Improvement of Well Stimulation Fluids by Including a Gas Phase
Authors: Foshee W.C. , Hurst R.E. 
Published: December 2007

Addition of liquified nitrogen and carbon dioxide to stimulation fluids to aid in well clean-up following treatment has become common practice in some areas. Unfortunately, in many cases, the added expense of the gas cannot be economically justified on the basis of improved fluid returns alone. However, recent laboratory and field studies have shown that multiphase oil-water-gas mixtures exhibit fluid-loss control and other characteristics which more than justify the use of gas. The purpose of this paper is to describe the properties of multiphase mixtures and show how and where they may be desirable.

Introduction

Gas has been used as an aid to stimulation treatments for many years. In its original form, this technique consisted of injecting natural gas (compressed by mobile field compressors) into a subsurface formation, together with liquid well-stimulation chemicals, to improve well productivity. The object was to lighten the fluid column sufficiently so that, during the "clean-up" of stimulation fluids following the treatment, the formation energy would be sufficient to flow these fluids from the wellbore, without using swabs or other mechanical clean-up tools. A more recent innovation has been the use of liquefied nitrogen or carbon dioxide gas for this same purpose. This has been particularly valuable in conjunction with acidizing or fracturing treatments, especially in reservoirs having low bottom-hole pressure. Results of treatments utilizing these techniques on several hundred wells have been encouraging, with stimulation fluids being returned to the surface following more than 75 per cent of the treatments, without the use of mechanical aids. Unfortunately, in many cases, the added expense of adding gas to the stimulation liquids cannot be economically justified merely on the basis of improved fluid returns following treatment. However, laboratory and field studies have revealed additional benefits which more than justify the use of gas.

Fluid-Loss-Control Properties

Laboratory tests have shown that a multiphase oil-water-gas mixture has definite fluid-loss-control characteristics even without conventional solid fluid-loss additives. Furthermore, the degree of fluid-loss control can be regulated by the proportions of gas and liquids used, and by the concentration of non-emulsifying surfactant included to improve phase dispersion. Fig. 1 is a photograph of the laboratory equipment used to conduct dynamic fluid-loss tests. The individual components are identified in Fig. 2, which is a schematic diagram of the system. In conducting these tests, the system was first cleaned out thoroughly and then filled with a liquid, suitable for conducting a permeability determination. Pressure was applied to the system through the accumulator (to prevent extraneous gas from entering the system), and the permeability of the core in the core holder was determined by the volume of fluid escaping through it. Circulation of the single-phase fluid, during these preliminary tests, showed no effect on the observed rock permeability values.

JPT

P. 768ˆ