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E Hydraulic Fracture Height GrowthShawn C. MaxwellSchlumberger, Calgary, Alberta, Canada

Introduction

Hydraulic fracture stimulations or ‘fracs’ are vital for economicproduction in low permeability tight gas and shale gas reser-voirs, and the frac height is a key factor for engineers to opti-mize the hydraulic fracture treatment. As unconventionalreservoir development has spread through North America andspecifically to regions unaccustomed to petroleum production,public awareness of hydraulic fracturing has increased.Environmental concerns have also grown, including protectionof shallow freshwater aquifers. Fortunately within the engi-neering and geophysical literature there are a number ofpublished microseismic results in numerous settings andvarious depths. These examples shed light on the factors thatcontrol height growth, and show the actual hydraulic fractureheights that are created in a variety of reservoirs.

Hydraulic fracturing involves fluid injection at sufficientpressure to create a tensile fracture in the formation, whichwill tend to grow perpendicular to the minimum stress direc-tion. At the depths of most petroleum reservoirs, theminimum stress is approximately horizontal, which results inthe creation of a vertical fracture. The vertical fracture heightgrowth is of interest for various engineering reasons, particu-larly related to coverage of the reservoir depth range and out-of-zone fracture growth. Hydraulic fracture engineers havedeveloped modeling tools to enable prediction and simula-tion of frac geometry, including the fracture height. Thecreation of these engineering tools has also spawned a needfor fracture imaging technologies tomonitor and validate fracturegrowth. Several techniques exist,including tracers, distributedtemperature and vibration sensorsthat monitor near-wellbore fracturegeometry. However, in order tomonitor the hydraulic fracturegrowth away from the wellboremicroseismic monitoring has provento be the most valuable technology.

In the early days of hydraulic frac-ture simulation, the fractures weremodeled as simplistic penny shapedcracks in infinite elastic media. Thesemodels tend to estimate approxi-mately circular fractures, as tall asthey are long. More advancedapproximations were able to accountfor depth varying material proper-ties and ultimately the associatedvariable stresses. As monitoringtechnologies began to provideinsight into the actual growth,evidence quickly mounted that fracs

are not as simple as initially thought. 2D hydraulic fracturemodels have been available for quite some time, and haveevolved in order to better match observed fracture geometries.As an example, consider the application to a project whichincluded microseismic data (Figure 1). Typically these fracturemodels match injected pressures and net volumes (includefluid leakoff from the fracture) to hydraulically model theresulting fracture profile. The mechanical material propertiesand stresses are also used to geomechanically constrain thehydraulic fracture mechanics. In this example, the initialmodeling (top of Figure 1) incorporated stress and materialproperty variations around a thin reservoir, and predictedsome fracture containment within the reservoir. This ‘uncali-brated’ model predicted both upward and downward growthwhich limited the length of the fracture. However, the micro-seismic monitoring results (overlain on both models) showedthat the fractures were contained entirely in the reservoir andthat the actual fracture length was much longer than origi-nally anticipated. The microseismic data was used to ‘cali-brate’ the model (bottom of Figure 1), by adjusting the inputparameters such that the predicted and observed geometriesmatch. The fracture model also predicts the proppant concen-tration in the fracture (contours) that will define the relativepermeability enhancement associated with the fracture. In thisexample the calibrated model predicts a geometry that is morefavorable from a reservoir contact point of view, since thedepth containment results in a longer fracture mostly in thereservoir with little out-of-zone fracturing. In order to matchthe microseismic geometry an additional material property of

Figure 1. Cross sectional view of a frac model and microseismic events. Yellow is the reservoir depth interval,and top panel shows the original model and bottom the model after calibration to the microseismic results.On the left are the input model parameters and right is the relative fracture width profile (after SPE96080).

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composite layering was included asshown by the blue track on the left sideof Figure 1. This composite layering (orits equivalent) has found its way intomany fracture simulation workflowsand has proven to be an important factorto match the fracture containment that istypically observed.

Geology Controls on Fracture Growth

In order to understand this compositelayering effect, it is informative toexamine geologic examples of fracturegrowth. Figure 2 is a downloadedgeologic section with annotated frac-tures. Notice that all of the fractureseventually stop at a bed boundary.Some fractures persist longer thanothers and cross some bedding planesbut all eventually terminate. Figure 3highlights one potential cause of the bed termination. Thisexample illustrates that the containment is related to changes inthe fabric of the rock and can be caused by thin laminationsbetween thicker layers. Consider possible causes as shown inFigure 4 depicting two alternate views or end members of thepossible effect of fractures growing between layers with differentproperties. The layering could correspond to fractures growingfrom a soft shale unit into more brittle layers (a common litho-logic section in many shale gas reservoirs surrounded by lime-stone layers). Horizontal stresses will tend to be amplified in thestiffer layers associated with the increased load bearing capabil-ities. Increased stress will tend to limit fracture growth forsimilar fracture pressures, due to a lower net pressure acting onthe fracture face. In addition, fractures in the stiffer rocks willopen less for a given pressure increase, limiting their ability toaccommodate fluid. Together the stress and compressibilitydefine geomechanical conditions that tend to restrict hydraulicfracture height growth. The alternate scenario depicted in Figure4 involves the fracture completely terminating at the interface.(Note that variations between these two scenarios are alsopossible including fracture ‘offsets’ or deflection where the fracpartially grows along the interface layer). If the bedding plane ismechanically weak and allows the two layers to move inde-pendently, the fracture opening in one layer may not translate tofracture opening in the second layer. If the bedding plane iseither not welded or contains thin laminations (such as Figure 3)the associated VTI strength anisotropy may hinder the fracturecrossing the interface. Instead the fracture may either terminateor grow along the interface depending on the geomechanical andhydrodynamic conditions. Bedding laminations are analogous toautomobile safety glass structure, where laminated layers ofglass are used to limit fracturing entirely through windshields(as known by many of us Alberta drivers watching fracturesgrow from stone chips!). Composite layering in rocks tends tolimit the overall hydraulic fracture height growth, and fracturesimulations have found these to be an equally important factortogether with geomechanical variations. The combined impact ofthese factors limits fracture height growth, both for natural andhydraulically created fractures.

Figure 2. Interpreted fractures within an outcrop. Notice that many fractures terminate at bed boundaries and thatnone persist all the way from top to bottom of the section (from lyellcollection.org).

Figure 4. Schematic diagram of end member scenarios when fractures grow betweenlayers of different properties.

Figure 3. Natural fractures within an outcrop. Note the fracture termination fromthe thick to thinly laminated layers (from agu.org).

Microseismic Evidence of Height Growth

Limited fracture height growth can impact the ability of a singlehorizontal well to contact a thick reservoirs section, resulting inengineering challenges to drain the reservoir with few horizontalwells. Consider an example from the Montney. Depending on thelocation there could be a thick potential target zone spanningfrom the Lower to Upper Montney and in places up into theoverlying Doig. Figure 5 shows microseismic data recordedwhile fracing a horizontal well targeting the Upper Montney. Inthis case, the microseismicity shows that the frac was success-fully contained within the Upper Montney (a few events locatein the Doig but are attributed to location resolution) and thatrelatively long horizontal fracture wings were created providinggood reservoir contact area. In this case the Upper Montney isthe target and so the fracture containment is favourable, but fordiscussion purposes consider a hypothetical example where theintention is to create a hydraulic fracture that grows upwardsand downwards. In these conditions, it would be an engineeringchallenge to contact a large depth interval which could be over-come by modifying the fracture injection parameters (often injec-tion rate is the control) or changing the horizontal well landingpoint and drilling the well at a different depth. In the Horn Riverfor example, microseismic monitoring is being used by variousoperators to look at containment in the Muskwa and Evie shalesin order to understand the depth containment and interactionsbetween wells drilled into the different layers.

How can geophysics help? Clearly the engineers want to be ableto characterize the geology sufficiently to robustly predictcontainment. There is a lot of effort being made to extract stressesand rock properties from seismic reservoir characterization tohelp quantify hydraulic fracture response and define sweetspots. While sweet spot identification is an important aspect forwell placement, in terms of height containment thinly laminatedbeds such as shown in Figure 3 may occur below seismic resolu-tion. Ultimately robust geomechanical characterization will

likely require an integrated approach of geology and geophysics,including high resolution wireline measurements of sonic andformation imaging. We face technical challenges as we travel thispath: such as up-scaling between log and seismic and extrapo-lating from high resolution well based imaging. To help bridgethis gap, microseismic is the key technology to validate andconfirm the geomechanical earth model. Imaging height growthand validating the fracture containment characteristics is animportant engineering application of microseismic and one ofthe reasons that microseismic is an important component in theboth the appraisal and development phases of unconventionalresources. Microseismic provides insight into a number ofaspects of the hydraulic fractures, but in particular has openedthe engineer’s eyes to issues of height containment as discussedearlier. Extensive industry knowledge has been built from tens ofthousand of fracs that have been imaged using microseismicity.While the results of all this monitoring are obviously proprietaryto the individual operators, it has provided insight into fracturegrowth as well as competitive advantages around the bestfracing practices for a given reservoir.

As the search continues to predict fracture containment, a largenumber of papers have been written showing the microseismi-cally imaged hydraulic fracture growth in many unconventionalreservoirs. These publications provide released examples of themicroseismic hydraulic fracture height (number of publications istoo large to provide a complete listing of the papers here). Foreach published example the depth of the upper and lower mostmicroseismic event and associated average perforation depth wasdetermined as illustrated in Figure 5. In some cases, multiplestages of a given project were included in the publication but hereonly a representative example is included intentionally using theexample with the largest height growth. Figure 6 shows a plot ofthe depth to the shallowest microseism, perforation (frac initia-tion depth) and deepest microseism. Several well known reser-voirs have been highlighted: including examples of the Montney,Horn River and Cardium. Perforation depths vary between about

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Figure 5. Map (left) and cross section (right) of a multistage hydraulic fracture stimulation in the Montney shale. Cross section shows average perforation depth withshallowest and deepest microseismic events, along with associated symbols used in figures 6 and 7. (Courtesy Mark Norton, Progress Energy).

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1200 to 3500 m, although for each of the reservoirs there will be avariety of perforation depths beyond that flagged in this plot.Note that while there is a large variation in the height growthfrom project to project the top of the hydraulic fracture tends toget deeper as the well or frac initiation gets deeper. Figure 7shows the relative upward and downward growth to better illus-trate the variation. Notice that there tends to be more upwardgrowth than downward growth, which does not appear to be adetection bias. The most probable explanation is that stress tendsto decrease upward, favouring upward growth into lower stressrocks which is accentuated by frac fluid buoyancy effects. Alsonote that significant upward growth is limited to a few cases andmost examples tend to be less than about 100 m. Figure 7 alsoshows the average upward and downward height along with ahistogram, which is summarized in the table below.

Upward Growth Downward Growth

Mean +/- stand dev 80 m +/- 87 m 73 m +/- 48 m

Maximum 295 m 190 m

Minimum 12 m 10 m

Table 1. Statistics of the data depicted in Figures 6 and 7.

The variation in height growth between locations is expectedwith the variation in injection rates, geomechanical variationsand reservoir settings depicted in this broad sampling. Within agiven setting, the overall height growth tends to be fairly consis-tent with the examples included here. For example, in theHaynesville and Wolfcamp/Spraberry upward fracture growthtends to be commonplace, while in formations such as theCardium sands hydraulic fractures are often verticallycontained. The height growth summarized here also providescontext regarding concerns around the possibility of fracing intoaquifers. While the depth to the bottom of an aquifer depends on

the geographical location, most tend to be relatively shallow. Theaverage top of the fracs shown here is more than 2 km below thesurface much deeper than freshwater aquifers. The data shows asignificant thick buffer of rock above the top of the hydraulicfracture. Obviously in particular cases where there may be ashallow frac or regions with particularly deep aquifers, targetedmonitoring may be advisable to ensure aquifer integrity.

Conclusion

In conclusion, examples of hydraulic fractures in many differentreservoirs demonstrate contained height growth which is directlyanalogous with natural fractures. As fractures grow vertically andpass through different lithologies, the geomechanical conditionschange and limit height growth. Composite horizontal layeringalso imposes additional height containment. From an engineeringperspective contained hydraulic fracture height growth can beeither good or bad news. In thin reservoir targets, containmentfavours longer fractures and increased reservoir contact area,whereas in thick or layered reservoir targets, containment canintroduce challenges associated with contacting the entire reser-voir depth interval. Ultimately this may lead to having to usemultiple horizontal wells at different depths to drain the entiretarget interval. However, containment is good news from an envi-ronmental perspective, because it results in limited hydraulicfracture height growth and an associated buffer between deepfractures and shallow aquifers. R

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Shawn Maxwell (SMaxwell@slb.com) isSchlumberger’s Microseismic ChiefGeophysicist and Advisor. Prior positionsinclude establishing microseismic serviceswith Pinnacle and ESG and teaching atKeele University in England. Shawn wasawarded a Ph.D. in microseismicity fromQueen’s University, and is currently

serving as the passive seismic associate editor forGeophysics. He has published numerous papers on applyingmicroseismic for various engineering applications, taught anumber of industry focused courses and served on a numberof industry technical and workshop committees.

Figure 7. Hydraulic fracture height growth depicted in Figure 6. Averageupward and downward growth are included (dashed lines) along with ahistogram of the overall height.

Figure 6. Depth to shallowest microseismic (green circle), deepest microseismic(orange circle) and perforation depth (red circle) for published examples in variousreservoirs. (Various sources).