Use Of Fine Salt As A Fluid Loss Material In Acid Fracturing Stimulation Treatments

Abstract

Abstract This paper discusses a new secondary fracture fluid-loss material that is being employed successfully in treatments of the San Andres formation (Slaughter Field, TX). The use of fine mesh salt in this reservoir containing less than saturated brine is presented as a substitute for fine mesh sand, providing equal or better production increases as providing equal or better production increases as obtained with fine mesh sand, without production equipment problems commonly associated with post-treatment fine mesh sand returns. Production results post-treatment fine mesh sand returns. Production results of fine mesh salt stimulation techniques are shown and compared with presently popular stimulation technique results. Introduction This paper presents a new stimulation technique that employs fine salt (30/60 and 80/120 mesh) in acid fracturing stimulation treatments. The fine salt acts as a substitute for fine mesh sand when stimulating reservoirs that contain secondary fractures. The purpose for the use of fine salt is to provide the same production increases as obtained by use of fine mesh sand, yet eliminate maintenance costs associated with subsurface and surface production equipment failures caused by production equipment failures caused by post-treatment returns of fine mesh sand. Acid fracturing post-treatment returns of fine mesh sand. Acid fracturing stimulation workovers using fine mesh salt presently are being performed in the San Andres dolomite limestone of the Slaughter Field in Hockley and Cochran Counties, TX (see Fig. 1). Ninety-day after workover results indicate comparable or better production increases than those attained by other production increases than those attained by other presently used techniques, such as 100 mesh sand/acid presently used techniques, such as 100 mesh sand/acid staged hydrochloric acid, and chemically diverting preflush/acid treatments. preflush/acid treatments. THEORY To increase acid penetration distance into the San Andres formation, the fluid leak-off rate into secondary fractures must be reduced. This can be accomplished through the use of fine mesh sand as a secondary fracture fluid-loss material. Local adaptation of this technique consists of pumping a pad of water containing fine mesh sand ahead of each pad of water containing fine mesh sand ahead of each acid stage. Theoretically, the fine sand particles enter the secondary fractures and create a pack or bridge. The fluid leak-off into these secondary fractures is reduced due to the low fluid conductivity of the fine sand pack. Thus the acid will penetrate farther into the formation via penetrate farther into the formation via hydraulically induced fractures. One hundred mesh sand/acid treatments work quite well in practice; however, returns of 100 mesh sand with the reservoir fluids often create production equipment failure. production equipment failure. The fine mesh salt prepack technique shares the same theoretical basis as the use of fine mesh sand. Instead of using fine mesh sand, fine mesh salt is added to a salt-saturated brine and is pumped as a preflush to each acid stage. Laboratory test results preflush to each acid stage. Laboratory test results indicate that fine mesh salt packs have a lower fluid conductivity than fine mesh sand packs. After treatment, any fine mesh salt returns will be dissolved by the less then salt-saturated reservoir water before arrival at the subsurface pump, thus eliminating production equipment failure due to returning fine sand. Also, dissolution of the fine salt removes the potential for flow restriction in the secondary fractures during production that may occur when using an insoluble material such as fine mesh sand. Although the fine mesh salt/acid technique is being applied to a waterflood situation, there is no reason why the treatment technique would not be beneficial to many primary recovery wells if the formation has mobile water that is less then salt saturated. LABORATORY FINDINGS Physical characteristics of pure sodium chloride salt are presented in Table 1. The various commercial salts used in fine mesh salt/acid treatments in the Slaughter Field contain 94 to 98% sodium chloride. Table 2 (Table 2A for SI metric units) compares the solubility of sodium chloride salt in various strength hydrochloric acids.