In Situ Treatment Technology - Chapter 10 pptx

Regardless of the carrier fluid, low permeability, fine grained soils and rockrepresent a significant challenge to in situ contaminant remediation alternatives.. By fracturing, not only do

Kidd, Donald F "Fracturing"

In Situ Treatment Technology

Boca Raton: CRC Press LLC,2001

CHAPTER 10 FracturingDonald F Kidd

CONTENTS

IntroductionApplicabilityGeologic ConditionsTechnology DescriptionHydraulic FracturingPneumatic FracturingScreening Tools

Geologic CharacterizationGeotechnical EvaluationsPilot Testing

Area SelectionBaseline Permeability/Mass Recovery EstimationFracture Point Installation

Hydraulic FracturingPneumatic FracturingTest Method and Monitoring

Fracture OrientationCarrier Fluid InfluenceProppants

Full-Scale DesignCase HistoriesPneumatic Fracturing Air PhaseEffectiveness—Hydraulic Fracturing Air PhaseReferences

Regardless of the carrier fluid, low permeability, fine grained soils and rockrepresent a significant challenge to in situ contaminant remediation alternatives Wehave already discussed the problems of moving air and water carriers through thesetypes of geologic conditions Without the movement of these carrier fluids, in situ

remediation methods are severely limited in their effectiveness Despite the lowpermeability of clays, silts, and competent rock, these geologic formations can stillbecome impacted Over time, organic contaminants can permeate throughout a widearea of the subsurface in vapor, nonaqueous (NAPL), and aqueous phases, migratingthrough natural fractures and by diffusion into the fine grained soils Once in thesezones, rapid or sufficient removal of the contaminants is difficult to achieve, if notimpossible

Within low permeability settings, excavation and above-ground posal or encapsulation are commonly selected remedies As with all above-groundremediation, the excavation process may actually enhance the potential exposure ofthe population to the subsurface contaminants during the process This is always anobjectionable consequence of the cleanup process The excavation process is alsodisruptive to ongoing facility operations, and impacted soil transported to a landfillposes some long-term liability

treatment/dis-Another technology, fracturing for permeability enhancement, is rapidly beingdeveloped to address these low permeability zones The limitations on achievablecontaminant reduction in situ are mainly due to inadequate carrier fluid exchangefrequency and/or nonuniform distribution of the carrier fluid The fracturing processseeks to increase soil permeability within discrete zones through the production ofhigh permeability fractures Both hydraulic and pneumatic fracturing are designedwith this purpose in mind

APPLICABILITY

Almost any rock or soil formation can be fractured, given enough time, energy,and determination The key aspects that have to be considered for remediationpurposes are: Will the benefit derived from fracturing offset the cost of the process,and what are the risks and benefits of the process? Armed with the answers to thesetwo questions, the decision to proceed with testing and, ultimately, full-scale appli-cation of the technique can be made on an informed basis

Fracturing is most appropriately applied to soils where the natural permeability

is insufficient to allow adequate carrier movement to achieve project objectives inthe desired time frame The following soil types and rock are generally treatablewith the fracturing technologies (Schuring and Chan 1993):

• Silty clay/clayey silt

• Sandy silt/silty sand

• Clayey sand

• Sandstone

• Siltstone

• Limestone

• ShaleFracturing a sand or gravel formation, while possible, is probably not justifiedbecause the increase in soil permeability would likely be incremental

Fracturing technologies are equally applicable to both vadose zone (unsaturated)soils and saturated soils within an aquifer The idea is to improve the flow of carrierfluids for contaminant removal, or delivery of nutrients or reactive agents

By itself, fracturing is not a remediation process There is no inherent advantage

to having contaminants in contact with a high permeability formation and, in fact,there can be disadvantages to this situation Fracturing has to be combined withsome other technology to be of benefit for reducing contaminant concentration,mobility, or both Essentially, fracturing serves solely to engineer changes in thesubsurface so that carriers can more effectively reach the contaminants of concern.Contaminant removal and encapsulation processes by degradation, volatilization,dissolution (leaching), and stabilization are still controlled by the characteristics ofthe contaminants and the impacted media

The important process of diffusion has been previously discussed in terms ofhow it impacts the spreading of contamination in the subsurface and the implications

on the cleanup process Controlling or limiting the role of diffusion on remediation

is perhaps the most promising aspect of induced fracture formation The importance

of this understanding cannot be overemphasized to those responsible for developingremediation programs Chapter 1 describes diffusion as a process by which dissolvedchemicals move independently of the primary, advective flow path of impacted water

or soil vapors The chemicals can move into materials of low flow or even noflow(stagnant) conditions Diffusion based flow occurs due to molecular movement and

is enhanced by differences in dissolved concentrations between areas of high flowinto the relatively clean, low flow materials When trying to reverse the contaminationprocess (cleaning the impacted aquifer or vadose zone), these areas of diffusioncreated pockets of contaminants can significantly extend the life of a project

By fracturing, not only do we create higher permeability zones for enhancement

of advective flow through the impacted material, we also shorten the pathway fordiffusion controlled flow of the carrier fluid The creation of advective flow channelsand shortened pathways for the lower velocity diffusive flow result in enhancement

of the carrier delivery (or recovery) process The final cleanup level attainable byfracturing and the associated remediation process will still be governed by charac-teristics of the soil/rock and contaminant Diffusion limited extraction will stillinfluence the rate of contaminant recovery even after fracturing, and contami-nant/media attractive forces will still influence the final concentration This is impor-tant to understand

As discussed in Chapter 3 (Vapor Extraction and Bioventing), the flow volume

of the carrier fluid has a significant impact upon the rate of contaminant removal.The flow volume of the carrier is in turn a function of soil permeability and the in situ pressure gradient (pressure values between points of reference) The vaporvelocity in the subsurface is then limited by the achievable volumetric flow rate at

the extraction well The following equation can be used to approximate this metric flow rate given knowledge or estimation of soil permeability and radialinfluence

volu-Q/H = π * (k/µ) * Pw* [1-(Patm/Pw)2]/ln(Rw/RI)

(Johnson, Kemblowski, and Colthart 1990)where, Q = flow rate (ft3/min); H = screen length (ft); k = soil permeability (ft2 ordarcy); Pw = well pressure (atm); Patm = atmospheric pressure (atm); Rw = well radius(ft); and RI = radial influence (ft)

Figure 1 illustrates the relationship between pressure gradient (applied vacuumlevels), sediment permeability, and vapor withdrawal rates (Johnson, Kemblowski,and Colthart 1990)

The vapor flow velocity diminishes rapidly with distance from the point ofextraction, again as discussed in Chapter 3 This occurrence results from expansion

of the area through which the vapors pass Mathematically, the vapor velocitythrough the soil or rock is calculated as flow (Q) divided by area (A) As an example

of this relationship, Figure 2 illustrates a typical extraction well constructed with a

10 foot screened section within a low permeability clayey sand (k = 0.1 darcy) Asillustrated on this figure, the area of flow is defined as:

A = 2 * π * r * Lwhere, A = area (ft2); r = radial distance (ft); and L = screen length (ft)

Figure 1 Vapor flow vs sediment permeability.

Note this estimation of flow area assumes that flow into the extraction well isprimarily horizontal such that kv << kh (Kv is vertical permeability and Kh is hori-zontal permeability.) This approximation is especially valid for materials with asignificant fraction of fine grained particles.

For our hypothetical situation, an applied vacuum of 12 inches of mercury (in.Hg.) is predicted to result in a flow of 1.1 scfm based on the above equation.Figure 3 illustrates the predicted vapor velocity versus radial distance from theextraction well, again assuming that the vertical flow component is negligible Ahorizontal line is also placed at approximately 0.01 ft/min which represents theminimum critical velocity described in Chapter 3 In summary, this critical velocityrepresents the minimum recommended velocity to optimize contaminant recoveryrates As shown on Figure 3, for a clayey sand, the critical velocity occurs atdistances less than two feet from the extraction well To maintain vapor velocity

in excess of the critical value, well spacings would be tight The benefit offracturing in the situation described above is that more vapor can be withdrawnfrom the subsurface and the desired velocity profile can be extended further outinto the contaminated formation

Fracturing can also expand the applicability of other in situ remedial ogies beyond vapor and liquid extraction As an example, hydraulic or pneumaticfracturing of a low permeability vadose zone overlying a more transmissive geologicunit can allow the use of air sparging (detailed in Chapter 5) in a geologic settingwhich is normally unsuitable Generally, the injection of air beneath a low perme-ability formation can initially result in organic-laden vapor accumulation beneaththis zone, and eventually the lateral migration of these vapors With the uncontrolledmigration of contaminant vapors beyond the influence of a collection system, errantemissions can result leading to unforeseen exposure routes and/or the contaminants

technol-Figure 2 Illustration of radial flow and area of flow.

may re-enter the groundwater through dissolution The latter case would result inthe expansion of the dissolved plume initially targeted by the remedial action Byinstalling a fracture network above the zone of aeration (sparging), the vaporcollection system can recover the stripped contaminates thereby avoiding these twoundesirable occurrences.

Figure 4 provides a conceptual illustration of this application of fracturing Forthe case illustrated, the formation overlying the impacted water producing sand zone

is fractured and connected to a vapor collection system Through the fractures, thevapors containing elevated contaminant levels are provided a capture zone

Geologic Conditions

As with all remedial techniques, fracturing is beneficial for environmental diation only for a range of site conditions In addition to the consideration of soil/rocktypes described in the previous section, the mode of deposition and changes occur-ring after deposition affect the effectiveness of fracturing Most notably, the state of

reme-in situ stresses has long been characterized as the primary variable in the orientation

of fracture formation (Hubbert 1957)

When fractures are formed by the injection of fluids, they are oriented dicular to the axis of least principal stress with propagation following the path ofleast resistance For environmental remediation, horizontal fractures are of thegreatest benefit Vertically oriented fractures offer limited additional benefit toremediation as the fractures will tend to reach the ground surface at a relativelyshort distance from the injection point Normally consolidated formations and/orfill materials have been found to produce vertically oriented fractures For vaporextraction technologies, described in detail in Chapter 3, the short circuiting of

perpen-Figure 3 Flow velocity vs distance.

vapor flow resulting from vertical fractures would actually be detrimental to thecleanup effort Essentially, the soil vapor will follow the path of least resistancethrough the fractured media with little influence occurring beyond these engineered,preferential pathways.

In situ stress fields are subdivided into horizontal (x and y direction) and vertical(z direction) components When initially deposited, sedimentary formations repre-sent essentially hydrostatic conditions whereby the three principal stresses are inequilibrium and are equal to the weight of overburden External forces (tectonics,burial/excavation, glaciation, and cycles of desiccation/wetting) after deposition thenmodify these stress fields

Over-consolidation is defined as compaction of sedimentary materials exceedingthat which was achieved by the existing overburden Again, changes to the in situ

stress fields after deposition have imparted a residual stress component to the mations Over-consolidation of soils specifically results in stress fields favorable forfracturing In this instance, the least principal stress in the vertical direction Theinduced fractures would again be created perpendicular to this stress and be hori-zontally oriented Figure 5 illustrates the concept of stress fields showing both equaland unequal stresses and the resulting orientation of fractures

for-The formation and later retreat of glaciers is one condition which results in consolidation The weight of the ice on the soil initially compacts the sedimentarygrains When the ice melts, the vertical stress is relaxed but the horizontal stress stillmaintains a residual component of the loaded conditions This is not the onlycondition which results in over-consolidation, however Erosion or overburdenremoval by excavation also presents conditions which relax the vertical stress field.Additionally, the cyclic swelling and desiccation of clay-rich formations can alsocreate conditions of over-consolidation

over-Figure 4 Sparging under low permeable soil.

TECHNOLOGY DESCRIPTION

With the current state of the technology described in this chapter, there are twotypes of fracturing methodologies employed for environmental applications Hydrau-lic (water based) and pneumatic (air based) fracturing variants of permeabilityenhancement are described in the following sections The selection between thesetwo types of fracturing are based on these considerations:

• Soil structure and stress fields

• Contractor availability

• Target depth

• Desired areal influence

• Acceptability of fluid injection by regulatory agencies.

Hydraulic Fracturing

Hydraulic fracturing was first developed as a means of enhancing oil and gasproduction The first successful fractures completed for this purpose are credited tothe Hugoton gas field in Grant County, Kansas in 1947 (Gidley et al 1989) Earlyfracturing fluids were a gasoline-based, napalm gel and contributed significantly tothe hazards of fracture installation Since its beginning, more than 1 million fracturetreatments have been completed Currently, 35 to 40 percent of all production wellsare fractured to enhance production rates The process is reportedly responsible formaking 25 to 30 percent of United States oil reserves economically viable In otherwords, many oil and gas reserves would not be produced with only naturally occur-

Figure 5 Pneumatic/hydraulic fracturing.

ring pressure distributions and gravity drainage controlling recovery rates Theparallels between economic recovery of petroleum hydrocarbons and viability of in situ treatment alternatives are evident.

As the names imply, the primary difference between hydraulic and pneumaticfracturing for permeability enhancement is the penetrating fluids used by eachtechnology to create the subsurface fractures Hydraulic fracturing fluids are char-acteristically viscous, produce minimal fluid losses to the formation, and have goodpost-treatment breakdown characteristics High viscosity is desirable for the fluids

to create a wide fracture and transport the proppants into the formation Low fluidloss is important to minimize the volume of injected fluid while achieving the desiredpenetration Finally, post-treatment breakdown is necessary such that the injectedfluids do not clog the formation Cross-linked guar is an example of a commonfracture fluid used for both petroleum reservoir stimulation and for environmentalapplications This fluid is a common thickener used in the food production industrywhich essentially breaks down to water with very little residual materials depositedinto the formation A food-grade carrier fluid minimizes the potential for regulatoryobjections to the process

Because of the characteristic high viscosity, fracture fluids are capable of porting particles (termed propping agents or proppants) through the fractures outinto the formation These proppants then support the fractures upon relaxation ofthe injection pressure and, to some degree, prevent closure of the fractures Silicasand is most commonly used in both environmental and petroleum applications due

trans-to its relatively low expense, range of particle size, and general availability The useand applicability of proppants other than sand are described in the section onproppants later in this chapter

Hydraulic fracturing is a sequenced process in which multiple fractures can begenerated within the impacted soil or rock formation The separation between frac-tures is dependent upon an economical evaluation and the physical characteristics

of the soil The desired result of the fracturing process is a formation which allowsfor the effective delivery of carrier fluids and results in either a more rapid reduction

of contaminant concentration, minimization of project costs, or ideally, both of theseoccurrences The mechanics of the fracturing process are described in a later section

Pneumatic Fracturing

Fracturing of soil or rock formations can also be accomplished using a pressed air or other gas source As with the hydraulic variant, pneumatic fracturingproceeds by isolating discrete zones of the formation and applying energy (in thiscase compressed gas) Inflatable packers with delivery nozzles within the isolatedintervals of the formation are typically used (Figure 6)

com-To create the fractures pneumatically, compressed air is supplied at a pressureand flow that exceed both the in situ stresses and the permeability of the material.This energy then fractures the material and creates conductive channels radiatingfrom the point of injection (Schuring, Jurka, and Chan 1991/1992) Injection pres-sures on the order of 150 psig and flow rates as high as 800 scfm or higher are used

to create the fractures

The pneumatic fracturing procedure typically does not include the intentionaldeposition of foreign proppants to maintain fracture stability The created fracturesare thought to be self-propping Essentially, disruption of the soil or rock structureduring the injection of pressure results in localized realignment of grains within thefracture preventing closure after relaxation of the pressure Testing to date hasconfirmed fracture viability in excess of two years, although the longevity is expected

to be highly site-specific (Schuring and Chan 1993)

Without the carrier fluids used in hydraulic fracturing, there are no concernswith fluid breakdown characteristics for pneumatic fracturing There is also thepotential for higher permeabilities within the fractures created pneumatically (com-pared to hydraulic fractures) as these are essentially air space and devoid ofproppants

SCREENING TOOLS

Fracturing success is dependent on the application of both sound engineeringand sound judgment The data base of cleanup sites for which fracturing has beenapplied for testing and more so for full-scale remediation is limited With continuedtesting and reporting of both successes and failures, our understanding of the tech-nology will develop to the point where geologic conditions favoring the technologywill become better understood

Screening a site for possible application of fracturing first requires that the projectteam not only understand the mechanics and applicability of fracturing to enhancepermeability, but also the implications for the ultimate cleanup technology By thepoint in the project when remedial alternatives are being considered, the extent of

Figure 6 Pneumatic fracturing schematic.

GAS RELEASE INTERVAL