Assessing Hydrofracing Success from Earth Tide and Barometric Response

Assessing Hydrofracing Success from Earth Tide and Barometric Response
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  See discussions, stats, and author profiles for this publication at: Assessing Hydrofracing Success from EarthTide and Barometric Response  Article   in  Ground Water · April 2010 DOI: 10.1111/j.1745-6584.2010.00704.x · Source: PubMed CITATIONS 4 READS 45 2 authors:Some of the authors of this publication are also working on these related projects: Currently I'm a Program Director of Hydrologic Sciences at the National Science Foundation   ViewprojectThomas J. BurbeyVirginia Polytechnic Institute and State Unive… 73   PUBLICATIONS   755   CITATIONS   SEE PROFILE Meijing ZhangVirginia Polytechnic Institute and State Univ… 6   PUBLICATIONS   18   CITATIONS   SEE PROFILE All content following this page was uploaded by Thomas J. Burbey on 29 September 2014. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the srcinal documentand are linked to publications on ResearchGate, letting you access and read them immediately.  Assessing Hydrofracing Success from Earth Tideand Barometric Response by Thomas J. Burbey 1,2 and Meijing Zhang 2 Abstract Identifying fracture pathways and connectivity between adjacent wells is vital for understanding flowcharacteristics, transport properties, and fracture characteristics. In this investigation, a simple, straightforwardmethodology is presented for assessing hydrofracing success and identifying possible fracture connectivity betweenneighboring boreholes, using water-level barometric response and tide signatures of individual fractures in acrystalline-rock setting. Water levels and barometric pressure heads were collected at two wells 27 m apart bothprior to, and after, hydrofracing one of the wells at the fractured-rock research site in Floyd County, Virginia.Vastly different barometric and tidal signatures existed at the two wells prior to hydrofracing as well EX-1 had nodiscernable fractures, while W-03 was connected to an identified fault-zone aquifer and produced a notable water-level earth tide and barometric signatures. After hydrofracing EX-1, new fractures were induced and the resultingwater-level tidal signature and barometric efficiencies were nearly identical to the W-03 well. Aquifer testingconducted from both wells verified this connectivity along the fault-zone aquifer. The small phase differencebetween the tidal responses in the two wells can be accounted for by the calculated differences in transmissivityand casing diameter. Introduction Characterizing fracture networks and potential con-nectivity across boreholes in complex crystalline-rock settings is difficult but a necessary requirement if rechargeprocesses, contaminant transport pathways, or wellheadprotection delineation is to be identified for the wells inquestion. Acquisition of borehole geophysical logs such asgamma, electrical resistivity, heat-pulse flow meter, tem-perature, and caliper are often used to identify productivefractures within a single borehole (Paillet 1994; Seatonand Burbey 2005). Cross-borehole flow testing (Paillet1993), pulse tests (Williams and Paillet 2002), or tracer 1 Corresponding author: Department of Geosciences, VirginiaTech, 4044 Derring Hall, Blacksburg, VA 24061; (540) 231-6696;fax: (540) 231-3386; 2 Department of Geosciences, Virginia Tech, Blacksburg, VA24061.Received June 2009, accepted March 2010.Copyright © 2010 The Author(s)Journalcompilation © 2010NationalGroundWaterAssociation.doi: 10.1111/j.1745-6584.2010.00704.x tests (Rugh and Burbey 2008) are commonly applied toidentify individual fractures or fracture networks that con-nect one wellbore to another. Conversely, these testingmethods can identify “dead” fractures characterized byboreholes that are likely to yield little or no water if theyare pumped because the fractures that intersect the bore-hole are not connected to a source of recharge. However,the equipment necessary for these testing methods can beexpensive, and the procedures to acquire the necessarydata are often difficult and time consuming.In complex crystalline-rock settings, where boreholesiting for domestic or municipal water supplies ischallenging and often unsuccessful, hydrofracing hasbecome an increasingly used technique to essentiallyexpand the areal extent of the borehole by hydraulicallylengthening existing fractures in hopes of intersectingother productive fractures and therefore increasing theyield of an otherwise poorly producing well (Sutphinet al. 2000). Hydrofracing has its srcins in the oil andgas industry and has been used extensively to enhanceoil or coal-bed methane production (Shock and Davis GROUND WATER 1  1969). More recently it has been considered as a meansof enhancing storage for CO 2  sequestration (Klara et al.2003). Others have used hydrofracing to assess in siturock stress (Thompson and Chandler 2004).Fracture connectivity and productivity between twowells 27 m apart and open only in crystalline rock arecharacterized in this investigation both prior to and afterhydrofracing one of the wells that previously had nohydraulically connected fractures. The methods for theevaluation of the newly produced fractures and possi-ble connectivity between the two wells are barometricpressure responses and earth tide analysis. Both baromet-ric pressure changes and earth tides are known to causewater-level fluctuations in wells that are open to arte-sian aquifers. These pressure signatures have been usedto characterize confined aquifers (Bredehoeft 1967; Hsiehet al. 1987; Rojstaczer and Agnew 1989), but they havealso been used to a lesser degree to assess and characterizeindividual fractures in consolidated rocks (Acworth andBrain 2008; Bower 1983; Burbey 2010; Hanson 1984). Analyzing earth tides and barometric loading responses inwells is far less labor-intensive than performing sophis-ticated cross-borehole tests and is also far less costly,requiring only the use of a few pressure transducers. Field Site Description and Water-Level DataAnalysis The study site is located in the Blue Ridge Phys-iographic Province in Floyd County, Virginia, andrepresents a highly complex metamorphic terrain char-acterized by numerous shallow thrust faults that tendto compartmentalize ground-water flow (Figure 1). Thehydrogeology of the site has been extensively investi-gated and the interested reader is referred to Seaton andBurbey 2002; Seaton and Burbey 2005) for a detailedanalysis of the geology and hydrogeology, to White andBurbey (2007) for more about the source recharge areasand vadose zone characteristics, and to Gentry and Burbey(2004) and Rugh and Burbey (2008) for a discussion of the recharge and flow mechanisms at the site. A brief overview of the hydrogeology is provided as necessaryfor this investigation.Suites of logs from 12 wells and numerous two-dimensional electrical resistivity profiles were used tocharacterize the geology and hydrogeology at the site.Figure 1 shows the deep wells that are cased through thesaprolite and are uncased to depths ranging from 50 to350 m. The logs and surveys reveal a two-aquifer systemin which an overlying highly heterogeneous saproliteaquifer (5- to 15-m thick) is separated from a fault-zone aquifer by a relatively impermeable hanging wall.The thrust fault exhibits characteristics of both ductileand brittle deformation. The shear zone is filled with afine-grained low-permeability mylonite with a fabric thatsuggests ductile deformation. The hanging and foot wallsof the fault are largely granulite gneiss and tend to fracturein a brittle fashion adjacent to the fault plane creating fluidpathways for flow. The individual fractures tend not to be Figure 1. (a) Location of the FRS (Fractured Rock Site)study area in the Blue Ridge Physiographic Province showinglocation of deep wells and spring. Gray shaded arearepresents subcrop of thrust fault. Land surface contoursshow general expression of topography; contour interval5 m; (b) conceptualization of the thrust fault and aquifersystems. continuous, but the fault-plane aquifer appears to representa series of interconnecting fractures that possess dip anglesof about 20 ◦ to 45 ◦ . The overall fracture network followsthe dip of the fault which is exposed at the crest of the hillat the study site (shaded gray area in Figure 1a), and dipssouthward and becomes relatively horizontal at the baseof the hill. Figure 1b represents a conceptualization of the hydrogeologic system. Where the thrust fault reachesthe surface it tends to be highly weathered, providing apathway for meteoric recharge to the fault-zone aquifer(Figures 1a and 1b). Discharge from the site occurs at alocal spring (Figure 1a) as well as at springs further downgradient to the south of the study site. At the identifiedspring site, flow from the fault-zone aquifer moves upwardthrough a weathered quartz rubble zone into the saproliteaquifer.The wells directly used for this investigation arelabeled in Figure 1. Table 1 lists the names, diameters, Table 1 Well Names, Diameters, and Depths Well Name Well Diameter (cm) Well Depth (m) W-03 14.5 158W-10 14.5 300EX-1 19.3 92 2 T.J. Burbey, M. Zhang GROUND WATER  and depths of these wells. Borehole logs between W-03and W-10 indicate that they intersect the fault-zoneaquifer. Heat-pulse flow meter, caliper, and opticalteleviewer logs suggest that all the water-level patternsobserved at W-03 occur due to flow in a 1-cm-widefracture intersecting the wellbore at depth of 62 m belowland surface and dipping to the north-northwest at anangle of 40 ◦ (Burbey 2010; Seaton and Burbey 2002).EX-1 was intended to penetrate the fault-zone aquiferwhen it was installed; however, after drilling and logging,it was clear that this well intersected no fractures atthe depth of the fault-zone aquifer (no fractures weredetected below 40 m). Because the fault-zone aquiferrepresents a network of fractures, EX-1 managed to missthem, whereas all the other wells shown in Figure 1a areknown to penetrate the fault-zone aquifer. Heat-pulse flowmeter logging during both ambient and pumped conditionsshowed only a small amount of inflow at a depth of 25 m at EX-1. A 4-h aquifer test in which W-03 waspumped (located 27 m from EX-1) continuously fromthe fault-zone aquifer at a rate of approximately 7 L/minyielded no measurable drawdown at EX-1. The maximumdrawdown in W-03 reached at the end of 4 h was 9.51 m.Hence, it was concluded that EX-1 and W-03 had littleor no hydraulic connection at depths below 40 m. Theother neighboring wells near W-03, on the other hand, allexperienced significant drawdown by the end of the test.Figure 2 shows the hydraulic heads measured atW-03 and EX-1 over a period of approximately 1 monthduring the summer of 2008. The nearly 3-m headdifference between these wells suggests that they are nothydraulically connected. Toward the end of the period,well W-10 (Figure 1a) was pumped at an unknown rate(homeowner used well only several times per year) andthe response in W-03 is clearly observed, which suggests Figure 2. Measured hydraulic head data for wells EX-1 andW-03 for the period from June 1 to July 8, 2008, prior tohydrofracing. a strong hydraulic connection between W-03 and W-10along the fault-zone aquifer. It should be noted thatthe static hydraulic head measured at W-10 is slightlyhigher than that at W-03, but nearly 3 m higher than thatmeasured at EX-1. The greatly delayed response of nearly2 days and the small water-level change occurring atEX-1 is further evidence of the weak hydraulic connectionbetween EX-1 and the other wells at the site.Figure 3a shows the relation between the water levelsmeasured in EX-1 (expressed as gauge pressures) withthe corresponding barometric pressure heads. Loadingcaused by changes in atmospheric pressure can belinked to diurnal and semidiurnal periodic changes inpressure from atmospheric tidal signatures associated withatmospheric heating and cooling and resulting ascendingand descending air masses. In addition, aperiodic loadingcan occur that are related to frontal systems passing overthe area. The barometric pressure record in Figure 3ashows both periodic and aperiodic pressure changes.An increase in atmospheric pressure acting on an openborehole will generally cause the measured water levels todecline so that there is an inverse response to atmosphericpressure. The measured water-level response does notappear to be significantly influenced by barometric loadingin any correlative capacity, either at the diurnal scale(periodic) or from shorter frequency frontal systems(aperiodic). The lack of well response from diurnal Figure 3. Pre-hydrofracing conditions at EX-1 showing(a) water levels (gauge pressure) and barometric pressureheads, (b) smoothed and reduced water levels and baromet-ric pressure heads. T.J. Burbey, M. Zhang GROUND WATER 3  barometric tides is further evidenced by the smoothedand reduced water level and barometric pressure changesas shown in Figure 3b. These data are smoothed andhave been reduced with a low-pass filter algorithm(Godin 1972) that uses a 24 h averaging of the observeddata to eliminate short frequency responses. This samealgorithm was used by Hsieh et al. (1987) when reducingwater levels and barometric pressures for tidal analysis.This filtering effectively eliminates aperiodic changes inboth records and normalizes the head changes aroundzero. The result reveals the atmospheric tides in thebarometric response and the combination of atmosphericand earth tide response in the water-level record. Ingeneral, if no earth tides are acting on the system thereshould be an inverse correlation between the periodicvariations of the atmospheric pressure and water-levelmeasurements in the well. However, this correlation isnot apparent in Figure 3b and the water-level recordalso does not possess periodic behavior associated withatmospheric or earth tides because no connection to thefault-zone aquifer exists. A time-lag response betweenearth and barometric tides and water-level response inthe well can greatly complicate possible correlations andalso make it difficult to separate earth tide signals fromdiurnal barometric tides. Toll and Rasmussen (2007)developed a software program (BETCO, v. 1.0) usinga regression deconvolution procedure that allows one toassess the response function (barometric efficiency or BE)at different lag times.The BE is defined as the ratio of the change inwater level (relative to gauge pressure) that results froma change in atmospheric pressure head). The BE has alsobe described as a partitioning coefficient (Acworth andBrain 2008) that relates the relative compressibilities of the formation and water as: BE  = θβα + θβ = p a p f  (1)where  θ   is the formation porosity,  α  is the compressibilityof the formation,  β  is the water compressibility, is thechange in barometric pressure and is the change in fluidpressure measured in the well and associated with thechange in atmospheric pressure. These pressures canbe readily converted to pressure heads for convenience.A graphical procedure for calculating BE developedby Gonthier (2007) uses lower frequency aperiodicbarometric pressure changes because these data are notinfluenced by earth tides and are usually easily identifiablein the corresponding water-level record if the time-lagsare small. No BE could be calculated from either theBETCO program or from the graphical procedure of Gonthier (2007) in the pre-fractured EX-1 well at anybarometric frequency scale. It can be concluded then thatEX-1 has a BE that is at or near zero prior to hydrofracing,which indicates that the well does not have hydraulicallyproductive fractures intersecting the borehole on the basisof barometric response.The measured water-level record shown in Figure 3bcan be compared to Figure 4 which shows the reduced Figure 4. Plot showing reduced water levels for W-03 (solidline) compared to the theoretical tidal potential (dashed line). water-level records for W-03, which is known to beconnected to the fault-zone aquifer. Here, we comparethe observable periodic response to the theoretical earthtide potential, which is obtained for the study site usinga program developed by Harrison (1971). Clearly thehydraulically active fractures connected to W-03 arereflected in a very strong correlation with the tidalpotential as the fracture network of this fault dilates andcontracts in response to the periodic earth tide potential.Thus, the water-level response in W-03 possesses a strongearth tide response. The differences in Figures 3b and 4are apparent and one can conclude that the two wellsare not hydraulically connected. The nature of the EX-1response is not periodic and therefore not responding toearth tides and therefore cannot be analyzed using earthtides. The increased wellbore diameter of EX-1 cannotaccount for the large amplitude reduction between theW-03 and EX-1 tidal signals. Furthermore, it will beshown later that the wellbore diameter does not affect themeasured water-level amplitudes produced by the tide-generating strains.To corroborate the results of the tidal signals andconfirm that W-03 and EX-1 were not hydraulicallyconnected prior to hydrofracing EX-1, an aquifer test wasconducted in which W-03 was pumped at a nearly constantrate of 7 L/min for 4 h. Water levels were monitored inboth W-03 and EX-1 once per minute using pressuretransducers during the test. The pumping well, W-03,experienced 9.51 m of drawdown during the test whereasEX-1 did not experience any drawdown. Hydrofracing EX-1 Hydrofracing is a term used to describe the processof hydraulically inducing or extending the radial extentof fractures in boreholes to tens of meters (or more) 4 T.J. Burbey, M. Zhang GROUND WATER
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