Insight Into Strontium and Calcium Sulfate Scaling Mechanisms in a Wet Producer

Abstract

Summary On-going field scaling tests have provided a unique opportunity to study how strontium sulfate and calcium sulfate scales are formed in an oil-producing well. Results indicate that development of nucleation sites on tubing surfaces is the most critical step. Scale profiles within a wellbore are determined mainly by kinetics of crystallization, well-fluid dynamics, and tubing surface characteristics and, especially, synergistic effects of these factors. Introduction Scaling is a problem frequently encountered in oil fields. Calcium carbonate, calcium sulfate, and barium sulfate scales have been cited often in the literature. More recently, strontium sulfate scaling problems have been reported from oil fields in the problems have been reported from oil fields in the U.A.E. and Algeria. Mixing of chemically incompatible waters as a result of waterflooding is a primary cause for concern. Literature studies relating to the prognosis, actual occurrence, prevention, and control of oilfield scaling have been reviewed. Many factors are involved in oilfield scaling. In addition to thermodynamic considerations involving temperature, pressure. and water compositions, other factors, such as adherence. kinetics of crystallization, fluid dynamics in a producing well, and the adsorption of foreign material onto growing crystal surfaces may all be important in scale buildup. Better understanding of scaling mechanisms will help improve methods for scale prediction and mitigation. Unfortunately, direct studies of scaling mechanisms under actual or simulated field production conditions are limited. The Arabian American Oil Co. (ARAMCO) has recently concluded a series of simulated field scaling tests. The tests were designed to determine the extent of potential scaling problems that may occur as a result of potential scaling problems that may occur as a result of the seawater flood initiated in June 1978. The seawater flood, the initial Wasia waterflood, and the connate Arab-D formation water, form a three-water system with a possible incompatibility problem. The study of this problem was reported by Lindlof and Staffer. Since problem was reported by Lindlof and Staffer. Since then, additional tests have been carried out to determine the optimal method for scale inhibition. The results have provided extensive insight into the scaling mechanisms provided extensive insight into the scaling mechanisms in a wet producing well. Field Scaling Tests The initial sequence of field scaling tests concluded that sulfate scaling encountered in one of the four test wells proved that there is a potential scale problem when the three incompatible waters are allowed to mix in the wellbore. Subsequent tests, designed to mitigate or to prevent scale formation, were carried out on the scaling well discovered in the initial tests. The test well (Fig. 1) is produced through a 4 1/2-in. production string landed at the midpoint of a production string landed at the midpoint of a 170 × 6 1/4-in. open hole interval. The production string is set in a 9 5/8 × 7-in. casing configuration without a production packer. production packer. JPT p. 1249