High Resolution Packer Testing in Fractured Sedimentary Rock
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Packer tests in boreholes in fractured rock involving injection or withdrawal of water in borehole segments have been standard practice in bedrock hydraulic investigations pertaining to geotechnical and water resource projects since the 1950’s. However in contaminant hydrogeology, the tests are conducted to assess groundwater velocity and contaminant fluxes and therefore, much improved resolution and measurement accuracy is needed. For this thesis study packer testing equipment was designed specifically for studies of contaminant behavior in fractured rock with the ability to conduct four types of hydraulic tests: constant head/flow injection step tests, slug tests, pumping tests and recovery tests, all in the same borehole test interval without removing the equipment from the hole while acquiring high precision data for calculation of transmissivity (T) and fracture hydraulic apertures (2b). This equipment records pressure above, within, and below the test interval to gain insights regarding open borehole flow patterns, and to identify short circuiting to the open borehole above or below the test interval. The equipment measures flow rates as low as 6 ml/min up to 20 L/min, and the temperature in the test interval and at the ground surface is measured to account for density and viscosity variations. Each type of test is conducted repeatedly over a wide range of imposed applied pressures and flow rates and the equipment was applied to assess performance of this new methodology for packer testing and gain new insights concerning fractured rock hydrology in 6 boreholes in the fractured dolostone aquifer underlying the City of Guelph, Ontario. In the first stage of the equipment application in the fractured dolostone aquifer, over 150 high precision straddle packer tests using constant rate injection (Q) were conducted to identify the conditions of change from Darcian (linear) to non-Darcian (non-linear) flow based on the Q vs dP relationship where dP is the applied pressure above ambient. In the Darcian regime, the linear Q vs dP relationship passes through the origin (0,0) where the ambient pressure represents static conditions (i.e. Q=0 and dP=0). After the onset of non-Darcian flow, proportionally less Q per unit dP occurs so that the interval transmissivity (T) calculated from the test results using Darcy’s Law based models is underestimated by as much as an order of magnitude. The Darcy-Missbach equation was found to be a robust conceptual model for representation of step constant Q tests in which the linear proportionality constant relates Qn vs dP. It was found that quantifying non-linear flow allows for a more accurate determination of the linear data to obtain better estimates of T and hence the hydraulic apertures derived from the T using the Cubic Law. In order to obtain hydraulic apertures from the packer test T values, the number of hydraulically active fractures in the test interval is needed. The only data collected regarding individual fractures was the core log created during the coring process and the acoustic televiewer log, both of which identify the location of fractures, but neither could tell if the fractures identified were hydraulically active. A sensitivity analysis concerning the effects of non-linear flow and the number of hydraulically active fractures on the calculated hydraulic aperture shows that the number of fractures selected as hydraulically active has the greatest effect on the aperture values. A new approach is proposed for determining apertures from hydraulic tests in fractured rock utilizing the onset of non-linear flow to aid in the choice of the number of active fractures present in the test interval. In the second stage of the equipment application, the four types of hydraulic tests (constant head, pumping, recovery, and rising/falling head slug tests) conducted in the same test interval at gradually increasing flow rates showed that non-linear flow can be most easily identified and quantified using constant head tests providing a higher degree of certainty that the data used to calculate T are from the Darcian flow regime. Slug tests are conducted most rapidly, but formation non-linear behavior is commonly exaggerated by non-linearity within the test equipment at large initial displacements. However, the equipment non-linearity can be accounted for using a Reynolds number (Re) analysis allowing identification of the non-linear flow in the formation. In addition, non-linear flow can interfere with evidence of fracture dilation. The pumping and recovery tests are the most time consuming because of the relatively long time required to reach steady state. However, these tests offer the most potential to give insight into the influences of the peripheral fracture network and rock matrix permeability on test results In addition to the actual transmissivity of the test interval T values obtained from packer tests can be influenced by several factors including non-linear flow in the formation and in the test equipment, aperture dilation or closure, hydraulic short circuiting or leakage from the test interval to the open borehole and dual permeability properties of the system (fractures and matrix). The equipment and procedures developed in this thesis provide an improved framework for identifying these influences and in some cases avoiding them so that the aperture values calculated from T measurements are more accurate than those obtained through conventional approaches. In the conventional procedures for packer testing in fractured rock as recommended in manuals and guidance documents, the applied head and flow rate can be expected, based on the results of this thesis, to produce transmissivity values biased low because of non-linear (non-Darcian) flow.