TetrakisHydroxyMethylPhosphonium Sulfate (THPS) is used as a bactericide in the Skjold/Gorm oil production systems in the Danish sector of the North Sea. A reduction in H2S generation downhole due to THPS treatment resulted in savings on H2S scavenger consumption. In addition, use of THPS provides other benefits which are discussed in the paper.
THPS treatments are compared with conventional bactericidal treatments and it is demonstrated how downhole injection of THPS retards the growth rate of Sulfate Reducing Bacteria (SRB) and the rate of H2S production. THPS treatment resulted in a change in the downhole bacterial population, evidenced by changes in the sulfur isotope ratio of the produced H2S.
Re-injection of produced water with low H2S concentrations increases the potential for iron sulfide (FeS) deposition, both in topsides and downhole. THPS changes both the physical form and the bacterial content of the FeS deposits. Removal of the deposits with acid is facilitated by TI-IPS which exhibits FeS dissolving capabilities. Field tests have verified the effect.
The Skjold oilfield is located in the Danish Sector of the North Sea and is operated by Maersk Oil and Gas AS. Production of oil and gas commenced in 1982. Two wellhead platforms are installed in the Skjold field but control and operation is conducted from the nearby Germ Platform where Skjold production is processed.
Water injection, to maintain reservoir pressure, was commenced in 1985. The producing horizon is the Maastrichtian Age Chalk,
Sulfate reducing bacteria (SRB) are commonly encountered in produced waters from deep oilfield reservoirs. The growth and activity of such bacteria are responsible for souring of the crude oil. Many early studies" revealed the presence of mesophilic sulfate reducing bacteria (mSRB) within sour oil production facilities, especially in the colder parts of the system. Subsequently, more detailed surveys of North Sea oil production systems demonstrated that therrnophilic SRB (tSRB) are likely to be the most important group of sulfide generating organisms in systems above 60°C 2" 3. This was later confirmed by other workers 4' ~ who have also demonstrated the existence of a wide ranging population of sulfide generating bacteria which contribute to souring. In addition to the souring problems caused by (thermophilic) sulfate reducing bacteria, microbially influenced corrosion (MIC) is also enhanced in hot production systems 6.
A conventional bactericidal formulation has, in the Skjjold Field, been used to control bioproliferation in the water injection pipelines and topside production facilities but was unable to control downhole hydrogen sulfide (H2S) generation. Hence, produced H2S has been increasing steadily, necessitating the use of increasing quantities of H:S scavengers. In addition to increased corrosion due to HeS, iron sulfide deposits in Skjold produced water reinjection wells caused loss of production by restricting water injection rates.
An alternative bactericide product, tetrakishydroxymethylphosphonium sulfate (THPS) was field tested in six campaigns during the period 1994 to 1999 and has yielded reductions in H2S generation downhole in the areas treated. Similar reductions in H:S generation had previously been observed in other fields 7 and, furthermore, other beneficial effects of THPS have been noted, for example, it has made iron sulfide deposits more porous, thus increasing the effectiveness of acid dissolution. It is anticipated that future use of THPS will eliminate the need for acidising the wells as THPS dissolves iron sulfide s .
THE SKJOLD FIELD
Skjold is a dome shaped chalk reservoir with