Barium and Strontium Sulfate Solid Solution Formation in Relation to North Sea Scaling Problems

Abstract

Summary This paper presents the results of laboratory experiments carried out to investigate the formation of barium sulfate and strontium sulfate [(Ba, Sr)SO4] solid solution in multi-pressure-tapped cores. Two brines, one barium- and strontium-rich and the other sulfate-rich, were mixed in a core plug. Pressure differentials were measured and the changing permeability distribution along the core length was calculated. The permeability distribution along the core length was calculated. The morphology and chemical analysis of scaling crystals as shown by scanning electron microscopy (SEM) are presented. Results show a large extent of permeability damage caused by (Ba, Sr)SO4 solid solution deposits on the permeability damage caused by (Ba, Sr)SO4 solid solution deposits on the rock pore surface. The rock permeability decline and morphology and size of the scaling crystals indicate the influence of the supersaturations of BaSO4 and SrSO4 and the concentration ratio of barium to strontium ions. Introduction Seawater injection is common in North Sea field developments. The often layered nature of the reservoir results in early water break-through. The chemical incompatibility between injected seawater and formation water makes BaSO4 and related scale deposition possible at various producing wells and facilities in North Sea operations. Injected water may also mix with formation water in the near-wellbore area, causing possible resistance to flow. The presence of strontium and barium ions in some formation waters necessitates the examination of the possible formation damage resulting from solid solution formation of barium sulfate and strontium sulfate. A laboratory investigation of (Ba, Sr)SO4 scale formation in a porous medium was conducted to give insight into the nature of the scale and its effect on rock permeability. Beaker tests were also carried out to compare the morphology of the scaling crystals formed under hydrodynamic forces in a core with that of the crystals formed in a static bulk solution. A novel injection face developed by Goulding was used for distributing two brines separately to the front face of a core plug to ensure that the two brines mixed uniformly immediately after entering the core. The core plug was bound in resin, and the sectional pressure differentials could be recorded along the core by pressure tappings, making a detailed study of permeability decline distribution possible. All the beaker and core tests were performed at a room temperature of 20 degree C. No attempt was made to simulate reservoir pressure in the study. The formation of (Ba, Sr)SO4 solid solution in a porous medium is a complex process that involves nucleation of the scaling crystals, scale precipitation from the solution, and deposition and growth on the pore surface. A number of factors influence this process and therefore the rock permeability decline caused by scale buildup in the rock pores. Analysis of the permeability damage is further complicated by the pores. Analysis of the permeability damage is further complicated by the interaction between the affecting factors. Several of these factors, such as the scaling ion concentration and rock initial permeability, were studied and their influences analyzed. Experimental Methods and Test Procedures Aspects of Investigation. The general purpose of the laboratory test was to gain insight into the scaling mechanism of (Ba, Sr)SO4 solid solution in a porous rock and its damage to rock permeability. Four aspects were considered: supersaturations of BaSO4 and SrSO4 in a brine, concentration ratio of barium to strontium ions, initial rock permeability, and hydrodynamic forces.