The Comparison of Non-Aqueous and Aqueous Scale Inhibitor Treatments: Experimental and Modeling Studies

Abstract

Summary This paper describes results from a series of comparative corefloods and static compatibility tests examining the differences in laboratory-test procedure, scale-inhibitor (SI) returns, and modeling approaches for nonaqueous and aqueous SI treatments. Two types of nonaqueous systems, one ethylene glycol (EG) -based and two oil-soluble products, each containing penta-phosphonate SIs, were investigated. Detailed compatibility and injectivity tests were carried out before coreflooding, and a carefully designed application/treatment process was required as a result of the hydrophobic nature of these products. To understand the SI-transport and -retention mechanisms for these nonaqueous systems, comparisons were made with the corresponding aqueous applications. These comparisions were made in terms of SI-return performance, flowback permeability, possible formation damage, and changes in the wettability conditions that might account for any post-treatment differences. In addition, approaches to mathematically modeling these corefloods were studied. This paper focuses on the application of a partitioning model in a standard reservoir simulator. An alternative two-phase mathematical model for such systems, which includes both interphase partitioning and adsorption, has been described in detail in another recent publication (Guan et al. 2004). All corefloods were performed with outcrop Clashach sandstone material rather than reservoir cores, and hence, the advantages of deploying nonaqueous treatments over the conventional aqueous treatments might not be evident. However, the experimental and modeling results help to capture the main transport and retention mechanisms of these nonaqueous systems in an understandable way. Results confirmed the existence of complex phase behavior during the corefloods with the two oil-soluble products. Examination of the core after flooding with an environmental scanning-electron microscope (ESEM) indicated decreased water wetness following the two oil-soluble SI treatments compared with the aqueous treatment. Numerical modeling results show that the behavior of this system is most consistent with the assumption that the SI in the nonaqueous system is only slightly soluble in the oil phase during the oil post-flush. Introduction In oil fields where seawater injection has been used for pressure maintenance and hydrocarbon sweep, scale formation has often been experienced in producer wells. Mineral scale may lead to significant production decline, and its removal once deposited is both difficult and expensive. The application of SI, often in a squeeze treatment into the near-wellbore formation, is regarded as a good method for preventing this problem. Downhole scale prevention generally is carried out by use of aqueous-based SIs (e.g., phosphonates, polyacrylates, and sulfonated copolymers). However, in situations in which relative permeability effects, water blocking, fluid lifting, or deep chemical penetration of the near-well formation are of major concern, aqueous SI squeeze treatments may not be desirable. Indeed, aqueous treatments may lead to impaired productivity, extended cleanup times, and process upsets during the flowback of the treatment fluids. Various researchers have developed an alternative treatment philosophy through the use of nonaqueous SIs, proposed initially in the late 1990s. Although the SI component in nonaqueous packages is usually based on a conventional phosphonate or polymeric species, their delivery systems are quite different, thus leading to transport and retention mechanisms that are also different, as reviewed in a recent publication (Guan et al. 2004). For example, (a) oil-soluble/miscible inhibitors (Guan et al. 2004; Wat et al. 1998a, 1998b, 1999a, 1999b; Jordan et al. 2000; Scott and Littlewood 2000; Jenvey et al. 2000) contain conventional SI products that have been manufactured to be inherently oil soluble before application; (b) in invert-emulsion systems (Collins et al. 2000, 2001; Lawless and Smith 1998; Jordan et al. 2006; Smith et al. 2000), the aqueous SI is deployed in a water-in-oil emulsion; (c) microencapsulated SIs (Scott and Littlewood 2000; Jenvey et al. 2000; Collins et al. 2000, 2001; Lawless and Smith 1998; Jordan et al. 2006; Smith et al. 2000; Bourne et al. 2000) involve the separation of chemical solution from the external environment by a wall or membrane, which is usually a polymer. One of the difficulties in applying nonaqueous SIs is that their detailed transport and retention mechanisms in porous media are much less well understood. This is true for each of these major types of nonaqueous products. Therefore, although several companies have performed successful field applications for their nonaqueous products, squeeze models that allow us to design such treatments in a systematic and routine manner are not yet routinely available.