Hydraulic Fracture

The electrical conductivity of a sediment depends on the soil type, porosity, water content, and pore water composition. Devices to measure conductivity have been combined with a standard electrical cone to give measurements of cone tip resistance, sleeve friction, and friction ratio (ratio of these two). 

Results of two plate load tests in a dense North Sea sand. (After Andresen, A. et al., Marine Geotech., 3,201-266, 1979. Reprinted with permission of Taylor Francis Group.) 

The electrical resistivity method of subbottom exploration/surveying is based on the relationship/correlation between the conductivity of soils and rocks and the ion concentration in their pore waters. The resistivity is, therefore, low in saturated clays, moderate in saturated sands/granular soils, and hi in dense rocks with a few voids, little moisture, and small amount of dissolved salts. 

The sediment resistivity, defined in Equation 4.27 as electrical resistance per unit length of a unit cross-sectional area of the sediment, is calculated from the voltage drop/potential difference of an electric current passing through the soil between two electrodes that are in line and placed at a known distance apart (a). 

A very low-frequency alternating or commutated/direct current, typically 50-100 mA, is normally used to avoid ac stray or dc polarization (i.e., accumulation of hydrogen ions at the negative electrode) effects. 

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Applications. The high heat tolerance and good salt compatibiUty of welan gum indicate its potential for use as an additive in several aspects of oil and natural gas recovery. Welan also has suspension properties superior to xanthan gum, which is desirable in oil-field drilling operations and hydraulic fracturing projects. It is compatible with ethylene glycol, and a welan—ethylene glycol composition that forms a viscous material useful in the formulation of insulating materials has been described (244). 

There are some rare situations where the gas turbine is used to power, through direct linkage, a mechanical unit, e.g., a hydraulic fracture pump. 

Hydroxypropylguar gum gel can be crosslinked with borates [1227], ti-tanates, or zirconates. Borate-crosslinked fluids and linear hydroxyethyl-cellulose gels are the most commonly used fluids for high-permeability fracture treatments. This is for use for hydraulic fracturing fluid under high-temperature and high-shear stress. 

A low-molecular-weight condensation product of hydroxyacetic acid with itself or compounds containing other hydroxy acid, carboxylic acid, or hydroxy-carboxylic acid moieties has been suggested as a fluid loss additive [164]. Production methods of the polymer have been described. The reaction products are ground to 0.1 to 1500 p particle size. The condensation product can be used as a fluid loss material in a hydraulic fracturing process in which the fracturing fluid comprises a hydrolyzable, aqueous gel. The hydroxyacetic acid condensation product hydrolyzes at formation conditions to provide hydroxyacetic acid, which breaks the aqueous gel autocatalytically and eventually provides the restored formation permeability without the need for the separate addition of a gel breaker [315-317,329]. 

Hydraulic fracturing is a technique to stimulate the productivity of a well. A hydraulic fracture is a superimposed structure that remains undisturbed outside the fracture. Thus the effective permeability of a reservoir remains unchanged by this process. The increased productivity results from increased wellbore radius, because in the course of hydraulic fracturing, a large contact surface between the well and the reservoir is created. 

In addition to hydraulic fracturing, there are other stimulation techniques such as acid fracturing or matrix stimulation. Hydraulic fracturing finds use not only in the stimulation of oil and gas reservoirs, but also in coal seams to stimulate the flow of methane from there. 

Table 17-3 summarizes the various types of fluids and techniques used in hydraulic fracturing. 

Certainly the optimal techniques depend on the type of reservoir. Reports that compare the techniques in a related environment are available. In the Kansas Hugoton field (Mesa Limited Partnership), several hydraulic fracturing methods were tested [403]. A method in which a complexed gelled water fracture was applied was the most successful when compared with a foam technique and with older and simpler techniques. The study covers some 56 wells where such techniques were applied. 

The fluid leak-off during hydraulic fracturing can be modeled, calculated, and measured experimentally. Procedures for converting laboratory data to an estimate of the leak-off under field conditions have been given in the literature [1426]. 

However, it has been established that an intense control of certain variables may improve the execution of a hydraulic fracturing job and the success of a stimulation. Therefore an intense quality control is recommended [552,553]. Such a program includes monitoring the breaker performance at low temperatures and measuring the sensitivity of fracturing fluids to variations in crosslinker loading, temperature stabilizers, and other additives at higher temperatures. 

Naturally occurring polysaccharides and their derivatives form the predominant group of water-soluble species generally used as thickeners to impart viscosity to treating fluids [1092]. Other synthetic polymers and biopolymers have found ancillary applications. Polymers increase the viscosity of the fi ac-turing fluid in comparatively small amounts. The increase in fluid viscosity of hydraulic fracturing fluids serves for improved proppant placement and fluid loss control. Table 17 summarizes polymers suitable for fracturing fluids. 

Acrylics, A copolymer of 2-ethylhexylacrylate and acrylic acid is not soluble either in water or in hydrocarbons. The ester units are hydrophobic and the acid units are hydrophilic. An aqueous suspension with a particle size smaller than 10 p can be useful in preparing aqueous hydraulic fracturing fluids [776]. 

Relatively small quantities of a bacterial cellulose (0.60 to 1.8 g/liter) in hydraulic fracturing fluids enhance their rheologic properties [1425]. Proppant suspension is enhanced and friction loss through well casings is reduced. 

Fluid Loss Additives for Hydraulic Fracturing Fluids... 

A defoamer and an antifoamer composition are described for defoaming aqueous fluid systems [1908]. The composition of a typical defoamer for hydraulic fracturing fluids is shown in Table 17-9. 

A hydraulic fracturing fluid containing guar gum or other natural polymers can be stabilized against bacterial attack by adding heterocyclic sulfur... 

Figure 17-12. Biocides for hydraulic fracturing fluids 2-mercaptobenzoim-idazole, 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercapto-thiazoline, 2,5-dimercapto-1,3,4-thiadiazole, and 2-imidazolidinethion.

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