Very Thick Crude Emulsions: A Field Case Study of a Unique Crude Production Problem

Abstract

This paper presents the results of a unique emulsion and sludge problem in one of the largest offshore oil fields in the world. The problem is related to heavy emulsion sludge deposition in the crude handling facilities. Very viscous emulsions containing 70–90% water were encountered in the offshore facilities where they were plugging up over 50% of the effective separator volume. As there is no bottom water drain at these crude handling facilities, the wet crude forms a very tight, complex emulsion that settles down in the equipment. This problem is anticipated to increase in the future as the water production increases in the field. A comprehensive study was initiated to diagnose and remedy the problem. Recommendations were made early on to conduct field trials by injecting a demulsifier at the wet crude handling facilities to control the sludge problem. Field trials were conducted at three offshore facilities. Samples were collected before and during demulsifier injection. A new method recently developed for testing emulsions was used to evaluate the removal of the thick emulsions. These field trials were successful and the emulsion/sludge problem has been brought under control. The sludge has been removed and the crude handling facilities are running with minimal demulsifier injection. This paper will describe the research work including a new method to test emulsions, comprehensive results from field trials, and highlight the teamwork between the Research Center and field personnel in successfully solving and controlling an operational problem. Introduction Saudi Aramco has encountered operational problems due to emulsions in several fields that have led, at times, to an increase in demulsifier usage, production of off-spec crude, and occasionally caused equipment upsets in the wet crude handling facilities (WCHF) and gas oil separation plant (GOSP). The problem is usually at its worst during the winter, due to lower temperatures. This paper describes a unique emulsion problem in a large, Saudi Arabian offshore oil field. This field produces Arab Medium crude oil (API 25) that is processed in four GOSPs offshore. The main producing reservoirs are sandstone. A schematic of the GOSP is shown in Figure 1 and consists of two high pressure production traps (HPPT) and two de-sanders. The traps and de-sanders operate at 250 psi and 80–90°F. The gas is separated and is transported to shore through a gas line and the crude oil is pumped to an onshore WCHF through undersea trunklines. The problem is related to heavy emulsion sludge deposition in the de-sanders. This is one of the consequences of the processing of wet crude in the field. Prior to 1997, the production from the field was dry oil and all the wet wells were shut-in. However, during the last four years, several wet wells have been put on-line. The opening of the wet wells has created an emulsion problem at the offshore GOSPs and the wet crude forms a very tight, complex emulsion that settles down in the de-sanders. The tight emulsion (viscosity in excess of several thousand cP at 80–90°F) plugs the de-sanders that have to be flushed periodically. This adversely affects plant operations. This problem is anticipated to increase in the future as the water production increases in the field. To tackle the sludge problem an Emulsion Team was established with members from the R&D Center, Operations and Production Engineering. Based on laboratory studies at the R&D Center, field trials were conducted at the offshore GOSPs with a recommended demulsifier. These are described in the paper.

Case Studies Of Emulsion Behavior At Reservoir Conditions

“…Another field has been facing emulsion challenges after the wells started to produce water 9 . It has three operating offshore GOSPs and one onshore GOSP to process the crude.…”

“…The wet crude forms a very tight, complex emulsion that settles down in the processing equipment. An investigative study 9 diagnosed the problem related to emulsions, which was brought under control by the installation of demulsifier dosing pumps. It is now confirmed that the emulsions at the wellhead (before demulsifier injection) are tight.…”

“…The section below describes emulsion experiments at reservoir conditions. Depressurized emulsion experiments were described in an earlier SPE paper 9 .…”

A study of the effects of phenolic de-emulsifier solutions in xylene on the de-emulsification of a Nigerian crude oil emulsion

“…Emulsion is a dispersion of one liquid in another liquid which is immiscible. Crude oil emulsions are formed when oil and water come in contact with each other, coupled with the presence of emulsifying agents like asphaltenes and resins, among others (Kokal, Al-Yousif, & Meeranpillai, 2001).…”

Optimization of Phenolic Based De-Emulsifiers

“…In most cases, crude oil emulsions result from the natural surfactants such as asphaltenes and resins contained in the crude oil which, when mixed with water, emulsifies the water into the oil (Sjoblom et al, 1992). This produces stable water-in-crude oil emulsion which often has a much higher viscosity than either the crude oil or water alone (Kokal and Wingrove, 2000;Kokal et al, 2001). The film formed by the adsorption of asphaltenes and resins around the water droplets, is generally strong and difficult to break.…”