Summary

Barium sulfate scale occurrence is a severe production problem in North Seaoil operations. Barium sulfate is often accompanied by strontium sulfate toform a completely mixed scale called (Ba, Sr)SO4 solid solution. This paperdescribes a laboratory study carried out at 70 degrees C to examine (Ba, Sr)SO4solid-solution scale formation in porous media and the formation damageresulting from the mixing of two incompatible waters. The paper is acontinuation of the previously reported room-temperature work. paper is acontinuation of the previously reported room-temperature work. Results ofexperiments carried out at the elevated temperatures again demonstrate thatsubstantial scale deposition can occur in a rock core and can causeconsiderable decline of rock permeability as a result of concurrent flowing oftwo incompatible waters.

Introduction

Scale deposition in waterflooding operations often results from theincompatibility between injected saline water and reservoir connate water. In North Sea offshore developments, sulfate scales, particularly barium sulfateand its mixture with strontium sulfate, are particularly barium sulfate and itsmixture with strontium sulfate, are the main scaling concerns. Sulfate-anion-rich seawater injected into the reservoir formation subsequentlymixes with formation water, which contains excessive barium, strontium, and/orcalcium cations. A limited number of works investigated potential formationdamage arising from the mixing of two incompatible waters. The work reportedhere was carried out as part of a major research program in oilfield scale. Ourearlier paper presented the results of laboratory experiments carried out atroom temperature to investigate systematically barium and strontium sulfatesolid-solution scale formation in rock pores. A large permeability reductionwas caused by scale deposition in rock cores. Permeability damage and scalingcrystal morphology were found to be influenced by sulfate supersaturations andscaling ion concentration ratios in the brines, and permeability loss also wasdependent on initial rock permeabilities.

This paper describes subsequent experiments carried out at 70 degrees C witha new multipressure tapped steel core holder to study (Ba, Sr)SO4 scaleformation in porous media under the influence of flow and in static bulksolutions. Two incompatible waters were injected concurrently into a core tostudy scale formation in porous media, and static scale precipitation testswere performed in glass jars. Besides simple precipitation tests were performedin glass jars. Besides simple artificial brines used in the room-temperaturetests, a full-component synthetic North Sea water and two formation waters wereused in elevated-temperature experiments to simulate field scale formation. Oneformation water (Water M) had medium scale precipitation when mixed withseawater; the other (Water ST) had severe scaling tendency. Permeability andporosi-ty changes caused by scaling, scale distribution, and scaling crystalmorphology were studied and brine effluents were analyzed to monitor thescaling ion concentration change during a test. Such influencing factors asbrine composition, temperature, and initial rock permeability are presented. Ascaling mechanism involved in scaling permeability are presented. A scalingmechanism involved in scaling crystal nucleation, precipitation, growth, anddeposition in rock pores is suggested from the experimental results.

Experimental Methods

Experimental Rig. Fig. 1 illustrates the experimental rig for scaleformation tests under dynamic conditions in porous media. The equipment usedwas the same as that used in the room-temperature tests, except the clamp coreholder was replaced by a multipressure tapped core holder installed in atemperature oven. The two water inlets connected to the core holder were3.5-m-long coils, so the injected fluids were heated to 70 degrees C beforethey entered the core holder.

Preparation. Clashach sandstone was the core material used in the dynamic Preparation. Clashach sandstone was the core material used in the dynamic coreflow tests; 76.2 × 25.4-mm core plugs were used. For each core test, asaturated core plug was put into the steel core holder and an injectiondistribution device was then placed in front of the core plug for distributingtwo incompatible waters across the front face of the core. Four pressuretapping holes on the core holder rubber sleeve were spaced 6, 28, 50, and 70 mmfrom the front of the core.

Brine Specification. The simple artificial brines used in the 70 degrees Ctests were of the same compositions as those used in the 20 degrees C study(Table 1). Each labeled brine indicates the equal-volume mixture of twoincompatible waters. One was made by dissolving sodium sulfate and sodiumchloride salts into distilled water to be sulfate-rich; the other was made bydissolving barium chloride, strontium chloride, and sodium chloride salts intodistilled water to be barium-and strontium-ion-rich. Table 2 gives thecompositions of the synthetic full-component North Sea injection water, whichcontains excessive sulfate anions, and the synthetic full-component fieldformation waters, Waters M and ST, which contain excessive barium and strontiumions. A comparison of Tables 1 and 2 indicates that more complex ions werepresent in the field waters.

The compositional characteristics of the brines are expressed by sulfatesupersaturation, solution ionic strength, and the concentration ratio betweenthe scaling ions. The supersaturation, S, of a scaling sulfate mineral, MX, ina brine is defined as the ratio of the square root of the concentration productof scaling Cation M and scaling Anion X in a solution to the square root of the solubility product of MX,

S = (mMmX) 1/2/QMX,

where mm and mx are the molalities of scaling Cation M and scaling Anion Xin the brine, respectively, and Qmx is the square root of the solubilityproduct of Sulfate MX. Solution ionic strength, I, is a measure of brinesalinity:

where Ci is the molal concentration of ith ion in the solution and zi is itselectrical charge.

Table 3 presents the compositional characteristics of the simple brines, each made of a 50:50 mixture of two incompatible waters. Table 4 gives thecompositional characteristics of the 50:50 mixed seawater and formation waters. Table 3 shows that initial BaSO4 supersaturations were always 8.4 at 70 degrees C or 15 at 20 degrees C, and molal ratios of the scaling cations (Ba2+ +Sr2+)to scaling anion SO42-, were unity in all the simple brines, while molalratios of Sr2+ to Ba2+ varied among 0.1, 1.0, 100, and 1,000 in Brines BSS0, BSS1, BSS2, and BSS3, respectively, to form a series of (Ba, Sr)SO4 solidsolutions.

Test Procedures. Beaker Test. For each experiment of (Ba, Sr)SO4 crystalgrowth in static bulk solution, 100 mL of two filtered opposite waters werepoured simultaneously into a clean glass jar followed by moderate manual pouredsimultaneously into a clean glass jar followed by moderate manual shaking. Themixed brine was left undisturbed for 24 hours to allow crystal growth.

SPEPE

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