Guido Cappetti', Paolo Fabio and Bruno. ENEL Via Andrea PISA Via G NOVARA

Transcription

1 Cappettiet al. OF ANTIMONY SULPHIDE SCALE BY CHEMICAL ADDITIVES: LABORATORY AND FIELD TEST RESULTS Guido Cappetti', Paolo Fabio and Bruno ENEL Via Andrea PISA Via G NOVARA Key words: Sulphide scaling, Inhibitors, Piancastagnaio field ABSTRACT The PC-26 well, first exploratory deep well in Piancastagnaio field (Mt. Amiata area), was at end of 1970s. It started production in Heavy scaling problems were soon experienced; se started a few meters downhole in production casing, and occurred mainly inside pressure separator. This scale consisted mainly of antimony sulphide and amorphous silica, with latter prevailing in low-temperature part of The aim of first laboratory tests was to screen possible chemical additives capable of inhibiting sulphide scaling and to develop effective concentrations of most promising products. In subsequent field tests two selected inhibitors were both effective in preventing antimony sulphide scaling, although with different side effects on silica precipitation. Since se positive field test results, an inhibitor addition has been used in PC-26 well, and it is no longer subject to any appreciable operating problem deriving from scale build-up in surface plant. 1. INTRODUCTION The first deep exploratory well in Piancastagnaio field (Mt. Amiata area) was drilled in 1979 with aim of verifying presence of fractured layers inside metamorphic basement underlying shallow reservoir (Triassic carbonate-evaporitic fomiation) on exploitation since The well, named Piancastagnaio 26 crossed a practically impermeable basement (mainly quartzite and phillite Paleozoic formations) up to a depth of where a fractured layer was found with liquid phase at a pressure of 20 and a temperature of 330 C. The chemical composition of liquid phase discharged by atmospheric separator is given in Table 1. Silica and antimony contents estimated at reservoir conditions are, respectively, about 700 and 50 Table Analysis of brine discharged by atmospheric separator The preliminary tests showed that, despite limited permeabilitythickness product of deep layer (about well was a commercial producer because of high reservoir pressure and temperature. A pressure separation plant was n installed and since 1980 well has been linked to Piancastagnaio 2 power plant for an extended testing period aimed at evaluating trend of production parameters and chemical-physical characteristics of produced fluids. The total production was about 15 with only 10% in weight of liquid phase, because flash occurred inside formation. After a few months of production, heavy scaling problems were experienced. These started a few meters downhole in production casings and mainly inside pressure separator, as well as in atmospheric separator. It was here that liquid phase was flashed before being stored in a settling tank and piped to reinjection wells. The scaling consisted mainly of antimony sulphide and amorphous silica, with latter prevailing in low-temperature section of Silica deposition was probably started by antimony sulphide precipitation, whose solid particles acted as deposition nuclei, thus favouring amorphous silica scaling in high-temperature part of The reduction of flowing area of pipes caused by scale and consequent pressure drops gave rise to periodical (after 8 to 10 months of operation) outages of plant for cleaning operations, which required closure of well. The associated production loss lasted about one week and some pipes had to be replaced. Moreover, operation of plant was severely hindered by effect of scale on valves, which remained blocked and leaked when shut. During above mentioned tests, two more deep exploratory wells were drilled in same area with positive results. The presence of an extended deep reservoir with a pressure and a temperature similar to ones measured in PC-26 well was thus confirmed. A development program of field was accordingly outlined, foreseeing an overall activity which included drilling of 50 wells and installation of 7 new power plants of 20 MW each. The scaling problems experienced during PC-26 well testing had yet to be solved, and it was expected that new wells would have same problems as PC-26, if chemical composition of deep reservoir were uniform, resulting in costly drawbacks in operation of plants. Based on positive results previously obtained in control of carbonate scale with chemical inhibitors (Corsi et al., 1985; et a study was started with aim of identifying possible chemical additives able to prevent precipitation of antimony sulphide and, consequently, of amorphous silica. Such research activity was carried out with laboratory tests as first step, and n with field tests. 2. LABORATORY TESTS 2.1 Experimental The laboratory tests were carried out as a preliminary screening of selected products that were currently used for scale control and water treatment: Albright BP Breox 3400, Cyanamid Cyanamer and Aerofloat 249 and 425, Xantate 317; Degussa POC 2020, POC P, 1189, 1389; Imco Poly Monsanto Dequest 2006, 2010 and and 5004; Nadar 1008, 1018, 1045, 1046, 4091, 4092,4093 and 4094; Aquaprox MD The following low-temperature batch test methodology was used: stock solutions M] of soluble salts, chloride 2503

2 I. Effect addition on precipitation antimony Table 2. resulrs. is shown 0 TEN I'hc was starred quick addition solution to an solution, additive. The resulting final of b.5. progress of monitored under a E Nadar suspension and filtrate for antimony Nadar additives dissolved, as in distilled water to sulphide solution. additives to allow heating ir concentrated solutions under (about 85 minutes to reach this temperature. for 15 minutes: to minutes). solutions used previously Nadar ! 20 2 to rhe is shown typical transmittance : in of inhihitor (curve many fine after of a of is thus completed after 2 minutes. of ttic larger n place. with an of value of particles with addition of (curve The at higher product nco Poly in commercial chemical Heating at (see text) Concentration in initial solution (without 5.0

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4 --, -- Separeted steam t o powcr plant tu Figure 3. plant 26 well arid field tests according or without of first uith concentration of of liquid) head. followed So.. a run, (at a liquid) head. n (run Nu. Dunng runs, were weeks; deposits sampled for Nu from pressure its leaked if shut blank test. During runs, brine steam produced well substantially, thus making conditions nor throughout various runs. I t refore decided to perform (one-day length) of previous runs, including 1602 inhibitor. A long-term test, about 3 with 4094 and months with was n performed ordcr to effcctivcncss of products terms of plant reliability (runs Nus. 4 and 5). well has in operation since February on continuous basic, with removed, and with of eir 125 of Nadar of inhibitor at No. 6). Test Inhibitor Downstream from pressure separator No I I! Thickness 2 3 Silica + (low) 0 5 Silica + (low) 0 N A Results In test. starting downhole production aiid down to connecting settling (stibnite, with scale still black was silica (Table blank sampling in a days had cleaned each time In runs 2 and with inhibitors, no scale upstream from atrnosphenc flash (Table Probably, limited amount of during 2 was due formed during blank aid coming lrom pipes S o of sampling valves se with inhibitor atmospheric separator, also with inhibitor addition, due high silica scale was and deposited inhibitor blank Using inhibitor, amount of soft yellow in atmospheric separator in line. between of two additives difficult. however. in this first runs flow decreased its composition not constant. as mentioned A precise indication Downstream from refore obtained during atmospheric separator following runs. when rate of brine did nut change Composition significantly. The chemical analyses of brine at of clearly showed that and silica eir without inhibitor or with Nadar 1602 addition, than in runs with and (Figure inhibitor!, sulphide. The absence deposition and 2506

5 Test run No. Inhibitor None Degussa Nadar 4094 Nadar 4094 Degussa Nadar 4094 Nadar 4094 Degussa 1389 Inhibitor Nadar 1602 No inhibitoi Figure 4. Chemical composition of brine discharged by pressure separator downstream from pressure separator, experienced during tests with use of additives, can be explained by inhibition of antimony sulphide precipitation, whose particles act as deposition nuclei for amorphous silica. The effectiveness of Nadar 4094 and Degussa 1389 inhibitors was ascertained in long-term test (six months duration). The results of this test confirmed those previously obtained: no scaling was detected in line upstream from atmospheric separator at end of test and operation of plant proved to be reliable. It was refore decided to use inhibitors on a continuous basis during production of well. The well and separating plant have been operating smoothly and no scale deposition has been detected to date in pressure separator. The problems related to valve operation and plant reliability have no longer been experienced. 4. CONCLUSIONS The or wells drilled in Piancastagnaio field showed chemical compositions slightly different from ones of PC-26 well and did not rise, as was initially feared, to sulphide scaling. PC-26 is refore only well in production currently treated with chemical additives. However, laboratory and field tests experience demonstrates that antimony sulphide precipitation can be. prevented with use of chemical additives. It is refore likely that deposition of or kinds of sulphides iron, lead), typical of or geormal fields, may be inhibited with similar products, too. Though no effort was undertaken during tests to determine minimum effective concentration of chemicals needed, this type of treatment proved to be economically effective, thanks also to limited flow rate of brine to be inhibited (typically ranging from 0.2 to 1.4 In fact, as cost of both effective inhibitors tested in field is around 3 assuming a separated liquid flow rate of 0.5 and an addition of 150 resulting cost of treatment is only 0.8 REFERENCES Corsi R., Culivicchi G. and Sabatelli F. (1985) and field testing of calcium carbonate scale inhibitors. Geor. Res. Counc. Trans., 9, S., Sabatelli F. and Tarquini B. (1988) Field testing of downhole scale inhibitor injection. Proceedings, Workshop on Deposition of Solids in Geormal Systems, Reykjavik. Geormics, 2507