this project because it is characterized by a significant number of anomalously pressured gas accumulations. Preliminary work within the Wind River Basin had demonstrated that there is a regionally prominent pressure surface boundary that could be detected by inversions in sonic velocity depth gradients in individual well log profiles and that could be seen as a velocity inversion on seismic lines. Also, Santa Fe Snyder Corporation provided the project with sonic logs, two 3-D seismic studies (40 and 30 square miles) and a variety of other geological and geophysical information.
The objective of the project was to test the validity of a new conceptual model and resultant exploration paradigm for “basin-center” gas accumulations in the Riverton Dome area of the Wind River basin of Wyoming. The strategy involves three-dimensional evaluation of the pressure boundary between normal and anomalous pressure regimes and the detection and delineation of porosity/permeability “sweet spots” below the boundary.
The project tested the validity of a new conceptual model and resultant exploration paradigm for “basin-center” gas accumulations such as are found in deep, gas-saturated portions of Rocky Mountain Laramide Basins (RMLB). The Wind River basin, specifically the Riverton Dome and Emigrant areas, was selected for this project because it is characterized by a significant number of anomalously pressured gas accumulations. This project opened the door for operators to revisit reservoirs once thought not to contain any producible reserves.
The project tested the validity of a new conceptual model and resultant exploration paradigm for “basin-center” gas accumulations such as are found in deep, gas-saturated portions of Rocky Mountain Laramide Basins (RMLB). The strategy involves three-dimensional evaluation of the pressure boundary between normal and anomalous pressure regimes and the detection and delineation of porosity/permeability “sweet spots” (areas of enhanced storage capacity and deliverability) below the boundary. The pressure boundary between normal and anomalous pressure regimes is typically expressed as a significant inversion in both sonic and seismic velocity-depth profiles.
Processed seismic data were analyzed using a ProMAX velocity analysis program. The Riverton Dome area 3-D survey consisted of 382 inlines and 577 crosslines, and the velocity analysis was done on every tenth inline (i.e., 1000 ft intervals) and at every ninth cross line (i.e., 990 ft). The vertical sampling for the velocity study was done at 100 ms intervals. Thus, the sampling grid utilized in this study (i.e.,60 points per inline) was considerably closer than that used in typical commercial processing (i.e., 10 to 11 samples per inline). The Emigrant area 3-D survey consisted of 330 inlines and 322 cross lines. Like the Riverton Dome study, the Emigrant velocity analysis was done on every tenth inline and at every ninth cross line, and the vertical sampling at 100 ms intervals.
In both the Riverton Dome and Emigrant areas, the following two tasks were accomplished: (1) evaluation and construction of the velocity fields according to the sampling strategy outlined above, and (2) isolation of anomalously slow velocity domains. Task 2 was accomplished by subtracting the normal regional velocity-depth gradient from the observed velocity-depth profile at 1620 sample points in the Riverton Dome 3-D survey, and at 705 sample points in the Emigrant 3-D survey. A typical normal velocity-depth gradient for each of the study areas was determined by modeling the compaction trend-sonic velocity-depth profile relationships from nearby well logs. The anomalous velocity profiles and volumes were thus the result of removing the typical regional normal velocity-depth profile from the observed velocity-depth gradients at each of the sample points. Using these procedures, it is possible to detect and delineate anomalously slow velocity domains in 3-D visualizations, as well as the regional pressure surface boundary/velocity inversion surface.
At Riverton Dome, velocity anomalies were detected within the intervals of the Cody, Frontier, Muddy and Nugget formations. These velocity anomalies were isolated and visualized in three dimensions to describe volumes that were gas saturated and anomalously pressured. Core observations in the area showed that these formations were not undercompacted, so the best explanation for the intensely slow velocities was that the fluid system within the volume contains significant free gas in the fluid phase. The anomalous velocity volume is thus an important lead as to where to explore for anomalously pressured gas accumulations in the Riverton Dome area.
During the time that the velocity field evaluation was progressing, six Muddy Formation tests were completed by Santa Fe Snyder Corporation, without input from the project. The test well results were made known to IER only after the final results of the Riverton Dome velocity evaluation were presented. There was a direct relationship between the anomalous velocity domain within the Muddy Formation and the IP values for the six recent test wells. Wells completed in rocks characterized by anomalous velocity values were 1500 ms slower than the regional velocity-depth gradient (three wells) which had IP values ranging from 3 to 4 MMcf/day. The one well at the edge of the velocity anomaly (anomalous velocity value of < 1200 ms slower than the regional velocity-depth gradient) had an IP value of 1 MMcf/day. The two wells drilled into rocks with a maximum anomalous velocity value of < 900 ms had IPs of less than 1 MMcf/day and were shut in.
Based on these promising results, a prospect was developed that was characterized by intensely slow velocities beneath CDP 124896, and this site was nominated as an ideal drilling location in the Riverton Dome 3-D seismic survey area, based on the IER exploration strategy. Unfortunately, due to the merger of Snyder Oil with Sante Fe Oil and the subsequent takeover of Santa Fe/Snyder Oil by Devon Energy, no verification well was drilled in the Riverton Dome field.
The same analytical techniques were utilized to study the Emigrant 3-D seismic survey. Graphical depictions of the anomalous velocity volume for the Emigrant survey revealed a highly irregular topography at the top surface of the anomalous region, suggesting that structural elements rather than stratigraphic boundaries were playing a larger role in determining its geometry. Aspects of the velocity anomalies in the Emigrant area suggested that the primary gas migration routes were vertical and that the gas was moving upward along conduits resulting from enhanced permeability that was the result of fracturing.
The Muddy and Frontier formations were the primary objectives considered in developing potential exploration targets, because they contained the most numerous and intense velocity anomalies. In order to determine the potential for enhanced porosity/permeability, two Event Similarity Prediction (ESP) horizontal slices were cut through the Emigrant ESP cube. Comparison of the anomalous velocity maps and ESP maps for both the Frontier and Muddy Formations revealed three overlaps that were selected as prioritized drilling targets with the highest potential for success in the search for anomalously pressured gas accumulations in the Emigrant area. Unfortunately, as was the case in the Riverton Dome area, corporate mergers precluded drilling of any test well by Santa Fe Snyder Oil Company. The blind test at the Riverton Dome area remains the best available public validation of the new technology.
This project is complete. A related, more recent project that further developed a detailed strategy for following this exploration model is described in the project summary for “Reducing Risk in Low-Permeability Gas Formations: Strategies for Avoiding Excessive Water Production and Optimizing Drilling and Completion Practices,” DE-FC26-01NT41325.
Aug. 1999 Final Technical Progress Report: Riverton Dome Gas Exploration and Stimulation Technology Demonstration, Wind River Basin, Wyoming [PDF-18.4MB]