78

Distinguished Author Series

APRIL 2003

Off-Balance Growth: A NewConcept in Hydraulic FracturingA. Ali Daneshy

AbstractThis paper offers a new fracturing concept that explainshow and why actual hydraulic fractures deviate from thecommonly accepted single-fracture growth models. Itdemonstrates that the fracture follows the local path ofleast resistance, not the global path, and this leads to sub-stantial branching, presence of extensive shear fractures,and a growth pattern that is dominated by conditions atthe tip of the propagating fracture, thus making its growthhaphazard and off balance. Within each fracture, branch-ing and shear fracturing create a complex path for the fluidflow and proppant transport, as well as production ofreservoir fluid. As a result, most hydraulic fractures aresubstantially shorter and narrower than the intendeddesign and yield a suboptimal production increase becauseof inadequate fracture length and conductivity.

Often, common well-completion designs also lead tocreation of multiple fractures at the wellbore, a situationfurther aggravated by prevailing fracture designs. Off bal-ance growth can obstruct proppant flow, which may leadto screenout.

IntroductionThe oil and gas industry has long recognized the inadequa-cy of existing theories to predict the behavior and outcomeof hydraulic fracturing treatments. Treatments require high-er pressures than predicted by theory. After-fracture pres-sure buildup tests often behave more like those of radialflow from wells with negative skin than of fractured wells.1Actual production increases from fractured wells are lowerthan predicted from fracture design computations. Severalfield observations show very complex fracture paths, pres-ence of multiple fractures, and random-appearing proppantdistribution. Warpinski et al.2-5 provide a comprehensivereview of some of the major experiments conducted by San-dia Natl. Laboratories at the Nevada Test Site and by the GasTechnology Inst. and U.S. Dept. of Energy at M-Site. Thesereports detail important features of these fractures, togetherwith an explanation of their cause. These results are usedextensively as background and support for the new off-bal-ance fracture growth concept.

The concept of fracture tortuosity was introduced toaccount for a nonplanar near-wellbore fracture path.Although this term is not scientifically defined, its use isvery widespread in fracturing literature and generallymeans any wellbore effect that complicates the near-well-

bore creation and extension of the fracture. The focus ofmost existing papers is more on recognition of the problemthan on the underlying cause. The general approach hasbeen to relate tortuosity to near-wellbore events and con-ditions. Most literature refers to two specific causes for tor-tuosity: near-wellbore change in fracture direction and ini-tiation of multiple fractures by use of perforations. Mahreret al.6 give a comprehensive account of events and publi-cations leading to the current industry understanding andhistory of tortuosity. Aud et al.7 attribute most screenoutsto near-wellbore events most likely caused by tortuosity.Cleary et al.8 provide a description of some of the eventsthat can cause tortuosity. Weijers et al.9 consider naturalfracturing as the main cause of multiple fracturing.Shlyapobersky and Chudnovsky10 attributes the complexfracture behavior to the presence of a near-tip “processzone” that increases the dissipation of energy duringhydraulic fracturing.

In this paper, the concept of off-balance fracture growthis introduced and used to show how it dominates thegrowth pattern of a large majority of hydraulic fractures,both near the wellbore and away from it. Two distinct frac-ture characteristics are defined and described. These aremultiple fracturing (a near-wellbore effect caused mainlyby well-completion details) and branching and shear frac-turing (which exist within the main body of the fractureand control its global growth pattern). Off-balance growthoccurs everywhere along the fracture and is not restrictedto the near-wellbore region. Its consequences are very nar-row fracture widths, short fracture lengths (created andpropped), severe branching, high pressure drops along thefracture, and preferential fluid and proppant movementdominated by shear fractures.

Fundamental Fracturing ConceptsFractures in rocks are created by any one or a combinationof three mechanisms: opening, sliding-mode, and tearing-mode fractures.11

Tensile Fractures. In this mode, the stress creating thefracture is tensile and separates the two faces of the frac-ture (Fig. 1a). Because rocks are particularly weak in ten-sion, this type of fracture usually requires the leastamount of force and energy for its creation. Tensile frac-turing is the dominant mechanism in hydraulic fracturing.The main resistance to fracturing comes from the least in-situ principal stress, σmin. Fracture opening (width) is theresult of compression of the formation adjacent to it.Thus, higher incremental pressures (fluid pressure insidethe fracture minus the least in-situ principal stress) createwider fractures. Fracture width is also directly propor-tional to its length and height and related to formation

Copyright 2003 Society of Petroleum Engineers

This is paper SPE 80992. Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology by describingrecent developments for readers who are not specialists in the topics discussed. Written byindividuals recognized as experts in the area, these articles provide key references to moredefinitive work and present specific details only to illustrate the technology. Purpose: toinform the general readership of recent advances in various areas of petroleum engineering.

79APRIL 2003

mechanical properties. Most fracturing models are basedon tensile fracturing.

The force requirement for creation of the fracture is:

Apf.pf=Af

.σmin, . . . . . . . . . . . . . . . . . . . . . . . . . . . .(1)

in which Af =fracture area perpendicular to the least in-situ principal stress, Apf =pressurized area of the fracture,pf =constant pressure equivalent of pressure in the frac-ture, and σmin=least in-situ principal stress.

Fracture opening (width), ω, is related to formationmechanical properties E and ν, the difference between fluidpressure pf in the fracture and σmin, and fracture lateraldimension l (length, height, or radius) through the equation:

(1−ν2).(pf −σmin)lω=κ . . . . . . . . . . . . . . . . . . . . .(2)

E

in which κ is a constant and depends on fracture shape(e.g., circular, rectangular, or elliptical).

Without proppant, tensile fractures close when pf<σmin.

Sliding-Mode Fractures. Fracturing occurs when theinduced shear stress along a given plane exceeds the shearstrength of the rock in that plane (Fig. 1b). Failure (fracture)occurs when the two faces of the fracture slide over eachother in opposite directions but without separating the facesby forming a gap between them. Thus, it has no volume. Ingeneral, because the shear strength of rock is greater than itstensile strength, shear fracturing requires higher pressure.The more common occurrences of sliding shear fracturesare along existing natural fractures and planes of weaknessin the formation (i.e., shear strength is very low), during thefracture reorientation process, or as a connecting mecha-nism to join two adjacent fracture branches. The latter twosituations will be discussed in more detail later.

Tearing-Mode Fractures. In this type of fracture, the fail-ure mode is a tearing action, in which the two faces of thefracture twist away from each other (Fig. 1c). The basic

tearing action does not create a gap between the two facesof the fracture and does not create additional volume.Again, this type of failure is more likely to occur along nat-ural fractures or existing planes of weakness during thefracture reorientation process, or as a mechanism to jointwo fracture branches.

Both types of shear fractures do not close as easily as ten-sile fractures. Closure involves reverse sliding of the twofracture faces. Although the shear strength of the forma-tion along the shear fracture plane is zero (already frac-tured), resistance to closure comes from frictional forcesalong the fracture faces.

Mixed-Mode Fractures. These fractures are created bycombined tensile and shear forces. Figs. 1d and 1e show thetwo opposite faces of the fracture separated from each otherwith a gap, as well as laterally shifted from each other.

Fracture Branching. Branching occurs when the fracturepropagates along two or more different but connectedpaths (same as the term “strand” used by Warpinski2). Abranch of a fracture may, and usually does, have its ownbranches. As discussed later, branching is caused mainlyby formation characteristics and treatment parameters.

Multiple Fracturing. In this paper, this term refers to sep-arate fractures created at the wellbore. Each of the multi-ple fractures has its own off-balance growth pattern.

Some publications have used branching and multiplefracturing interchangeably. However, significant differencesexist between the two. For example, the path of multiplefractures can be divergent, such that they gradually growout of each other’s influence zone while branches of a frac-ture are always within its influence zone. Fluid distributioninto multiple fractures is highly influenced by near-well-bore characteristics, including the number of perforationsconnected to each fracture, whereas branches compete forfluid within the same fracture and are somewhat indepen-dent of the near-wellbore behavior. Multiple fracturing isthe result of well-completion design (e.g., borehole inclina-tion; number, size, and distribution of perforations; etc.)and details of the treatment (e.g., injection rate and fluidviscosity), whereas branching is controlled mostly by for-mation properties. From a practical point of view, it is easi-er to influence multiple fractures than branching.

Off-Balance Fracture GrowthAlmost all theoretical developments of hydraulic fractur-

ing are based on an idealized assumption of a single frac-ture in a plane perpendicular to the least in-situ principalstress with a simple periphery (mostly rectangular, circu-lar, or occasionally elliptical) that extends progressivelyaway from the wellbore. As more fluid is pumped into thefracture, its dimensions grow, and the fracture becomeslarger in its original plane. The fracture is created by sim-ple tensile stresses, and it exists in a single plane andgrows in an orderly and predictable manner. Such agrowth pattern is defined as “balanced.”

Consider a hydraulic fracture with the dominant extent,l, and fluid pressure, pf, where pf and l are at equilibrium(Fig. 2). As more fluid is injected into the fracture, its pres-sure will rise until it causes instability and extension of thefracture. The extension path of the fracture is along itsFig. 1—Different fracturing modes.

80 APRIL 2003

local path of least resistance and depends on the state ofstress, as well as the strength of the formation at its tip. Ina homogeneous and isotropic material, the most likelypath is toward Point A, Fig. 2a. But material weaknessessuch as natural fractures can change this path: for exam-ple, Paths B or C in Fig. 2a. Each of these paths will havea new and different pressure for onset of its next instabili-ty, as shown by Points A, B, and C in Fig. 2b. Point A inthis example is perpendicular to the least in-situ principalstress and in purely opening mode. It will require the leastincremental pressure for its next extension and is, there-fore, the least stable of these paths. Point C requires thehighest incremental pressure for its next extension and is,therefore, the most stable. Whether the fracture willundergo another extension at Point C will depend onwhether fluid pressure reaches the required value for onsetof its next instability. The opening of the fracture is also afunction of its path, widest along Path A and narrowest

along Path C, as shown in Fig. 3. Failure along any pathother than Path A will involve shear. Furthermore, thenext growth step of the fracture from any point other thanPoint A will also involve shear failure, and in a differentpath than the original path. Thus, one shear failure leadsto a progression of other shear failures, which result in arugged and discontinuous fracture plane.

The tip of a hydraulic fracture spreads over a large area(from tens to several thousand feet). Because of materialanisotropy and heterogeneity, the resistance to fracturingvaries along the tip. A fracture grows wherever and when-ever the induced stresses exceed the correspondingstrength of the formation. Fracture growth occurs at small,discrete local steps and randomly moves along its tip. Ateach local extension increment, the fracture follows itslocal path of least resistance, which can lead to creation ofmany interdependent branches along its tip, as showngraphically in Fig. 4. These branches can be very close toeach other, and some of them may link through local smallshear fractures. An example of this situation is shown inFig. 5.3 In this experiment, the front face of the fracturehas been removed to show details on the other face. Thedyed fracturing fluid helps to identify extension sequence

Fig. 2—Fracture growth along local path of least resis-tance.

d = cos( )ω θ

d

ω

ωω

θPfσmin

σmin

Fig. 3—Effect of fracture orientation on width.

Fig. 4—Fracture branching.Fig. 5—Branching caused by discrete fracture growth.(Courtesy of N.R. Warpinski)

81APRIL 2003

through differences in color intensity. Each of the branch-es on the left tip of the fracture corresponds to a differentspurt in fracture growth and clearly demonstrates its dis-crete nature. While these have the same general directionand are linked to the main fracture, a very short distancealso separates them from each other. Closer examinationof the fracture shows it to have at least three very close,larger branches. The darker colors correspond to olderfractures and show that, even though adjacent to eachother, these branches were created at different times, andthat the main branch connected to the fracture tip is theyoungest. Because of this closeness, all branches are with-in each other’s influence zone, increasing the effectivevalue of σmin. Furthermore, the limited continuity of eachbranch will result in a small value of l (lateral branchextent). Thus, referring to Eq. 2, it is clear that the widthof the fracture at each branch will be quite small, substan-tially smaller than computed from any of the existing mod-els. Measurements of fracture width reported by Warpins-ki et al.3 confirm this conclusion. The fracture in Fig. 5contains many smaller shear fractures, but they are noteasily visible in the photograph.

In the above example, if σint is horizontal, then branch-ing will more likely be vertical or subvertical. If σint is ver-tical, then branches will tend toward horizontal.

From the above discussion, once a hydraulic fracturedeviates from its natural plane (normal to the σmin direc-tion), its growth becomes associated with branching andshear fracturing. The nonplanar geometry and presence ofshear fracturing and branching exert shear stresses at thefracture tips such that growth is not in simple openingmode and not strictly in the plane perpendicular to σmin.The outcome of this process is a fracture created in mixedmode (in varying degrees at different locations) that con-tains numerous branches. The extensive tip of the fracturehas many areas that lie within the “fractured zone” and notat its leading edge farthest away from the wellbore. Fluidand proppant travel along a very complex path very differ-ent from the piston-like motion assumed in many fractur-ing models. The fracture growth staggers from one part toanother in a random and unpredictable manner. Thus, theterm “off balance” is used to describe the fracture growth

pattern. Larger fluid volumes do not guarantee alonger fracture length. More proppant does notmean longer propped length. Once a fracturebegins to grow off balance, its extensionbecomes progressively more off balance and cre-ates a network of randomly distributed andpropped branches.

Multiple Fracturing. Usually, multiple frac-tures are the result of an excessive supply offluid and occur when the initial fracture is notable to accept the total injected rate. The excesssupply of fluid causes the wellbore pressure torise until it exceeds the formation strength atthe next weakest perforations, at which pointanother fracture initiates and begins to acceptfluid. The process continues until enough frac-tures are created to balance the volume injectedinto and out of the wellbore.

As noted by Cleary et al.,8 the breakdown pres-sure of each perforation is as related to its own

characteristics as it is to its alignment with the preferredfracture direction. Therefore, it is possible for some perfora-tions to stop taking fluid after other more preferably orient-ed perforations break down and provide an easier link totheir associated fracture.

Causes of Off-Balance GrowthThe following offers some of the more common causes ofoff-balance growth.

Borehole Inclination With Respect to Principal Stresses.Fig. 6 shows a laboratory sample with an inclined hydraulicfracture created from an open hole.12 It shows extensiveshear fracturing and branching. Inclined boreholes also arelikely to create multiple fractures, each with its own com-plex behavior and growth pattern. The number of perfora-tions linked with each fracture will control the distributionof the injected fluid between them. Abass et al.13 recom-mend substantially reducing the length of the perforatedinterval to avoid creating multiple fractures. Reducing oreliminating multiple fractures increases the effective lengthand productivity of the fracture, as well as increases theprobability of successful completion of the treatment.

Perforation Pattern. The capacity of the initial fracture toaccept fluid is limited by the number of perforations linkedwith it. If the injection rate is higher than this capacity, anincrease in wellbore pressure will cause breakdown ofother perforations, leading to initiation of multiple isolat-ed fractures from the same wellbore. Multiple fracturesmay manifest themselves by multiple breakdowns, whichcan be detected on the treatment pressure record. This sit-uation is quite common in the industry.

Fracture initiation from a perforated interval can bequite complex. For example, a fracture may initiate fromonly one side of the borehole, the two sides of the fractureon the opposite sides of the borehole may not be in thesame plane, the fracture plane may deviate vertically tolink with adjacent perforations, or combinations of these.Furthermore, in cases of multiple fractures, creation ofwidth in one fracture may disrupt the alignment betweenadjacent perforations and their respective fractures.

Fig. 6—Branching and shear fracturing in included holes.12

82 APRIL 2003

Natural Fractures and Planes Of Weakness. Most forma-tions contain natural fractures and planes of weakness. If apropagating hydraulic fracture intersects these, dependingon the relative magnitude of the formation tensile strengthalong the original fracture plane compared with the shearstrength along the plane of weakness, the fracture mayextend along the plane of weakness, thus temporarily devi-ating from its path. Although the fracture width (ω) mea-sured perpendicular to its original direction may stay thesame, its width for fluid and proppant flow is reduced toωcosθ, where θ is the angle between the old and new planesof fracture (Fig. 3). As θ increases, the real fracture openingdecreases and may, in fact, be insufficient to allow proppantentry into this portion of the fracture. Fig. 7 shows a view oftensile fractures joined together by shear fractures in themine-back experiments at Nevada Test Site.* It clearlyshows that shear fractures are narrower than tensile frac-tures, and that the width has decreased with increase in theangle θ. The sharp turns in the fracture path are effective fortrapping the proppant and preventing its forward move-ment, thus causing a pressure increase inside the fracture.

Even when the deviation in a fracture plane is very local,the reorientation process extends to a larger area than thenatural fracture and requires additional shear fracturing,some of it in intact rock with larger shear strength than thenatural fractures. If fracture pressure and dimensions do notcreate enough leverage to overcome the formation shearstrength, then part of the fracture will become dormantunless increases in fluid pressure and fracture growth else-where provides the necessary leverage to overcome forma-tion shear strength. These pockets of unbroken rock“pinch” the fracture width and obstruct movement of theslurry. Thus, even small natural fractures can create off-bal-ance growth that extends beyond their immediate area. Thiseffect also shows that the extending edge of the fracture isnot necessarily at its farthest points from the wellbore. Infact, two points geometrically very near each other in thefracture may have been created at very different times.

Consequences of Off-Balance GrowthFracture Dimensions. Conventional fracture dimensions:length, width, and height take different meanings in off-bal-ance growth. Each branch of the fracture will have a differ-ent set of dimensions, all of them different from those of theoverall fracture. Fracture length will refer to the farthestpoint reached by any of the branches, and height means the

extreme vertical dimension of the branches. Width willvary greatly along the fracture and may restrict the prop-pant entry into very narrow segments of the fracture.

Fluid Leakoff. Off-balance fractures have very high ratiosof fracture surface area to volume. Thus, fluid leakoff con-stitutes a large percentage of the injected fluid. The com-plex fracture geometry makes application of existingleakoff computations made on the basis of simple fractur-ing theory (Nolte14 and Lee15) questionable. Small branch-es with narrow width may, in fact, get plugged off with gelresidue left behind after fluid leakoff into the formation.

Preferential Fluid and Proppant Movement. The narrowopening of shear fractures makes fluid movement betweenadjacent branches more difficult than along each of them,resulting in a preferential direction for fluid movementand, consequently, fracture growth. This situation is exac-erbated with proppant. As the slurry tries to move throughthe shear fractures, proppant particles can get trapped inthe narrow width and cause further decrease in the areaopen to flow. The net effect is development of a dominantflow path along the length of the branches and restrictionperpendicular to it, thus creating a de facto barrier in thedirection normal to their major growth dimension. Therestriction in fluid flow also creates a pressure differencebetween branches that may cause further shear fracturingand associated effects on growth pattern.

Because all of the fluid and proppant has to enter the frac-ture through perforations, their shape and distributionaffect treatment execution. Narrow fracture widths (morelikely caused by shear than by branching near the wellbore)can trap the proppant and form a nucleus that can spreadand block an entire part of the fracture. Shear fractures par-allel with the borehole will have a bigger effect on proppantdistribution than those perpendicular to it. The shear frac-ture in Fig. 8 imposes a minor obstruction to fluid flow.However, when proppant enters the fracture, the flow areacan be substantially reduced if proppant particles gettrapped behind it. The slurry will have to find an alternatepath for flow, which will also be at a higher pressure withthe possibility of a similar blockage. Screenout results whenthese alternate paths are insufficient to transmit the inject-ed fluid under allowable fracturing pressures. Under simi-lar conditions, shear fractures perpendicular to the bore-hole will have much less effect on the fracture.

Fracturing Pressure. The complex nature of fracturegrowth and slurry path result in larger frictional pressuredrops along the fracture. This is not limited to the near-wellbore region and spreads across the entire fracture area.The pressure increases associated with shear fracturing andbranching are more prominent early in the fracturing treat-

Fig. 7—Narrow width of shear fracture.

Fig. 8—Proppant blockage by shear fractures parallelwith borehole.

* Personal communication with N.R. Warpinski

83APRIL 2003

ment. With short fractures, the main mechanism for over-coming shear resistance of the formation is fluid pressure.When fracture dimensions become larger, they can exertsufficient leverage to break the formation under shear withlower pressures. This explanation is much more plausiblefor the early pressure increase seen in some fracturing treat-ments than the commonly used reasoning of Perkins-Kerns-Nordgren type treatment with limited height.16 Infact, as discussed earlier, in off-balance growth, the termsfracture length and height have different meaning, fromthose in classical theory of balanced fracturing.

Production Increase. The complex connection betweendifferent parts of the fracture creates an undesirable pathfor reservoir fluid flow. Because proppant is distributedinside many small branches, the local flow capacity is alsomuch smaller than computed. The result is large produc-tion-pressure drops along the fracture. This pressure dropis why hydraulic fractures very seldom provide the expect-ed theoretical production increase and after-fracture welltesting that often indicates the behavior of a well with asmall negative skin rather than a fractured well.

Fracture Closure. Closure of shear fractures is more diffi-cult than tensile fractures and requires a larger force.Therefore, some shear fractures will not return to theiroriginal position, which results in keeping some of thebranches open even without proppant. Whether this willprovide the fracture with some intrinsic flow capacitydepends on the interconnection of these branches with thedominant path of reservoir fluid flow inside the fracture.

The other aspect of this phenomenon is that closure in itsclassical form does not take place in off-balance fracturing.This aspect casts some doubt on the validity of the tech-niques currently used by the industry to define closure stress.

Screenout. Screenouts are caused by obstruction to prop-pant movement to such an extent that the fracturing pres-sure exceeds the allowable maximum pressure of the treat-ment. The narrow width of shear fractures and branchescan block or hinder proppant movement. Away from thewellbore, the disruption increases the fluid pressure,which in turn increases the local fracture width. As pres-sure increases, some of the dormant tips of the fracture canreactivate and extend, creating new avenues for slurrymovement. Increase in fluid pressure means these obstruc-tions have moved closer to the wellbore where the fracturehas fewer tips for reactivation, and the increase in fluidpressure acts on a smaller area. Thus, the probability of anyparticular combination of shear fracturing or branchingcausing screenout is inversely related to its distance fromthe wellbore. This distance also affects the time and speedof screenout; the closer the obstruction is to the wellbore,the earlier and faster the screenout.

Beneficial Applications of Off-Balance Growth. One ap-plication in which off-balance growth may be superior toconventional fracturing is stimulating high-permeabilityreservoirs. In these situations, multiple fractures and off-bal-ance growth create a short, high-permeability zone aroundthe wellbore that will bypass the damage and provide suffi-cient conductivity to promote efficient radial fluid flow. Thissituation can help delay or reduce sand production.

Hindering Off-Balance GrowthThere are two key issues—how to decrease the number ofmultiple fractures and how to decrease the amount of off-balance growth in each fracture.

Two main contributors to creation of multiple fracturesare fluid pressure inside the wellbore (which is controlledby rate, viscosity, proppant concentration, etc.) and num-ber and type of perforations along with details of the wellcompletion, especially well inclination. Longer perforatedintervals increase the probability of multiple fracturing.The most desirable scheme for perforating is hydrojettinga short slot. Another option is to use larger-diameter con-centrated perforations, as recommended by Stadulis,1 Audet al.,7 Cleary et al.,8 Weijers et al.,9 and others. Even whenthe intent is to create several fractures in different sectionsof the producing interval, it is better to keep each perfo-rated interval as short as possible. Some authors recom-mend perforating normal to the σmin direction (Weijers etal.9). The dilemma is trying to determine the orientationbefore performing hydraulic fracturing.

The probability of multiple fracturing also increaseswith injection rate. If the initial number of fractured per-forations is insufficient to transmit all the fluid, pressurewill increase and cause breakdown of other perforationsuntil a balance is reached between pressure, rate, and thenumber of perforations linked to multiple fractures.

Some authors recommend the use of high breakdownrates with the intent of reducing near-wellbore “tortuosi-ty,” Weijers et al.9 This technique will work if all the per-forations are connected to one fracture. Perforation geom-etry has a bigger effect on control of fluid supply for eachof multiple fractures than the injection rate.

Some authors recommend use of high-viscosity fluids toreduce “tortuosity” (Aud et al.7 and Weijers et al.9). Use ofhigh-viscosity fluids increases the wellbore pressure andthe possibility of multiple fractures. Also, it increases thefrictional pressure caused by slurry flow in the fracture andprobability of internal branching. The drag force on theproppant caused by high viscosity is useful only if prop-pant is able to move forward inside the fracture.

Borehole inclination is another source of multiple frac-turing. Even in vertical wells, doglegs and deviations fromvertical within the pay zone can result in misalignment ofthe borehole with the principal stresses and cause multiplefractures. The severest situation occurs during fracturinghorizontal holes, especially if fracture orientation is per-pendicular to the borehole direction. In the latter case, sev-eral authors have pointed out that creation of multiplefractures is a near certainty.13,19

Cleary et al.8 recommend injection of a small volumeproppant slug during the minifracture treatment or thepad stage to clear the path for the main treatment.McDaniel et al.17 also cite case histories in support of thisapproach. Stadulis1 extends this approach to include sev-eral high-concentration proppant slugs (5 to 12 lbm/gal)separated by small volumes of clean fluid. It is difficult toexplain why the proppant slug will have a different effecton the fracture than the proppant that is pumped duringthe main treatment.

Because of the many unknowns at the start of a fractur-ing treatment, it is best to design the job for maximumflexibility, namely the ability to change rate, fluid viscosity,volume of each stage, and proppant concentration. A mini-

84 APRIL 2003

fracture treatment provides the first glimpse into the frac-ture. The first step in the recommended treatment consistsof determining the fracture reopening pressure. This deter-mination can be made by injecting at the lowest possiblerate and observing fluid-pressure variations. The rate canbe increased gradually to observe its effect on computed(or preferably measured) bottomhole treatment pressure.The maximum treatment rate can be established by thisapproach. If the job data indicates off-balance growth, thenit is wise to adjust the treatment accordingly. Large padvolumes do not provide a safeguard and may reduce thepropped length of the fracture (and may increase cost). Inthis case, one should be prepared for possible screenout.At this stage of the treatment, it is too late to modify com-pletion details for this treatment, and it is recommendedthat notes be made of the results for use in designing thecompletion of the next well in the same formation.

Higher fluid viscosity increases the drag force on theproppant and helps move it deeper into the fracture. Thisproperty is more essential for off-balance fractures becauseproppant has to move inside individual branches with verynarrow widths. The increase in drag force also increasesthe fluid pressure inside the fracture and causes morebranching, which will result in a shorter fracture length. Itis difficult to determine the net effect of viscosity on prop-pant transport and, in particular, on effective fracture con-ductivity. It seems reasonable to expect that moderate-vis-cosity fluids (10 to 50 cp) will yield a more conductive,albeit shorter, effective fracture.

Closure Stress. Reasonable determination of closure stressis very critical for on-site analysis of job data and establish-ing the occurrence and extent of off-balance growth. Exist-ing techniques of closure stress determination on the basisof after-fracture pressure falloff data do not help in this sit-uation. The simplest way to determine σmin is to estimate itfrom fracture reopening pressure. The recommended prac-tice is to break down the formation at the lowest rate allow-able with the equipment on location, pump fluid for a veryshort time (2 to 5 minutes), shut the well for a few minutes,and reinject at the same low rate. The pressure at which thefracture opens is a reasonable estimate of σmin and is suit-able for future diagnostic work.

Case HistoryExample 1. The treatment consisted oftwo back-to-back minifractures followedby the main treatment. The minifracturedata is presented in Fig. 9. During thefirst minifracture, the data show at leastthree distinct pressure peaks, shown asPoints A, B, and C, indicating multiplefracturing, with the first and the third tak-ing a larger percentage of the fluid thanthe second. The large pressure drop afterPoint C indicates initiation of a dominantfracture at this point. The rise and fall influid pressure around Points D and Ecould indicate removal of large-scaleobstructions to fracture growth, possiblyby shear fracturing, to create better link-age within the fractures. Although thereare several sharp pressure drops afterPoint E, the associated changes in rate do

not allow confident interpretation of their cause.The rapid pressure drop after Point F confirms the earli-

er diagnostics. The frictional resistance of shear fractures issupporting part of the closure force, thus relieving the fluidfrom providing full support.

The initial pressure decrease during the second mini-fracture (Point I) indicates easing of fluid movement dueto better linkage between more branches. At this stage,shear fracturing can take place at lower pressures becauseof larger fracture extent.

The surface pressure and rate, together with computedbottomhole pressure and proppant concentration data forthe main fracture treatment, are presented in Fig. 10. Therelatively constant pressure between Points A and B indi-cates steady fracture growth (which includes consistentbranching and shear fracturing) during this period. Thecomputed bottomhole pressure shows an instantaneousincrease as soon as the proppant enters the fracture. Thisis consistent with the earlier diagnostic. The surface andbottomhole pressure increase indicates steady growth ofthe blockage to proppant flow. At Point C, there is amarked increase in the rate of pressure increase. Althoughthis pressure change appears to be related to proppant con-centration, the two events may not be connected to eachother. The more likely cause of the increase in pressure isthat one of the multiple fractures may have screened out.Shortly after this point, the job screened out, and attemptsto re-establish injection failed to produce a positive result.The very high pad volume in this job did not help over-come the obstructions created by shear fracturing.

ConclusionsOff-balance growth creates fractures that are short, nar-row, and contain many branches and shear fractures. Theproppant distribution inside the fracture is suboptimaland not suited for generation of sustained high well pro-ductivities. Therefore, industrial hydraulic fractures donot achieve the production increases estimated from the-oretical computations.

NomenclatureAf = fracture area perpendicular to the least in-situ

principal stress

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

110 120 130 140 150 160 170 180 190 200

Time, min

Su

rfa

ce

Pre

ss

ure

, p

si

0

10

20

30

40

50

60

Slu

rry

Ra

te,

bb

l/m

in

Tubing Pressure, psi

Slurry Rate, bbl/min

A

B

C

D

E

F

I

Fig. 9—Minifracture chart for Example 1.

JPT

85APRIL 2003

Apf = pressurized area of the fracture E = Young’s modulusl = fracture lateral extent (height, length)

pf = constant pressure equivalent of pressure in thefracture

θ= angle between hydraulic and natural fractureplanes

κ= constantν= Poisson’s ratio

σmax= Maximum in-situ principal stressσint= Intermediate in-situ principal stress

σmin= least in-situ principal stressω= fracture width

AcknowledgmentsThe author would like to thank Norm Warpinski, RobertLangedijk, and PDO Oman, and Buddy McDaniel, Hal-liburton Energy Services.

References1. Stadulis, J.M.: “Development of a Completion Design to Con-

trol Screenouts Caused by Multiple Near-Wellbore Fractures,”paper SPE 29549 presented at the 1995 SPE Rocky MountainRegional/Low Permeability Reservoirs Symposium, Denver,20–22 March.

2. Warpinski, N.R. et al.: “Direct Observation of a Sand-ProppedHydraulic Fracture,” Sandia Report SAND-81-0225 (May 1981).

3. Warpinski, N.R.: “In-Situ Measurement of Hydraulic FractureBehavior, PTE-3 Final Report,” Sandia Report SAND-83-1826(July 1985).

4. Warpinski, N.R. et al.: “The Interface Test Series: An In-SituStudy of Factors Affecting the Containment of Hydraulic Frac-tures,” Sandia Report SAND81-2408 (February 1982).

5. Warpinski, N.R. et al.: “An Interpretation of M-Site HydraulicFracturing Diagnostic Results,” paper SPE 39950 presented atthe 1998 SPE Rocky Mountain Regional/Low PermeabilityReservoirs Symposium, Denver, 5–8 April.

6. Mahrer, K.D., Aud, W.W., and Hansen, J.T.: “Far-FieldHydraulic Fracture Geometry: A Changing Paradigm,” paperSPE 36441 presented at the 1996 SPE Annual Technical Con-ference and Exhibition, Denver, 6–9 October.

7. Aud, W.W. et al.: “The Effect of Viscosity on Near-WellboreTortuosity and Premature Screenouts,” paper SPE 28492 pre-sented at the 1994 SPE Annual Technical Conference andExhibition, New Orleans, 25–28 September.

18. Cleary, M.P. et al.: “Field Implementation of Prop-pant Slugs to Avoid Premature Screenout ofHydraulic Fractures With Adequate Proppant Con-centration,” paper SPE 25892 presented at the 1993SPE Rocky Mountain Regional/Low PermeabilityReservoirs Symposium, Denver, 12–14 April.

19. Weijers, L. et al.: “Simultaneous Propagation ofMultiple Hydraulic Fractures—Evidence, Impact,and Modeling Implications,” paper SPE 64772 pre-sented at the 2000 International Oil and Gas Con-ference and Exhibition in China, Beijing, 7–10November.

10. Shlyapobersky, J. and Chudnovsky, A.: “FractureMechanics in Hydraulic Fracturing” presented atthe 33rd U.S. Symposium on Rock Mechanics,Santa Fe, New Mexico, 3–5 June 1992.

11. Daneshy, A.A.: “Hydraulic Fracture Propagation inthe Presence of Planes of Weakness,” paper SPE4852 presented at the 1974 SPE European SpringMeeting, Amsterdam, 29–30 May.

12. Daneshy, A.A. “A Study of Inclined Hydraulic Fractures,”SPEJ, (August 1973) 61.

13. Abass, H.H., Hedayati, S., and Meadows, D.L.: “NonplanarFracture Propagation From a Horizontal Wellbore: Experi-mental Study,” SPEPF (August, 1996) 133.

14. Nolte, K.G.: “A General Analysis of Fracturing PressureDecline With Application to Three Models,” SPEFE (Decem-ber 1986) 571; Trans., AIME, 281.

15. Lee, W.S.: “New Minifrac Procedure for Simultaneous Deter-mination of Early- and Late-Time Fluid Loss During Fractur-ing,” paper SPE 21463 presented at the 1990 SPE EasternRegional Meeting, Columbus, Ohio, 31 October–2 November.

16. Nolte, K.G. and Smith, B.S.: “Interpretation of FracturingPressures,” paper SPE 8297 presented at the 1979 SPE Annu-al Technical Conference and Exhibition, Las Vegas, Nevada,23–26 September.

17. McDaniel, B.W., MeMechan, D.E., and Stegent, N.A.: “ProperUse of Proppant Slugs and Viscous Gel Slugs Can ImproveProppant Placement During Hydraulic Fracturing Applica-tions,” paper SPE 71661 presented at the 2001 SPE AnnualTechnical Conference and Exhibition, New Orleans, 30 Sep-tember–3 October.

18. Daneshy, A.A.: “Three-Dimensional Propagation of HydraulicFractures Extending From Open Holes” 15th U.S. NationalCommittee for Rock Mechanics, Application of RockMechanics Symposium Proc. 157–179.

19. Abou-Sayed, I.S. et al.: “Multiple Hydraulic Fracture Stimula-tion in a Deep Horizontal Tight Gas Well,” paper SPE 30532presented at the 1995 SPE Annual Technical Conference andExhibition, Dallas, 22–25 October.

Ali Daneshy, SPE, is President of Daneshy Consultants Intl.He holds a PhD degree in rock mechanics from the U. ofMissouri-Rolla. Daneshy has been involved in hydraulicfracturing for more than 30 years including laboratory andtheoretical research, numerical simulation, fracture design,and field execution of treatments. His publications includemany technical papers as well as chapters in several bookson hydraulic fracturing. Daneshy's other areas of technicalinterest include well completion, control of excess waterproduced with oil and gas, and intelligent wells. He is arecipient of SPE Distinguished Member and DistinguishedService awards.

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

0 10 20 30 40 50 60 70 80 90

Time, min

Bo

tto

mh

ole

Pre

ssu

re, p

si

0

10

20

30

40

50

60

Rate

, b

bl/m

in,

or,

Pro

p C

on

c., lb

m/g

al

BHP, psi

Surface Pressure

Slurry Rate, bbl/min

BH Prop Conc, lbm/gal slurry 3.5 lbm/gal

A B

C

Fig. 10—Main fracture chart for Example 1.