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Applications & Calculations
In this chapter the student will learn:
How foamed fluids are different from commingled fluids. Benefits and calculations of acids when foamed. How foams can be used for diverting. Benefits and calculations for proppant laden fluids when foamed. Effect nitrogen has on cement when foamed Foam generators, what they are and how to use them.
Background of Foam in the Oilfield
Foams are being used in a number of petroleum industryapplications that exploit the foams' high viscosity and low liquidcontent. Some of the earliest applications for foam dealt with its useas a displacing agent in porous media and as a drilling fluid.Following these early applications, foam was introduced as awellbore circulating fluid for cleanout and workover applications.In the mid-1970s, nitrogen-based foams became popular for bothhydraulic fracturing and fracture acidizing stimulation treatments.In the late 1970s and early 1980s, foamed cementing became aviable service, as did foamed gravel packing.
The early widespread use of foams as fracturing fluids was to helplow-pressure gas reservoirs in returning the liquid phase of the
2foam. The internal phase of the foam typically consisted of 65 to80% by volume (quality) of nitrogen gas, with an external phase ofwater and a foaming agent (surfactant). These simple nitrogenfoam fluids, coupled with the pumping technology of the 1970s,were able to transport sand concentrations of 1 to 2 lb/gal intofractures. Such low proppant concentrations gave beneficial resultsin low-pressure sandstone, carbonate, and shale reservoirs. Muchof the success of the early treatments was due to the capability ofnitrogen gas to expand and remove substantial quantities of theliquid phase from the reservoir.
The viscosity of a fracturing fluid is important because of itsinfluence in creating fracture geometry and in transportingproppant. Adding linear polymers or crosslinked polymers towater increases its viscosity. Viscosity of the fluid mixture is alsoincreased by adding nitrogen gas to create an internal phase (gasbubbles), when a stabilizing surfactant (foaming agent) is present.High-viscosity foam fluids can be prepared using low amounts ofwater and gelling agents, thereby minimizing the liquid loadplaced on a formation.
Foam viscosity depends on a number of variables, includingquality, viscosity of the external phase, and texture. The mostimportant parameter is foam qualitythe percent volume occupiedby the internal gas phase. Since gas volume is a function oftemperature and pressure, downhole conditions must be known.As quality increases, foam viscosity increases. In addition, the yieldpoint characteristics of foams are an exponential function ofquality.
Higher quality foams have better transport properties, particularlyat very low shear rates, because of high yield points. The viscouscharacter of the external liquid phase is also a major parameter.Flow of high-quality foam may be visualized as gas bubbles slidingpast one another on thin films of the liquid external phase. If theliquid film contains a viscosifying agent, then the bubbles willundergo greater drag forces because of the viscous thin films, andflow will be more difficult, resulting in higher bulk viscosity.Texture, or the bubble size distribution, plays an important butlesser role in determining foam viscosity. Foams exposed to shearfor a sufficient time will equilibrate to a bubble size distributionthat is characteristic of that shear rate. Texture is also influenced by
3the surfactant that must be present in sufficient concentration tostabilize the foam under dynamic conditions.7,8
Q - Quality - This term is used mainly when foaming a fluid. This like VLR has no unitsdue to being a comparison of the total mixture to just one additive (gas) in the mixture. Itdiffers from VLR in that the mixture is in the bottom part of the equation rather than thetop. Due to this, the Quality will always be less than one. The Tables in section III of theNitrogen Data for Oil Well Servicing manual uses Q. It can also be calculated using theTables in section VI.
As oil and gas wells age, many of them show similarcharacteristics. One of the most obvious is, of course, reducedbottomhole pressure that can contribute to the formation ofparaffins, asphaltenes, and scales. Many old wells have hadrepeated acid treatments. Following conventional acid treatments,large amounts of insoluble fines such as quartz, gypsum, andfeldspars may reduce fracture conductivity. All of these factorsrelated to old wells can be controlled through foamed acidstimulation.
Treating wells with characteristics as outlined above with aconventional nonfoamed acid treatment will probably bebeneficial. However, the high liquid content of conventional fluidsmay increase clay problems. Also, low viscosity of the spent acidmay leave a large amount of insoluble fines in the well. Inaddition, low bottomhole pressure may require swabbing to cleanup the well.
Nitrogen (nitrogen) is the most widely used material in foamtreatments. Volumetric gas content (foam quality) is generallybetween 65 and 85% (comprising 65 to 85% gas and only 15 to 35%liquid), although qualities as high as 95% have been used. Theliquid phase of the foam may contain 0.5 to 1.0% surfactant and 0.4to 1.0% inhibitor.
4Advantages of Foamed Acid
Foamed acid has widespread applications in both oil and gas wellsand offers the following characteristics to virtually eliminate theproblems mentioned in the previous section:
Low liquid content- Foamed acids used in fracture acidizinggenerally range from 60 to 80 quality. The low liquid content isextremely important when treating a liquid-sensitive formationwhere large amounts of liquid may cause swelling in the formationand reduce the permeability of the formation to the producedfluids.
Reduced fluid loss- The high apparent viscosity of the foamedacid results in reduced fluid loss, allowing deeper acid penetrationthan a comparable nonfoamed or conventional acid system. In lowpermeability reservoirs, the bubbles of the foam may be sufficientto prevent leak-off to the matrix. This can reduce the affect ofwormholing (channeling). Also, since no fluid loss additive isnecessary in low permeability reservoirs, there is a reduced chanceof impairment of formation conductivity due to the solids in someadditives.
High apparent viscosity- Viscosity is difficult to obtain in anonfoamed acid system since the acid used frequently is notcompatible with the gelling agent. A viscous acid provides theadvantage of better pumpability, wider fracture, and improvedfluid loss when used in fracture acidizing. Increasing the viscosityof the acid before it is foamed will give these benefits plus help toincrease foam stability.
Better cleanup- The built-in gas assist derived from using afoamed acid treatment now makes recovery of treating fluids fromlow-pressure reservoirs more effective than nonfoamed treatments.The built-in gas assist plus the high apparent viscosity of thefoamed acid enable the acid insoluble formation fines to bereturned to the surface on flow back rather than stay in theformation where they could hamper production. This means afaster cleanup that reduces liquid damage to water-sensitiveformations. Also, it may eliminate the need to swab the well afterthe treatment.
Improved solids transport- Another advantage of foamed acid isits capability to suspend fines. Often in conventional acidtreatments, large amounts of insoluble fines such as quartz,gypsum, and feldspars will be left behind because of the low
5viscosity of the spent acid. This may reduce fracture conductivity,but with the additional viscosity provided by foaming, more ofthese fines are suspended and removed from the well duringcleanup.
Less formation damage- Foamed acid has a low liquid content.Normally, foamed acid is 60 to 80 quality. Less liquid contacts theformation, thus reducing the opportunity for damage to occur.
Minimum well shut-in time- Foamed acid treatments should haveminimum well shut-in time after pumping. The foamed acidshould be flowed back as soon as possible following the treatmentto reduce the chance of liquid and nitrogen separation. The longerthe foamed acid is allowed to remain in a static, nonflowingcondition, the easier it is for liquid to drain from the foam bubblesand for suspended fines to settle out of the foamed acid.
Better control- Foamed acid also provides better control. Flow canbe better controlled by adjusting the amount of nitrogen, therebychanging the acids density. Because acid is normally heavier thanthe formation water, acid treatments tend to sink. Foamed acid canbe made to stay higher in the fracture by being less dense than theformation water. Foaming the acid also helps control the reactionrate by reducing its diffusion. Foam increases the viscosity of anacid system, so the acid can be prevented from entering morepermeable or low-pressure zones. This allows for more uniformcoverage without the use of other diverters. Foamed acid can alsocarry any of the conventional diverting systems such as Perf Pacball sealers or granular diverter.
Foamed acid offers other advantages. It has less thermal demand,causing less thermal contraction in the tubing. In cold treatmentconditions, this can save having to reset the tubing due to tubingshrinkage. Nitrogen-foamed acid systems reduce asphaltenesludge by diluting the concentration of carbon dioxide (CO2)formed from acid reactions. In addition, foamed acid treatmentscan be displaced with straight nitrogen, leaving the hole with nohydraulic column to impede load recovery.