Wells – Processes – Placing fluid into the formation
Reexamination Certificate
2002-07-10
2004-11-23
Neuder, William (Department: 3672)
Wells
Processes
Placing fluid into the formation
C507S903000, C507S922000
Reexamination Certificate
active
06820694
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates generally to a method for forming a fracturing fluid and a method for fracturing subterranean formations. More specifically, the invention relates to a method for fracturing oil, gas and/or water bearing formations using a novel method of preparing a suitable fracturing fluid.
2. Description of the Prior Art
Fluids based on crosslinked polymer solutions are commonly used to create hydraulic fractures in subterranean formations. Typically a solution of the polymer is made in the mix water, a crosslinking agent is added to the fluid along with any pH buffers, activators, or delay agents required for the specific job. This fluid is subsequently pumped down a wellbore.
In practice one of the most difficult steps in the preparation of a fracturing fluid is the dispersion and hydration of the polymer in the mix water. A dry polymer powder, a liquid emulsion, or a liquid suspension of the polymer in a carrier fluid, is added to the mix water in a high shear environment—such as in a centrifugal pump, or in a high-speed blender. The high shear is required to rapidly disperse the polymer throughout the mix water—and to assist in the stripping off any hydrophobic oils or surfactants that are stuck to the polymer particles surface. If the dispersion is incomplete, clumps of partially hydrated materials can form (commonly referred to as “fish-eyes”) and seriously impede the full development of viscosity. Large fish eyes can cause plugging problems in the process stream and potentially damage fracture conductivity.
Once the polymer is dispersed, the hydrophilic polymer slowly unravels until it is fully hydrated (i.e., it reaches an equilibrium solvation which is determined by the particular polymer concentration and polymer properties, fluid pH, fluid ionic strength, etc.). The kinetics of hydration are typically slow—on the order of many minutes—and also depend significantly on the temperature, particular chemical composition, ionic strength and pH of the mix water. As the polymer hydrates the fluid builds viscosity. In general it is best that the polymer is fully hydrated, or close to full hydration, prior to the addition of crosslinkers and/or crosslinker activators. If the polymer crosslinks prior to full hydration the overall performance of the fluid is significantly reduced. Operational procedures, equipment design and the order of addition of chemical components must accommodate the relatively slow kinetics of polymer hydration—especially when continuous mix procedures are being followed. In order to accommodate the relatively slow polymer hydration kinetics, fluid blenders and work tanks with large volumes (150-300 bbl) are used to hydrate the gel. This requirement increases the equipment requirements of a hydraulic fracturing treatment. Furthermore, if the treatment is not pumped to completion—the left over fluid in the hydration unit becomes a disposal problem.
Additives that have limited impact on polymer hydration, such as biocides clay stabilizers, and temperature stabilizers, may be added to the mix water prior to the addition of the polymer. Sometimes pH buffers or modifiers (usually on the mildly acidic side) may be added to accelerate the rate of polymer hydration. However, with previous polymeric fluid systems, additives such as crosslinkers and crosslink activators are not added prior to polymer hydration or partial hydration or they are added in an inactive state, such as boric acid.
FIG. 1
shows a schematic of the equipment required for performing continuous mix hydraulic fracturing treatments. In these treatments large volumes of fluid (~100-20,000 bbl) are pumped at rates up to 100 bbl/min. Typical rates are in the range of 10-50 bbl/min (420-2100 gal/min). The triplex pumps used in hydraulic fracturing treatments can generate extremely high discharge pressures, but are not very efficient in suction—that is they have a net positive suction head requirement when pumping at high rates. Therefore, the triplex pumps have to be primed with fluid prior to a treatment, and it is critical that prime be maintained throughout a treatment (i.e., there is no interruption in fluid flow). If a pump looses prime, it can be extremely difficult to bring it back on line in order to successfully complete the treatment. If many or all pumps loose prime—it can mean the early termination of the treatment.
Centrifugal pumps and/or blenders that act as pumps are used to prime the triplex pumps during a treatment. Usually they develop less than 200 psi on their discharge sides (60-100 psi is typical) to feed the triplex pumps. Although somewhat better at suction then the triplex pumps, these pumps are fed primarily by gravity from the water storage tanks on location (occasionally booster pumps and/or head tanks are added in line to assist in moving fluid and keeping the prime of the blender).
Therefore, flow assurance throughout the surface equipment is critical during hydraulic fracture execution. Any interruption of fluid transfer during execution, anywhere in the system, can and often results in early and unsuccessful termination of a fracturing treatment. Flow assurance problems are exacerbated when the fluids have high shear and/or extensional viscosities, or if by their physical properties they are difficult for the pumps to handle. Hydrated polyacrylamide solutions at concentrations greater than 10 lbm/1000 gal active polymer weight are an example. These fluids have very high extensional viscosities, which makes them difficult to pump with conventional surface equipment such as centrifugal pumps, vortex pumps, and blenders.
The primary criteria for designing and formulating fracturing fluids are set by the physical properties of the formation. The temperature, permeability, fluid saturations, salinity, mineralogy, and mechanical properties of the rock in the target formation are prime considerations when selecting or formulating a fracturing fluid for a given treatment. For example, most fluids are selected to have sufficient viscosity to transport proppant and create fracture width for a time equal to or greater than the duration of the treatment. Considerations related to flow assurance—ease of mixing and compatibility with equipment—are usually of secondary importance. That is, when faced with a choice, blending, pumping equipment, or operational procedures are modified to handle the fluid. The more demanding the bottom hole conditions—the less flexibility there is in optimizing the ease of well site delivery of the fluid. Fluids designed for formations with bottom hole static temperatures (BHST) in excess of 300° F. are especially challenging with respect to flow assurance. These fluids typically have high polymer loadings (in excess of 20 lbm/1000 gal), pH buffering packages and added high temperature stabilizers in order to maintain viscosity at these high temperatures.
SUMMARY OF THE INVENTION
The present invention describes a method for preparing a fracturing fluid, which method may be used to continuously mix fluids that meet the requirements for use high temperature formations, while at the same time significantly improving the flow assurance during the well site delivery. In this method the polymer is added to a fluid stream and hydrated subsequent to or simultaneously with other additives, which may be present in the fluid stream. The fluids prepared by this method have excellent performance and problems associated with flow assurance are minimized.
Initially, a water stream is provided as the basis for the fluid. Any suitable water source may be used in conjunction with the present invention. The specific type of water required, such as brackish water or “city” or municipal water, is determined by the specific type and characteristics of the fluid being prepared. For example, the salt content of the water may affect the rheologic characteristics of the fluid. Therefore, the water source should be compatible with the desired characteristics of the fracturing fluid. Additives may be used to alter or modify
Nagl Michaela
Willberg Dean
Nava Robin
Neuder William
Schlather Stephen
Schlumberger Technology Corporation
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