Hydraulic fracturing involves literally breaking or fracturing a portion of the surrounding strata, by injecting a specialized fluid into the well bore directed at the face of the geologic formation at pressures sufficient to initiate and extend a fracture in the formation. Certain commonly used fracturing treatments generally comprise at least three principal components: a carrier fluid, a polymer, and a proppant. Many further comprise a crosslinker.
The purpose of these fracturing fluids is to first create and extend a fracture, and then once it is opened sufficiently, to deliver proppant into the fracture, which keeps the fracture from closing once the pumping operation is completed. The carrier fluid is the means by which proppant and breaker are carried into the formation.
A typical fracturing fluid can be prepared by blending a polymer, often a polysaccharide, with an aqueous solution. The purpose of the polymer is to increase the viscosity of the fracturing fluid and to thicken the aqueous solution so that solid particles of proppant can be suspended in the solution for delivery into the fracture. If a crosslinking agent is added to the fracturing treatments, the agent further increases the viscosity of the fluid by crosslinking the polymer.
In addition to being useful in fracturing, fluid that comprises a polymer and crosslinker can also be useful in the workover of a hydrocarbon production well to improve production. After the treatment, a gel formed by the workover fluid can be intentionally degraded or remain as a permanent plug.
Fracturing fluid must be chemically stable and sufficiently viscous to suspend the proppant while it is sheared and heated in surface equipment, well tubulars, perforations and the fracture; otherwise, premature settling of the proppant occurs, jeopardizing the treatment. Crosslinkers join polymer chains for greater thickening, but in certain instances, a delay in crosslinking is advantageous.
For example, a delayed crosslinker can be placed downhole prior to crosslinking; the gel fluid is prepared on the surface, then crosslinks after being introduced into a wellbore which penetrates a subterranean formation, forming a high viscosity treating fluid therein. The delay in crosslinking is beneficial in that the amount of energy required to pump the fluids can be reduced, the penetration of certain fluids can be improved, and shear and friction damage to polymers can be reduced. By delaying crosslinking, crosslinkers can be more thoroughly mixed with the polymer fluid prior to crosslink initiation, providing more effective crosslinks, more uniform distribution of crosslinks, and better gel properties.
U.S. Pat. No. 5,145,590 to Dawson discloses a solution and method of use for providing controlled delay and improved high temperature gel stability of borated fracturing fluids.
Some of the primary delayed borate fluid systems used in United States, currently the largest fracturing market, have several deficiencies. Certain fluids are crosslinked with suspensions of finely ground anhydrous borax in petroleum distillate carrier fluids, suspended by polymers. These systems, due to their suspension flow characteristics, cannot be pumped with most equipment and are generally unsuccessful when used with existing blenders and mixers. Modifications of existing equipment or the use of skids specifically designed for pumping specific high viscosity fluids have typically been required. Still, a recurrent problem is posed by the formation of scale and borate deposits on the surfaces of lines and equipment that can result from the presence of moisture in the system leading to the crystallization of borate hydrates. There have been numerous major failures in field operations relating to the difficulty in pumping such systems.
To avoid failures of the pumping and mixing equipment handling these borate fluid systems, it is important to transport and place the fluid systems downhole before they begin crosslinking. However, a crosslinker that is suspended in a hydrocarbon-based oil will raise environmental concerns. For example, oil-containing liquids are likely to fail two key US Environmental Protection Agency (EPA) tests for use in the Gulf of Mexico: EPA Method 1664, Oil and Grease, and EPA Part 435/Appendix A/Supbpart 1: Static Sheen.
Borate systems that pass the key EPA tests described above can still present pumping and mixing problems if the crosslinker suspension is too viscous. U.S. Pat. No. 6,743,756 to Harris teaches liquid suspensions of particles in non-aqueous liquids such as polyglycol that are said to resist settling or separation of the suspended solids over long periods of time. However, polyglycol based suspensions are not sufficiently flowable to permit the use of pumps, mixing equipment and tanks that are typical in well treatment fluid service.
It can thus be seen that there is a need in the art for a crosslinker that can be processed without introducing oil into the treatment fluid, that is in the form of flowable liquid or pumpable suspension, that can pass the two key EPA mandated tests for use in the Gulf of Mexico, and that can inhibit scaling and plugging associated with borate crosslinker systems.