Removing Hydrogen Sulfide From Sour Crudes In The Field
Authors: Wolf J.R. , McLean T.C. 
Published: December 2007

A chemical treating process is described that reduces dissolved hydrogen sulfide gas in crude oil to a concentration of less than 50 ppm. Such a process system is readily available for field installation with a minimum of surveillance being required. The treating takes place between the emulsion-separator equipment and production tankage. An aqueous ammonium hydroxide solution is used to convert the hydrogen sulfide to a water soluble ammonium sulfide salt. The aqueous phase separates in the production tank carrying with it the hydrogen sulfide as ammonium sulfide. The water is conventionally withdrawn from tankage and combined with waste water for injection into a water disposal well. A test procedure has been developed to monitor the treated crude oil hydrogen sulfide concentration. Process variables and compensations are discussed.

Introduction

Recently, the maximum levels of hydrogen sulfide gas (H2S) dissolved in crude oil were established for the shipping and storage of sour crude oil (12 degrees API) in the Santa Maria Valley. The criteria is indirectly based on medical exposure studies which have resulted in federal, state, and local regulations, both environmental and OSHA. The correlation is such that the vapor space of a storage tank, containing acceptable levels of H2S for a nonprotected person working for eight hours (10 ppm maximum), would be in equilibrium with the stored crude oil (50 ppm H2S maximum). The equilibrium data and limits were established by the shipping company. The H2S level in the crude oil was measured using the UTM method No. 504-75 of Union Oil Company.

This paper describes a simple field chemical treating procedure and subsequent analytical procedure which Husky Oil Company and Oilfield Engineering and Consulting, Inc., developed for use. The chemical treating is compatible with conventional oil field production equipment and practices. The analysis that production equipment and practices. The analysis that has been developed is correlated to the UTM determination, thus offering a quicker, reliable testing of the crude oil for H2S. The complete scheme has been proven and accepted by the crude oil shipper. Recently, the treating and testing procedure has been successfully employed by a Central American crude oil producer experiencing high levels of H2S in the vapors producer experiencing high levels of H2S in the vapors from production and storage tanks.

HISTORY

Several materials known to react with hydrogen sulfide were tested on crude oil to determine the most effective for the specific set of field conditions found in Husky's Santa Maria Operation. All but two chemicals were discarded as unsatisfactory. Of the two remaining, sodium hydroxide was selected over ammonium hydroxide because of the stability of the produced by-product.

The primary consideration for treatment is the reduction of the hydrogen sulfide content of the oil phase from 900 ppm to 50 ppm or less. This must occur phase from 900 ppm to 50 ppm or less. This must occur without the formation of additional emulsion and within a time frame of thirty minutes at temperatures of 150 degrees F to 220 degrees F and pressures of 20 to 40 psig. The salts of the reaction should be stable in order to prevent dangerous hydrogen sulfide build-up in water disposal facilities. In addition, the salts should be soluable in water and thereby disposable in water injection wells. Salts must also prefer the water phase over the oil phase to prevent contamination of oil shipments. Of course the corrosivity of the resultant solutions must be held to a minimum to allow use of "off the shelf" production facilities. Last but not least, the material used has to be economically feasible.

Laboratory work indicated sodium hydroxide could be used, so the necessary field installations were made. Sodium hydroxide was obtainable in liquid form as a fifty percent solution, seventy-six percent saturated basis. Transportation in 2000 to 3000 gallon lots was effected by stainless steel tank trucks from points 300 to 500 miles from Santa Maria. It was points 300 to 500 miles from Santa Maria. It was stored in above ground plastic tanks and pumped directly from those tanks into the produced oil stream by the use of plastic diaphram chemical pumps.

Figure 1 is a schematic showing the typical oil handling facilities for the crude requiring treatment.