Abstract

A new crosslinked fracturing system has been developed for use in both oil and gas producing reservoirs. The system is designed for rapid viscosity reduction upon completion of the treatment. This allows well turnaround and cleanup to be initiated with a minimal shut-in period. This system's compatibility with CO2 (carbon dioxide) and nitrogen make these gases effective additions to stimulation treatments designed for rapid well cleanup. The system's basic chemistry, rheological properties, proppant transport capabilities, viscosity breakout and other data necessary for treatment design are discussed. It is also compared with the more conventional crosslinked and uncrosslinked guar gum fluids. The comparison deals with both fluid properties and differences in the fracture geometry that occur when using each of the hydraulic fracturing fluid systems under identical well conditions.

Introduction

As the demand for oil and natural gas continues to grow, efforts to achieve maximum recoverable reserves increase in importance. These efforts require greater engineering emphasis in all aspects of drilling, completion and well stimulation, especially in the design of any fracturing treatment. Both laboratory studies and field results have verified the importance of fracturing fluids selection. [n addition, field results have shown the need to recover broken fracturing fluid from the well as quickly as possible. These techniques are particularly applicable in tight gas reservoirs containing significant amounts of water-sensitive clays. Positive well results have encouraged the practice of rapid well turnaround (sometimes in less than one hour) and the use of CO2 or nitrogen to boost the recovery rate. To take full advantage of information from case histories and laboratory studies, fracturing systems, compatible with these two gases and capable of rapid breakout, must be developed. A new hydraulic fracturing system, designed to fulfill these requirements can be described in terms of the following aspects;

  1. Fluid chemistry

  2. Fluid rheology

  3. Fluid loss control

  4. Friction pressure

  5. Proppant transport capabilities

  6. Fluid breakout

  7. Fracturing fluid residue

  8. Computer comparisons

FLUID CHEMISTRY

The exact chemistry of this system, although difficult to describe) can be postulated from information that is available on polymers and their cross linking mechanisms. One possible explanation of the chemistry involved in this system is as follows.

The base polymer used in this system is a chemically modified polysacchande (CMP) in which negative or anionic functional groups have been added onto the polymer chain. One advantage of using this particular polymer system is its ability to rapidly hydrate once in solution. Eighty percent of this polymer's peak viscosity is obtained within the first 40 sec of hydration. This phenomenon may be explained by examining the nature of polymers. Wost polymers approach maximum viscosity when the polymer chains are uncoiled and fully extended. Since like charges repel like charges, the anionic functional groups that are substituted onto the base polymer cause the branches of the polymer to uncoil, and the result is a rapid viscosity increase. This rapid hydration rate opens up the possibility of continuously mixing the system.

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