Mitigation of Calcium Sulfate Scale Formation When Seawater Is Used To Prepare HCl-Based Acids

Abstract

The damaging effect of calcium sulfate precipitation on the permeability of carbonate cores when mixing hydrochloric acid with seawater for matrix acid treatments has been identified in our recent work (SPE 143855). The objective of this work is to mitigate calcium sulfate precipitation by using a suitable scale inhibitor in hydrochloric acid. Another objective is to determine the scale inhibitor type, concentration, and whether it is needed in the preflush or post-flush stages. Core flood tests were conducted using Austin Chalk cores(1.5 in. × 6 in.) with a permeability of 5 md, to investigate the effectiveness of scale inhibitor. A synthetic seawater was prepared according to the composition of seawater in the Arabian Gulf. Calcium, sulfate ions, and scale inhibitor concentrations were analyzed in the core effluent samples. The minimum concentration of scale inhibitor was determined over a wide range of temperatures (77 to 210°F). A scale inhibitor (sulfonated terpolymer) was found to be compatible with hydrochloric acid systems, and can tolerate high concentrations of calcium (30,000 mg/l). Analysis of the core effluent indicated that the new treatment successfully eliminated calcium sulfate scale deposition. The concentration of scale inhibitor ranged from 20 to 250 ppm, depending on the scaling tendencies of calcium sulfate. This work confirms that an appropriate scale inhibitor can be added to acid, to avoid calcium sulfate precipitation when seawater is used to prepare hydrochloric acid for matrix acidizing.

Fines Migration: Problems and Treatments

“…It can be governed by mineralogy and permeability of the rock, salinity of the water (either injected or produced water), pH value, or even by the drag forces, which is a combination effect of flow velocity, turbulence and fluid viscosity [10][11][12][13][14][15]. Figure 1 shows a typical scenario of fines migration.…”

“…In the realm of formation damage, fines have been widely to be considered one of the main formation damage problems. Because they are loose in nature and cannot be placed in place or be bound together with sand grains physically by cementing materials [2][3][4][5]. Under certain circumstances, those formation fines, initially located in the interior surface of formation matrix, will migrate along flowing fluids in the formation.…”

“…Commercial service companies have developed and applied UTTA very successfully in recent years. This paper discussed the mechanisms of UTTA and experimental data and field cases were present for better understanding [10].…”

Effective Strategies for Selection of Suitable Clay Stabilizers to Control Clay Swelling

“…Osmotic repulsive pressure exists between clay particles when the total amount of ions in the double layer of clay minerals is more than that of in the solution. In this case, water in the pore space will enter into double to dilute the concentration [16]. As the electric filed acts as a semi-permeable membrane, water can enter into the double layer but the exchangeable cations cannot leave the layer.…”

“…In most cases, illites are interlayered, which makes them hard to stabilize and more easily to be dispersed in the formation. Smectite is the most sensitive expandable clay mineral to brine salinity, which can cause severe loss of permeability and micro porosity [2][3][4][5]. The formation damage issues caused by clay minerals can be due to chemical reactions or physical processes, which are determined by many factors, including mineralogy and chemical composition, mineral abundance, mineral size, etc.…”