Friction Pressure Reducers in Well Stimulation

Abstract

Reducing friction pressure by the deliberate use of additives was stimulated by field observations of this phenomenon in fracturing and acidizing operations. The most commonly used additives have been used for other advantages for nearly 10 years. The mechanics of friction pressure reduction are explained. A brief outline of the laboratory testing of friction pressure reducers is presented, and a method of translating these data to field operations is discussed. A survey is made of the five types of friction reducers now in use. These agents are compared according to their erect on apparent viscosity, leak-oft control and friction reduction. It is concluded that friction reducers should be chosen on the basis of all properties affecting well stimulation. Introduction Friction pressure reduction, resulting in lower surface treating pressures or increased injection rates, is a comparatively new technique in well simulation. The advertisement and application of materials specifically for this purpose is about five years old, although the phenomenon had been reported in the literature a few years earlier. Also, the additives first used (and still occupying a predominant position in the market) have been observed by field personnel to exhibit low friction pressure since 1954. However, these additives were then being used as sand suspending and leak-off control agents. The realization that the use of friction reducers would permit the higher injection rates required for more successful stimulation treatments at reasonable equipment costs instigated the search for better and cheaper agents. At least two significant developments have resulted from this search; the discovery and use of several new additives, and new knowledge of this phenomenon has been gained. Also, the use of the original additives has increased greatly. The major points of this discussion are:a look at the mechanics involved;an explanation of one method of translating laboratory results to field use;a comparison of the additives presently available; andsome considerations in choosing the right additive. Mechanics of Friction Reduction The characteristics of fluid flowing through a conductor can be compared to traffic on a highway. Visualize a multi-lane superhighway with a bumper-to-bumper flow of traffic in all lanes. The surface of this highway is iced over. So long as the speed of the automobiles does not exceed a certain velocity, the drivers can keep them proceeding straight ahead. Above this velocity, however, enough traction is lost to cause the cars to swerve. As speed is increased, swerving increases with a resultant increase in collisions caused by cars sliding into other lanes. More cars travel along the highway under the latter conditions, but energy losses due to random motion are much greater. This is analogous to the change from laminar through transition to turbulent fluid flow patterns. Figs. 1-A and 1-B, depicting longitudinal cuts through pipe sections, represent laminar and turbulent flow patterns. If flexible dividers are placed between the lanes of the above highway, nearly all of the swerving automobiles are retained within their own lanes. Also, most of the collisions are between cars and flexible material. This tends to keep the vehicles proceeding straight ahead and greatly increases the flow of traffic. Less energy is lost to random motion and to collisions. This represents the effect of friction pressure reducers in dampening turbulence by the orientation of long polymers parallel to the fluid laminae. This type of flow might be represented by Fig. 1-C.Another factor to which it seems logical to credit some of the efficiency of friction reducers is the minimizing of pipe roughness. It is in the turbulent velocity region that friction reducers are most effective; and this is the how pattern under which pipe roughness becomes the most important factor in friction pressure. It has been observed that friction reduction in galvanized pipe is greater than in the smoother stainless steel drawn pipe. Fig. 2 illustrates the effect of relative roughness on friction factor f. Predicting Friction Reduction From Laboratory Data Laboratory data generally cannot be translated with extreme accuracy to field operations in well stimulation. JPT P. 865ˆ