Date of Award

12-2017

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Environmental Engineering and Earth Sciences

Committee Member

Dr. Lawrence Murdoch, Committee Chair

Committee Member

Dr. Stephen Moysey

Committee Member

Dr. Ronald Falta

Abstract

Hydraulic fractures are often used to stimulate fluid flow from wells in low permeability geologic units. This capability has been beneficial to the oil and gas industry as well as the remediation industry. The shape and extent of a hydraulic fracture and the distribution of granular proppant affects the performance of a hydraulic fracture during well stimulation. Several methods of tracking the location of hydraulic fractures have been developed (tiltmeter mapping, microseismic detection, and electric potential), but none have the ability to image the distribution of proppants. Electrical resistivity tomography (ERT) has been proposed as a new method to track the location of proppants containing electrical or magnetic contrast agents. The objective of this study is to evaluate the ability of ERT to determine the location of proppant injected into hydraulic fractures at depths of a few meters. This shallow depth is intended to provide a proof of concept for applications where proppant is injected into much deeper reservoirs. To achieve this goal, hydraulic fractures were created, and fine grained coke breeze or steel shot were injected during the fracturing process to create a proppant layer with an electrical conductivity that contrasts with the enveloping formation. ERT, soil core, and excavation data were collected and compared in order to evaluate the ability of ERT to image the proppant. This contrast was intended to increase the ability of ERT to resolve the proppant layer. A field site in Powdersville, SC was chosen, and it was divided into six 9 meter by 9 meter cells. One fracture was created in each cell at a depth of 1.5 meters (5 feet). Two fractures contained a coke breeze proppant. Two fractures contained a coke breeze and sand proppant mix. Two fractures contained steel shot proppant. Different amounts of proppant were injected into each cell. In Cells 1 through 4, about 2000 N of proppant were injected into the fractures, and in Cells 5 and Cell 6, about 5500 N of proppant were injected into the fractures. The slurry volumes injected into the cells varied from 0.33 m3 to 0.83 m3. ERT data were measured prior to proppant injection and then again after injection. Then the difference in these two data sets was inverted to estimate the changes in electrical conductivity caused by proppant injection. The ERT inversions were then compared to results from soil core data and direct inspection in excavation. Uplift was measured with optical levels, and these results were interpreted to estimate the locations of the fractures. The created fractures had an average fracture uplift length of 7 meters at the surface of the earth based on field site measurements of the uplift. Approximately 130 soil cores were collected from the field site. The cores were analyzed to determine the presence and location of the fractures. The soil core data were compiled to create maps and cross sections that were compared to the ERT data. Trenches were dug in the vicinity of two cells, and the fractures were described where they were exposed on the trench walls. The uplift, soil core, excavation, and ERT data show similar results. Comparisons with the soil core and trench mapping data show that the ERT inversions consistently match the direct observations. For example, in 83% of the soil cores that contained the fracture, the ERT data also indicated the fracture was present. In 96% of the cores where the fracture was absent in the core, the ERT data also indicated that the fracture was absent. Comparisons with the soil core and trench cross sections demonstrate that the ERT results overestimate the fracture depth in approximately ¾ of the instances. However, the error is likely due to sampling since the p-value of 0.90 indicates that the ERT and soil core data are from the same population. The average and standard deviation of the error in the depths is 0.17±0.19 meters with the negative sign indicating that the ERT indicated depth was deeper than the soil core depth on average. The close match between the ERT data and the ground checked data all point to the ERT method as a potentially new, non-invasive tool that can be employed to determine fracture shape, extent, and depth. This new non-invasive method could benefit the oil industry by allowing them to determine the extent of the fractures they created without the high costs of invasive methods such as coring.

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