Continuous process for hydraulic fracturing with foam

Xf~;~()4C) ~ ' A CONTINUOUS PROCESS FOR HYDR~ULIC FRACTURING WITEI FOA~ The present invention refers to hydraulic fractur$ng with foam. More speci~ically, the present invention refers to hydraulic fracturing with nitrogen foam gen~rated in''s'i'tu, by means o~ chemical reaction be~ween a~ueous solutions of nitro gen salts. ~ , _. . . __.,. ._______._ _ , As is well-known, nitrogen gas is widely u~ilized in wel'l-treatment operations,-nitro. gen being available'from cr~ogenic cylinders. Theapplicant verified now that it is possible to conduct hydraulic fracturing of a forma~on with nitrogen foam g~n'erated in 'si'tu~ utilizing ~onventional equipment and pumping and mixing to conduct the treatment, without re~uiring facllities for~ uid nitrogen storage and conventional high-pres'sure'unit, ¢ryogenic pumping and vaporization unit. ~ : Formation fxacturing in whi'ch'the fracturin~ fluid consists o~ foam is a widely explored technique. Foam~ o~ntain up to 95% of a gaseous phase, ~he most usual range being 65-85% of gasebus content. Faams o~fer Yarious advantages' as a fracturing flui.d: high sand carrying and suspension capacity, low filtrate loww, low hydrostakic pres ~ure, lc~w pressure drop ~y frlction, fast fluld rec:overy, lc~w formatlon damage, arld absence of fra ~ ture c~nductivity reductlon due 1:o fluid lngredients.

Although ~st ~oam ~pplicationl; have been ~nducted in l~w-permeability gas ~reserv~ir~, oil reservolr~ h ve ~n eucces~fully E;~ected to thi6 type of treatmentO lt~ operat~onal (co~t 2ieir~g egual to ~r sllghtly lw~er thar~ that with ocnventlonal flulds. Foam6 ~ fracturing fluid~ are ~lspersion6 of ~a~, u~ually nltrogen, in a llquld, usually wat~r, with a 8mall proportion of a E;urfactant ~o~mlng ~gent.

The volt~metri~ gas oontent ~re~erred to Eozlm quality~ u~ually ~n the range betsdeen 65 ~ ~5~ he ~urfas:t~t represeDt6 0.,5-1.0~ of liguid ~olu~e. me E~bility 1~ lncrea~ed ~ the addition of ~ore ~;urfactant, whe~er M ~Eoal~n9 or a gelling agen~. ~hoRe ~oams ~re homGge~eou~ mixt~re~ wi~ a r~arrow range of ~ubble ~ize, average b~ le ~lze under 200 ~nicra, ~elng stable during several h~ur~. 5'he scient~fic~ te~hnical litera~ure deals wlth the various ~spects ~onnecte~ to the phy~ erb~al and rheologlcal propertie of th.e foar~ nd their appl~e~ati~n ln well traat~ent.

On th~ ~ub~e~t, re~er t~ ~O~:.Blauer ~nd C.A.~o~lhaa~ ormatlon Fr~cturlng with ~?oam", SPE ~003, 1974s D.R.Davl~s 2~d E.A.Ri~ihardEon, "Field Appllc:~*lon of 1~ situ Nltr~ge~i Ga~ Gerieration ~y~;~emn~, SPE ~' u 9653, 1981; S.H.Raza, "Foam ln Porous Media: Characteristics and Potential Applicat1sns", SPE Journal, December 1970, p. 328-336; W.A.Abbott and H.F.Vaughn, "Foam Frac Completions $or Ti~ht Gas Formatlons", Petroleum Engineer, April 1976, p. 38-50; S.R.Grundmann and D.~.~ord, "Foam Stimulation", Jour~al of Petroleum Technology, March 1983, p. 597-602; V.L.

Ward, nNitrogen and Carbon D~oxide in the Oil Field:

Stimulation and Comple~ion Appllcations", SPE No.

12594, 1984, and references mentioned ~n the papers above. ~ Basically, foam is defined as a coars dispersion of gas in a liquid, each bubble b~ing encapsulated by a thin liquid ~ilm. The foaming agent acts at the system's interface, having preferably the polar portion of the molecule oriented towards ~he liquid phase and the non-polar portion oriented towards the gaseous phase. The ~tability of a given foam is bas~cally associated with two factors: the tendency of liquid dr~inage and the tendency of film breaking as a con~equence of randomic disturbances. ~ --One physlcochemical characteristicwhich easily identifies a given foam is the qual1ty.

F~am quality (r) i~ defined as the ratio between the Yolume of gas in the ~ispersed phase and th~ total foam v~lum~ ~~~~~~~~~~~ 04C) r = ~= ~'g .

V~ Vg ~ Vl where ~he total volume includes the aggregate volume of the gas and li quid whlch form the foam. ~ Under given pressure and temperature conditions the compressibility of the foam or, more accurateIy, of the gaseous phase, can be defined.

Such compresslbility, in its turn, affects foam yuality in Case of pressure and~or temperature ~ariation. Boyle's. law can be directly applied to a given foam, at a given temperature, negLecting gas solubility and liquid compresslbility~ ' :1 ' r = ~ 1 *PP~ 1) . .

where r ls ~he foam quality at a given pressure P and ra ~s ~he foam quallty at atmospheric pressure.

The texture Of a given foam is associated to gas-bubble size distribution. m e small er the bubbles and the more unifoxm their distribution, ~he moxe viscous ~hall ~he foam be. ~ ~ he rheologi~al properties of a foam are a function of iiquid phase viscosity, foam quality and ~hear rate. According to somb hypo., '.

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: ~ 04~) thetical modelst a foam with quality between O and0.54 presents a viscoslty value near that of the liquid phase, and its ~ehavior shall be newtonian. Foams with quality range between 0.54 and 0.96 present a viscosity value higher than that of ~he liquic phase, increasing with ~uality and decreasing with shear rate, that is, ~hey present a pseudoplastic behav~or. ~ For quality values above 0.96, the system starts being classified as a mist, and the viscosity decreases down to the value of gas viscosity, that is, ~ero. ~ F~acturing foams are presently generated by means of simultanebusly pumping the aqueous fluid which contains the foaming agent and ~he inert gas, -usually nitrogen obtained from cryogenic cylinders. The dispersiDn of gas in the liquid phase is achieved by means.of a di~fuser which receives ~he components in a turbulent regimen9 ~ In-~itu nitrogen generation with the purpose of stimulating the production of formation gas or oil has al~o heen described in patents. ~ US Patent No. .3,937,283 describes well fraoturing with foam generated from li~uid nitrogen and sur~actant, process parameters being adjusted so that foam quality ~e in the range of 0.52-0.99~ US Patent~o.

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040 4,178,993 describes, in lts turn, what i8 technically referred to as 'ibacksurgingn, ~hat is, a gas well which cannot produce due to the hydrostatlc pressure of the liquid whlch lt c:ontains, ~tarts producing again when an aqueous solution with reagents capable of producing nitrogen gas i~ injected in the well, ~he gas generated displaclng a sufficient volume of llquid out of the well, 60 hat the hydrostatic pressure becomes lower than the fluid pressure in the adjacent portion of the reservoir and the fluid is displaced from the reservoir towards the well. ~ US Patent No. 4,232,7~1 describes the temporary bIocking of parts o~ an underground r~servoir by the injection of an a~ueous liguid ~olution containing gaseous nitrogen generating reagents, a foaming surfac~ant and a pH-controlling agent which acts towards ~ainta~ng the solution less reactive (delayed ~ystem) lnside the well but forms a relatively immobile foam lnside ~he pores or other reserv~ir openings. Foaming is achIeved within ~he formation.

US Patent No~ 4~219,083 describes another backsurging process with in-situ foam generation, ~he gas-generating ~olution containing also an alkaline delaying ~uffer and a pH-reducing reagent wh~ch eventually destroys the buX~er effect, so that thè ~ .

t) fast productlon of gas and heat promotes-ba~ksurging.

US Patent No. 4,330,037 ~escribe6 the lncrease in permeability of a ~ormation to oil by the injectlon of a nitrogen-forming solution, wlth exothermiclty, and an alkal~ne buffer controlling the reaction rate. ~ The present invention, slmilarly to sQme references mentioned, i8 based on the reaction between ammonium ions and nitrite ions of an aqueous solution of these salts, producing nitrogen gas and heat. On the other hand, the distinctive characteristics of the presPnt invention as reIated to the state-of-the-art rPfer t~ the concentration of the reagents, to.the fact ~hat ~he reaction of the pxesent iRventi~n ~ s acceIerated (whereas the literature mentioned employs reactions usually delayed by alkaline buffers), does not require any foaming a~ent and foam generated is achieved at the surface, instead of being achIeved inside the formation.

Other peculiar aspects a~ the present inYention refer to the half-life time o~ the reaction, which in the present invention is 60 minutes (in US patent No.

4,232,741 this parameter can reach 1,0Q0 minutes~, and the gas volume/ll~uid volume ratio is here 72/1, whereas, for lnstance, in US patent No. 4,232,741 such ratio ~s, ~t most, 44/1. ~ `.~ .. , :

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:' ~ 040 ~ Thus, the present invention relates tv a process of formation hydraulic ~ractuxi~g based on the product~on of nitrogen foam generated by the exothermic reactio~ of nitrogen salts. The'aque~us solution generatin~ gaseous nitrogen contains: a) a compound b~aring at least one'nitrogen atom to which is connected at least one hydrogen atom and which .

can be rapidly and exothermically ''oxidI~el' in an acid a~ueous solution, for the'productio~ of heat, nitrogen gas and liquid,or dissolved'by-products substantially inert to wel'l pipe and reservoir components; b) at least one'oxidizing'agent capable of oxidizing the nitrogen compound o~ a); c) a bu~fer ~y~tem capable'of maintaining the'pH o~ the solution nearly equal to or less than 5.0, a ~iscosifylng compo~d, whi'ch'may ~e'any water-solubl~'polymer or . ~., gel capable of increasing the'effecti~e viscosity of the foam generated. ~ - ~ he'compounds'a)-b) which''in an aquebus solution form the'oxi.dation-reduction couple can be'constituted, for instance, by urea-sodium hypochloride, ~mmonlum hydroxide-sodium.hypochloxide~ urea-sodium nitrite,'~mmonium ~hl'oride-sodium hypo chloride'or ammonium chl'orlde-sodium nitrite. The' latter ~as the'couple ~eIected ~or the'reaction of nitrogen foam formation of the'present invention, due ~o the'easy ki'ne ic control of the reaction, .. , ~, ~ ' ~, 040- strong exo~hermicity, sensitivity to the pH o~ the medium and to the temperature. ;~ The buffer system c) consists of an aqueous solution of acetic acid at 40~ v/v and the viscosifying compound ls, preferably, hydroxyethylcellulose (HEC). One of the advantageous aspects of the present invention is that, ln opposition to what is usually practicPd, the applicant employs~ in stead of surfactant, the v~scosifying compound.HEC.

m at is so because the surfactant employed can cause, ln contact with the formation: a) a c~ange ~n rock wettability; b) oil emul~ification;-and c)~preclpita~on due ~o ~he incompatibility with formation water. On ~he other hand the viscosi~ying compound (HEC~ promotes higher foam viscosity wlth lower concentration than the ~urfactant. ~ . me wat~r utilized ~or.dissoluing.the .

nitrogen reagents i5 any good-quality industrial water, free from contaminants ~uch as mineral ac~d, alcohol, alkalies, dichromates and salts of trivalent iron. m e solution of sodi.um nitrite must be free from strong acid to pr~vent ~he generation o nitrous acid, which breaks. down to nitrous oxide, a compound ~hich ha's an irritatin~ smeil. The solution of ammonium ~hloride must be free from alkaline substances to prevent the release o~ ammonia. ~ . ~ ' ` ' ` .

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The reaction between the nltrogenogen erating compounds comprises the steps of reagent dissolution, formation o~ the NH4 N02 complex and further dissolution of the complex into nitrogen and water. The decomposition reaction may be subject to variation ln ~ts rate, in case the acid hydrogen species ~H~) ls introduced in the medium. Thus, the equation which describes the rate of the reaction depends upon the concentration of ammonium, nitrlte and acid hydrogen ions. Other influenclng parameters as temperature, agitation and viscosity, are associated to the degree of proximity o~ the chemical species nitrite and ammonlum in the formation of the complex. Pressure, temperature and solubllity, on the other hand, shall rule the state of the gas (PVT) produced. It shbuld be pointed out that th2 removal of the reactlon product from the reactional medium favors the reaction towards the products. ~ On the other hand, ~he nitrogen-yenerating reaction indicates a 1:1 stoichiometry (e~uimolar) be~ween ammonium chlor~de and sodium nitrite.

It may be verified that, ~aintaining the e~uimolar pr~portion between the reagents and varying the molarity between 1 and 3, ~he rate of the reaction increases si~nificantly. The p~ o~ ~he reaction medium affects as well the rate o~ the reaction, )40 8ince the mechanism ~f the reacti~n implie8 ~he actlon o~ the species H ln the ~tep of the actlvated complex.

The appli~ant found that the .

optlmum pH range for the nitrogen gas generating reaction is around 5. 0 and, more ~pecifically, between 4.75 and 5.50~ m e visc:osity of ~he medium affects the rate of the reaction: the higher the vlscosity, the longer ~he lifetime of the reaction.

As far as the calorlmetry of the system 1~ concerned, it may be observed that the dissolution of the reagents in water is endothermic, while the generation of nitrogen is strongly exothermic, with release of nearly 70-75 Kcal per mol of reagents, such strong exothermicity being extremely advantageous to the formation fracturing prccess, which occurs without cooling, when in contact with the fluid injected. ---Fiyure 1 is a general flow diagram illustratingthe hydraulic fracturing process of the present invention.

Figure II is a diagram of the experimental apparatus to simulate the hydraulic fracturing process of the present invention.

Figure III is a graph relating to the viscosity characteristics of the foam of the present invention.

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m e flow dlagram of the present hydraul~c fracturing pr~cess wi~h ~ foam geneEation is outlined on Fig. ~, where: (1) is ~he tank containing the ~iscosified solution of ammonium chloride; (2~ is the ~ank containing the 601ution of sodium nitrite; (3) ~s the funneI utilized for adding up sollds; (4) i~ the silo ~or frac sand; (5) is the blender or mixer; (6).is a triplex pump; (7) is the tank containing ~etic acid; (8) is a metering pump; and (9) represents the o~l well. ~ With the purpose of obtainlng a maximum rate of reactlon, -and since the maximum ' ~''`'1 0 solubility of ammonium chloride in water, at ambient temperature ~nearly 30C), is approximately 32.1~ by weight, this provides a solution of molar concentration equal to 6.0, whereas sodium nitrite can reach a molar concentration equal to 9.0 (62.1~ by weight) in the same conditions. -------~ Thus, the composition of the solutionsof NH4Cl and NaNO2 must maintain a mass ratio of 1:1.934 for molar concentrations between 6 and 9, respectively, and the mixture of same a volumetric (or flow rate) ratio of 1.5:1 so that equimolar concentrations of the salts may occur, observing thus the stoichiometry of the reaction. --------------~ The present invention resides in acontinuous process of hydraulic fracturing of a well with ln-situ nitrogen foam generation, comprising the steps of: a) preparing an aqueous solution of NH4Cl having a concentration of from 2.0-6.0 M; b) preparing an aqueous solution of NaNO2 having a concentration of from 6.0-9.0 M; c~ preparing an agueous solution of acetic acid having a concentration of about 40~ w/v;

d) adding hydroxyethylcellulose (HEC) to said NH4C] solution at a concentration of from 0.3-1.2~ w/v to form a viscosified NH4Cl solution; e) simultaneously pumping said viscosified NH4C1 solution and said NaNO2 solution to form a mixture having equimolar amounts of NH4Cl and NaNO2 thereby initiating an equimolar reaction f ~B ._ - , ~: ' ' ' . .

, 040 12a between the components of said mixture; ) pumping the acetic acid solution into the equimolar mixture of NH~C1 solution and NaN02 solution, so as to .

initiate foam generation, while keeping the pH of the mixture between 4.75 and S.50; g) injecting a cushion of the generated foam into the well (preflash); h) initiating the fracturing of the formation by injecting foam to which is added sand as a propping agent, said agent being added to the viscosified NH4Cl solution, in amounts varying from zero to 0.92 kg/l of the foaming mixture; and i) after the fracturing, injecting in the well a cushion of sand-Eree foam (over-flash) oE the same composition as the fracturing foam. ----------------------- The continuous process of hydraulic fracturing with in-situ nitrogen foam generation developed by the applicant is characterized by the following steps: -----------------------------------a) prepare solutions ~ and B by dissolving the nitrogen salts NH4C1 and NaN02 in the mixing tanks (1) and (2), by means of adding said salts to the mixing water maintained under permanent circulation through the addition funnel (3) of the pumping unit, the concentration of the salts being in the range between 2.0 and ~.0 M (10.7 and 32.1% w/v) for NaN02, maintaining always the equimolarity of the salts; ------- ------------------------------ ? ~ . ~ . .

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13 ~ 04~) b) prepare the ~olution C by dllu~ingcommercial acetlc acid in the acid ~ank (7) down to the concentration of 40% w/v;

c) viscosify the solution A by mean~ of adding hydroxyethylce~lulose ~HEC) to the ~olution of NH4Cl in a concentration of 0.3-1.2% w/v;

d) pump s~multaneously the ~olutions A, B and C in flow rates proportional to the concentrations of each solution in the mixture, first to form a cushlon (pre-flash) of foam with quallty between 0.50 snd 0.98, then with the addition of a propping agent-frac sand ~4~ ~n increasing concentrations from ~ero to 0.92 Kg/liter of solutlon (7 lb~gal), calculated as a function of the foam volume in the pressure and temperature conditions reached during the operat$on, and completing the treatment wi~h the cushion of ~oam (over-flash); ~ e) recoYer the fluids in~ected, bymeans of sw~bbing, if necessary, after the fracture accommodation period and the foam viscoslty drop per~ iod (for reasons of defoaming and/or breakdown of the viscosifying agent~: and ~ f) put the well in production, performing thus the productivity test~ The present hydraulic fracturing pro~cess was tested by means of laboratory methodology for tha preparat~on,-characterization and field :'f ~1, U~ 14 simulation of nitrogen foam generation. ~ l~us, ~etails o fracturing fluid pre paration, physicochemical and rheological ~oam characterization, ~nd physical simulation ~ foam generation are supplied as ~ollows. -~ As previously described, nitrogenfoam originates' ~rom the mixture of three'agueous ~olutlons. ' ~able'I below describes the compos~tion of thes'e'solutlon the'concentration of ammonium chloride may be'expres'sed as 6 M, that of soqium nitrite as 9`M, and ~hat of acetic acid as 40~ v/v, such'concentrations bein~, as a matter of fact, preferential compositi,ons; other compositions exist which ar~, equally susceptible of producing good results in the ~rocess of the present lnvention. As a matter of fact, the .concentrations and volumes of the sa].t ~lutions t only~observe the equimolarity of the reacti~n.

But, as the'concentrations increase (limited to 6 M for NH~Cl and 9 ~ for Na~)2,~ the highe~ is the'potentialit~ of the mixture'in N2 generation .. . . . _. . _. .. . .. ..._ .. _ . . . .

and consequently the lar~er is the amount of heat and the higher is the'~uality of ~oam. ~ _ __~__._________________________ _ __,._____ ~ _______ _______,___..___~________________ ______ ________.____ ~~~~~~~~~--~''~~~ 304(~ I~LE_I CONCENTRATION SOLUTION/COMPONENT /1000 ml /1000 gal A Water 755 ml 755 gal ~mmonium ChIoride 321 g 2,676 lb Hydroxyethylcellulose 12 g 100 lb B Water 750 ml 750 gal Sodium Nitrlte 62I mg 5,177 lb C Water 600 ml 60Q gal ~ce~ic Aci~ 400 ml400 gal The mixture o~ solutions A, B and G ~n volumetric amounts proportional to the stoichiometric ratio of the inOrganiG salts in the chemi~al reaction ~or nitrogen generation con~titutes the foaming fluid, according to ~cheme I and Table II, beIow, said Table ~ummarizing the composition of ~he fluid, expressed in unit~ per mililiter and per gallon. ~ S~HEME I NH Cl ~ NaNO ~ H~ N ~ N Cl 2 ____ _________.________________________ ___________ ______________.____.______.__________ ______________ - ' . . . '' .

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:` 04~:) 16 TABLE II SOLUTIûN VOLUME ~1000 ml /1000 gal .

A 600 ml 600 gal B 400 ml 400 gal C 40 ml 40 gal l~it}l the purpose of generating the foam, the mixture of ~olutions A, B and C is trarlsferred to a graduated cyllnder with capacity for 2,000 ml which i~ inunersed ln a therm~static bath at te~t temperature (60 C) . ____________________ ~ ~_~___ An important parameter for la~orat~ry tests ls the calculation of the ~oam generation reaction yield. ~ me the~re~ical yield ~cQn~iderirlg103% of ~hè reactisn) of foam generatic)n was ca~ ate~ as a function of the effectlve molar concentration of the ~alts irl the mixt:ure, the volume of the mixture arld the volume of nitrogen gas generated per volume ~ mixtu.re~ Vfoam Ymixt ~ NH,~Cl or NaN02 x Vmixt x ~*2 W~ th the purpose of chara~eriz~ ng ~oam ~eneration from a phy~;icochemical and rhec-log~ al point of view, varlous parameter~, ~escribed as - ' .

o~ 17 follows, were studied J ~ Reactlon Kine~ics , . , m e rate of the foam generati~n re~ .

action was determined from the evolution of foam volume during the test ~b~ecting 100 ml of mixtur~ to a temperature of 60~C. The values ~ reaction rate were expressed in ~erms of the percentage of volume r~lative to the theoretical value ~toichiometrically calculated. --~ Table III below -~ummarizes the kinetic data of the foam generation reaction.

~ABLE `III .- REACTION TIME FOAM VOI.UMEFOAM QUALITY YIELD (min) (l) (r~ (V%) _ ~ .

O 0.1' ' .~ ~ l/4 ~.2: 0.916 14.7 l/2 2.4 0.958 29.4 3/4 2.8 0.9~4 34.3 l 302 0.968 39~2 2 3.8 0.973 46.5 3 4.3 0.976 51.4 4 4.6 0,978 56.3 5.0 0.980 61.2 ..~ __ __ __ . ____. ____.____________________ ._______________ 18 It should be pointed out that the theoret~cal calculatio~ ~or the 100% yield afisumes a foam volume equal to 8.16 liters per 100 ml of mlxture and a ~oam quality e~ual to 0.988 at atmospheric pressure. ~ -Foa~ ual:i~y The calculat~on o~ foam quality ( at atmospheric pressure and at the temperature of 60-C was performed based on values of foaming li~uid volume (Vl) and foam volume IVe) meas--red af~er test completion~ ....V..

Vg ~ V where: Vg = Ve ~ V thenO .

Ve ~~ V e m e physicochemi~al properties of the foam-generating ~olutions were measured as well, such as densi~y, ~iscosity, Behavior Index and pHo ~ Foam Density m e determination of foam density at atmosph ric pr~ssure and a~b$ent temperature was per~ormed-in a density ~cale and compared to ~he value . L~ .-, ,, ~33~140 19 calculated theoretically ~rom the data of llquid density and foam ~uality~ De Dl x tl - r ) me values of these prvperties are shown in the Tables below, where Table IV records these values for the 601utions and ~heir mixtures, and Table V record~ these ~alues for the ~oam formed.

' T~Iæ,,~V A B C MIXTU~E Dbnsity (g/cm3) 1.0836 1~3~89 1.~448 1~1~97 vi~x~ity(a)(cæ) 810 1.9 1.0 148 Ebhavior ~ ) 0.61 1.0 1.0 0.84 pH 4.45 9.16 1.74 4.73 (a) FANN-35A ~i~cometer 12 rpm and 25C ` TA:BL~3 - V __ PROPERTY VALUE Quality 0.98 Densit~ (y/l) 25.02 pH 5.06 . . .

The rheological properties of the foam were measured as well~ ~hese properties are listed on Table ~I below, determlned at a~mospheric precsure and 60CC, ~n a Brook~ield rotating visco meter, model LYTD, coupled to a vertical path ' 1 *Trademark **Trademark :

elevator w~th ~pindle hellpath A~ ~BLE ~I .

PROPERTY READING Apparent ~iscosity (cP) s 12 rpm 1.450 6 rpm 2,450 3 rpm q~240 1.5 ~pm 7~240 0,6 rpm14,510 0.3 rpm24,460 Behavior lndex ~-) 0.23 - As far as ~he solidb carrying abllity by the ~oam ~s eoncerned, it was determln~d from the settl~ng rate of 1.6~ mm (10 mesh) ~and in ~at~c condition and at the temperature of 60-C. The data related to the settling rate are tabulated b~low (Table VII~ T~BLE VII ~AND SETTLING RATE mm (me~h) ~m/m~ ft/min 2,38 (8) 0.0666 0,00218 1.41 (12) 0-0555 0~00182 0.84 (20) 0.0434 0.00142 me ~arrying ability of foam may be ~ . al~o expre~sed as 0.92 Rg o~ 1.68 ~m-0.84 mm (10-20 'J'~ mesh~ 6and per liter o~ oaming mixture (or up to ... : .. ~ ~ 04~ 21 7 lb/gal). ~ Another important parameter in the evaluation o~ ~oam generated in the control of fluid loss. This measurement was achieved in a ~ilterpress, at the'pressure of 7 Kg/cm2 (100 psi) and ambient temperature. ' The'filtering element utllized was ~Wha~ ilter paper no. 50. During the test, the' variation in the volumes of li~uid filtrated and ~oam remanes'ceit in the cell was observed.

Table ~III bel'ow lists the'results relatlng to this parameter~ --- T~L~ ~III ~ L U. I D PROPERTY FOAM LIQUID Q 7 1/2 (cm3) 146 ~O9' Q 30 (cm3~ ~Q2 5.8" m ~cm3/mi~l/2) 1.06 Cw (cm3/minl/2) - 0.0384 .. . .

where:

Q 7 1/2 is the vol~me'of liquid o~ foam :Eiltered in 7.5'min ~ Q 30 is the volume'o~ liguid or foam :Eiltered in , 30 min'-------- ~~ -~~~~~~~~~~~~~~~~~ m i~ the ratlo between ~v and ~tl/2 ________________ * Trademark 040 where: v c volume (cm3) t = time (mln) Cw i~ ~he fluld loss ~oef~icient ~ ~ hermal stabil~ty i~ another importan~ ~oam characteri~tic. It has been evaluated at atmospheric pressure and 600C as a fu~ction of the change in the volume of foam wi~h time. In th'i~ stability test were determined al~o the foam break~ down ~ime, the viscosity reduct~on in the liquid phase, the insoluble residue amount, the ~oncentration o~ ~odium chloride, the density and p~ ~f the liquid phase. ~ In the conditions of atmospheric pres6ure (adver~e ~ondition) and temperature of 60C, th~ foam po ~esses a oertain stability, represented . by lts resistance to de~oaming, which ls ade~uate ~or its utillzation in hydraullc fractur~ngD with the reduction of 50% of the Yolume of ~oam in one hour. m e liguid phase r~aches a f1nal viscosity value equal to l.~ cP in three hour~, when subjected to the temperature of 600C, presenting ~nly traces of insoluble residues and density of 1.163 g/~m , due to the presence of ~iod~um chloride - a ~y-~roduct o~ the chemical reactlon - in a ~oncentratlon egual to 210 y~ . The data on thermal stability and foam breakdown ~re Ii~ted respectiYely on Tables IX .

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)4~) 23 and X below, ~e latter contaln~ng propertles of ~oam a~ter brea~cdown, ~uch as hal~-life time, flnal visc~sity, resldue amount, NaCl c,oncentr~tion, densi ty and pH O ~BL~ IX TIME STABILITY , (min) (~ vol) ' 100 53 38 . 180 19 2~0 g ~æ ~ I?ROPERTY VALUE .

~alf-iife (mirl) 65 Final ~.riwosity ~c:P) ~,5 Res$due ~mount traces NaCl ~oncentration (g/l~ 21~ Density ~g/ml) 1.163 pH (-~ 5 ,. ~ One ma~or ~haracteri~tic of ~rac foam i~ its compre~sib~l~ty. me c~ompre~sibility of frac foam at a given pressure and temperature ~ondition wa~ c:alculated fxom the ~ompressii:~llity factor 304~ 24 of nitrogen gas, ~ssumlng the lncompres~billty of the li~uid phase, the ins~lubility of ga~ ln the liguid and the yield of the reaction as belng equal to 100%, These data ~re listed on Table XI as follows. ~ __ TABL~ ~I PRESSURE ~EMPER~TU~E COMPRESSIBILITY FOAM FAC~OR QU~LITY psi (Kg/cm2) QF(~C) (~) (r) 14.751.D33 l00(37~7~ 0O96 0.988 140 (60.0) 0.90 0.988 180 (82.2) ~.84 0.989 220 ~10~.4) 0 79 0 gg0 ~00(35.~) l00 137~ 31.66 0.718 14~ (60.0~ 29.3~ ~.733 180 (8~.2) 27.57 0.745.

220 (104.4) 25.79 ~.756 l000(70.3) lO0(37.7) ~3.15 0.5~1 ~ 0,0) 58.35 0.580 l~0 (82.2) 54.43 0,597 220 1104.4) 48.~2 0.62~ 1500(105.4~ l00(37.7) 93.75 Q.462 140 (60.0) 86063 ~.482 180 ~82.0 80~59 ~.500 220 (104.4) 74.36 ~.5~0 2000(140.6) l00(37.7) 123.46 0.395 140 (60.0) 113.8~ 0.414 lB0 (82.2) 105O85 0O432 220 (104.4~ ~8.2~ ~.451 The adequacy o~ the COntlrlUCUS prccess o~hydraulic fracturing with in-situ mtrogen foa~ generation was tested by means of physica~ simwlation in the laboratory, the apparatus used to s~l~ula~e the process beLng shown in Figure II, where the ~lasks (1) a~d (2) contain, respectively, the solution of NH4Cl, thickened with HEC, and that of NaN02, in the appropriate concentrations, flask (3) contains the solution o~ acetlc aci~, ~4j represents peristaltic pumps, (5) is a ~eathed glass tube, (6) is the heating water inlet, ~7~ 1~ the heating wat~r outlet~ and (8) is the ~oam-collecting recipie~t. ~ The ~omposit$on o~ the ~aline ~olutions utilize is deta~lea in ~able XII below~ ' T~BLE - X T I SOLUTIONCONCENTR~TIONVOLU ~ FLOW ~ TE (mol/l), ~%~(ml3 (ml/min~ NH4Cl~a) 6 150 15 NaN02 9 100 10 AcOH /10/ 5 0.5 (a) Viscosi~ied with 100 :lb/l1000 gal of ~EC QP-100M-H. ~ . l~j .

"'" ' (; 4V 26 ~ he ~imulation ~est performed ln the apparatus of ~igure II produced i:oam with the charac terlsti~s li~ted on Table XIII.

T~BLE ~III CHARAC~ERISTIC ~LUE Inlet ~low rate (mixture)25.5 ~l/min Outlet ~lcw rate tfoam) .1~840 ml/min Inlet temperature (mix~re) 30~C Outlet ~emperature lfoam) 92~C Quallty 0.986 Density 25.0 g/l p~ 5.03 Volume 18.4 1 Y~ld 90% m ese data make it possible to.~eri~y that the physical simulation of ~oam generatlon by a continuous process sh~ws ~hat the adaptation o~ the process to a fiel~ scale ~or u ilizati~n in the hydraulic ~xacturing of the produc~ng ~rmatio~ i6 perfectly feasible, llm1ting the pumping pressure a~ the wellhead~ inltially, to Yalues below 140 Rg/cm2 ~2,000 ~si). ~ Thus, bo~h the preparation of ~ea~ent.

and catalyst ~olutions an~ ~he mixin~ ame may be conducted wlth conventional field resouxces, that o 27 is, pumps, tanks and mixer's, and in a relatively short tlme. ~ - One'of the' characteri~tlcs of the NH4Cl solution is lts high ~iscosity (810 cP), due to the addition of hydroxyethylcellulo~e.' ~Xter the mixin~ with the NaN02 solut:Lon this figure is.reduced tq 1~8 cP. The o~her phy~lcochemical properties of the mixture'reach intermediate figures relative 'to those 'of the saline solutions, and the pH of 4 .73 shall define the rate of the reaction at a ~iven temperature.' ~ The curve of ~he nitr~gen'-generation reacti~n rate under test conditions is characterized by the fast inductio~ in the'~irst two minutes with the generation of 46.5~ o~ the N2 volume, followed by a per'iod of slower ~oa~ing, the yield reaching only 61.2% in the following three'minute~. ' This is .

due 'to th.e'fact that the'rea:ction rate is directly proportional to the 'concentrations o~ NH4~ and NO2 ions consumed during the'generati~n of N2. ~~~~~~~~~ The'~uality of the foam ~enerated by this process reaches a value o~Ø98 under atm~spher$c conditions, whi'ch''corres'ponds to a 99~ yield. - The'rhe~lo~ical properties of the ,~oam are'significantl~ better th~n those of the generatin~ mixture.

The'''ps.eud~plasticity of the foam (n' ,=:0.23) i~ higher than that of the mixture .... ~ :

4~) 28 ~n' = 0.~4), and ~he ratio between the values of apparent viscosity of the formex (~e) and o~ the latter (~m) increases exponentially with the reduetion in ~hear rate. This fact ~s ~hown on ~igure The pre~ent proce6s prov~des benefits already expected: hiyher ~peed and ef~iclency in t~e recoYery of the ~luid ln~ected, wlthout the u~ilizatlon, aXter ~ractur~ng, of the systems of li~uid N2 or ~wabblng, ~nd lower ~olume o~ liquid utilized, re duclng thus the formation of emulsions with the oil of the reservoir. ~ In additlon, there i~ ~he a~vantage of th~ increased temperature of ~he in~ect~d ~luld, promQted by the exo~hermicity of the nitrogen-gener atiQn reaction~ It ic assumed that the increased temperature is advan~age~us to wells cvntaining paraffinic oil and ~hat the foam reaches quality levels highar than tho&e expected at formation temperature. ~ Duriny the ~olution pumping operation, it be~omes necessary tQ have a per~ect control ~f the l~w r2tes, particularly in relation t~ the acetic acld meterlng pump. PreYious tests may be conducted ~or be~st adjusting the rate ~f ~he nitrogen generatiQn reactlon as a ~uncti~n o~ ~he concen~ra~ion of ~ce~lc: acid. me propping agent shall ..

: ., . : '` ' ''"'' ` ~ . : '~ ' C3 29 be lncorporated in the'foam-generating mixture ln the blender, at a concentration calculated for the volume'of foam in bottom-hole condition. ~ After completing the operation, well opening for cleanout purposes ~hall be'~low and gradual, to avoid removing the'propping agent near the weIl, until the 'fracture is fully closed.

~ he curve of foam ~tability, determi~ed at the temperature'of 60C and at atmospheric pressure,' indicates' that its half-life time ~s 65 minutes. ~ -It i~ antici~ated that ~he pressurereduction of the foam during its recovery shall promote a significant expansion thereof at the surface.

Depen'ding upon the'~u~lity and the volume of the foam recovered, the'technigue of defoaming by means of an ethyl alcohol shower may be utilized, ~hus reducing considerably the volume of fluid. -~ Thus, ~he present invention proYides a process for hydraulic fractur~ng with in-situ ni trogen foam generation~ said process making possible a significant penetration on the part of the fracturing flu~d, with low fluid loss and consequently with a nearly null settling of the fluid and low formation damage.'' The '~uality of the foam formed is high, that is, the ratio between ~he volume of `~ 4~ , ` .

o ga~ and the volume of gas plus li~uids in the fluid ls high. The fact o~ the settllng rate of the propping agent (sand) being close to zero makes it pos6ible to avoid ~eparatlon by ~;ettling between the propping ayent and the fluid~ In addition, the high ga~ content ~n foams produced by the present ln vention makes it possible to remove mo~t o~ the llquid from th2 formatlon a.fter having completed the fracturing treatmentO ~ ====== Q C=====--=========_======--========-========= .

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