Propped fracturing

The fracture must be wide enough to permit entry of proppant to a distance sufficient to stimulate production. Tons of proppant are normally required to fill this void. Therefore the fracturing fluid must suspend the proppant long enough for it to be transported and placed, by flow, throughout the fracture. To preserve the maximum accessible flow area, the proppant should be uniformly suspended inside the entire propped fracture area while the fracture closes. Kaspereit(15), as well as Smith(16), has made the point that fracture conductivity can be a limiting factor. If the... 

The migration of iron mineral fines, primarily hematite and magnetite, is a common occurrence in portions of the Appalachian Basin. The phenomenon often occurs after well stimulation and can result in the continuing production of iron mineral fines which pose a significant disposal problem. The migration of iron mineral fines through propped fractures can substantially reduce the fracture flow capacity. Many of these are mineral fines are native to the formation and are not formed by precipitation of acid-soluble iron salts present in injection waters during or after acidi-... 

Cipolla, C.L. and Lee, S.J. "The Effect of Excess Propped Fracture Height on Well Productivity," SPE paper 16219, 1987 SPE Production Operations Symposium, Oklahoma City, March 8-10. 

One of the most important factors in the effectiveness of the hydraulic fracturing treatment is the ability to predict the settling velocity of proppant under fracture conditions. The transport of proppant and the final distribution of proppant in the fracture highly depends on the accurate estimation of settling velocity of proppant. The length of the propped fracture, the conductivity of the propped fracture, and height of the propped fracture are consequently affected by the settling velocity. 

Many shale gas service companies use groundwater pumped directly from the formation or treated water for their fracturing jobs. In some well stimulations, proppants are not needed to prop fractures open, so simple water or slightly thickened water can be a cost-effective substitute for an expensive polymer of foam-based fracturing fluid with proppant (Ely, 1994). Hydraulic fracturing performance is not exceptional with plain water, but, in some cases, the production rates achieved are adequate. Plain water has a lower viscosity than gelled water, which reduces proppant transport capacity. 

Low acid volumes should be used in poorly consolidated formations, low-permeability formations, and formations with high clay content. In poorly consolidated sands, excessive HF can cause sloughing or collapse of perforations and, possibly, sand production. Excessive HF in low-permeability formations and in formations with high clay content can cause severe plugging because of reprecipitation of dissolved solids. Formations with native permeability less than 10 mD can be acidized with care. However, hydraulic (propped) fracturing may often be the stimulation method of choice. 

As mentioned in chapter 2, fracture acidizing is an alternative to hydraulic (propped) fracturing in carbonates. With fracture acidizing, fracture conductivity is achieved by using acid to etch the walls of the created fracture. With hydraulic fracturing, fracture conductivity is achieved by filling the fracture with solid proppant to hold it open. 

Fracture acidizing can be used to bjqjass formation damage in a carbonate formation. For damage b)qjass, a long fracture may not be needed. The purpose would be similar to that of a frac-and-pack procedure in a sandstone formation. In that case, a propped fracture with length and conductivity suffident to effectively extend the wellbore radius beyond a damage zone is all that is needed— not an extensive, propped fracture. 

Most carbonate acid treatments are conducted to remove or bypass formation damage, either real or perceived. If damage is not present, fracturing with acid or with proppant should be considered. If damage is present and believed to be very severe or very deep, fracture acidizing and propped fracturing are the preferred options. 

There are no set guidelines for choosing between acid fracturing and propped fracturing. Historically, the choice often has been based on individual or collective logic. This results from experience with previous treatment response in the same field or under conditions that might be considered similar. Production response is the best criterion for deciding between the two stimulation methods. Relative cost-effectiveness (value) is also a factor, as it should be. 

A predominantly naturally fractured carbonate formation, potentially leading to propped-fracture compHcations... 

Propped fracturing has an advantage for many carbonates. The leak-off coefficient, fracture shape, and proppant conductivity can be estimated or measured with a greater degree of confidence than for an acid fracture. This is because the fluid is nonreactive. A pretreatment data measurement frac, or mini-frac analysis, is an established technique used to generate appropriate design parameters for propped fracturing. 

In deep formations with high closure pressure, proppant may create a more conductive fracture than can be retained after closure of an add-etched fracture. This is also true in shallow, soft carbonates. A propped fracturing treatment may create a longer effective fracture length because fluid properties, especially leak-off, are not compromised by reactivity. 

In naturally fractured carbonates, propped fracturing may not be appropriate because of the difficulty in placing necessary amounts of proppant. The tortuous paths often present and complex stress properties can result in fracture geometry that is convoluted that it becomes impossible to maintain proppant injection. 

It is important to evaluate the effectiveness of every stimulation treatment. Evaluations of matrix addizing and add fracturing treatments are usually based on production increases or comparison with other wells. With matrix acidizing, comprehensive pretreatment and posttreatment well testing and interpretation are usually not economically justified. Comprehensive evaluation can be justified most of the time with acid fracturing and propped fracturing. 

Unfortunately, evaluation of an add fracturing treatment is more difficult than evaluation of a propped fracturing treatment. This is because of the complexity of leak-off in add fracturing. With propped fracturing, leak-off of the nonreactive fracturing fluid can be more easily predicted and modeled. 

---