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Abstract

This paper discusses a recently completed FEED and post-FEED flow assurance study for the development of two satellite crude oil producing systems in West Africa. A 33 km 24 in. trunkline will transport the satellite field production to the main field. 

With a gas/liquid two-phase trunkline, terrain induced slugging required a slug catcher that was too big to be cost-effective. To mitigate the severe slugging problem, a gas/liquid separator was recommended at the inlet of the satellite manifold and the liquid surge volume could be completely eliminated. A 4 in. separate gas pipeline was adequate to avoid surge capacity requirements at the outlet.

The demulsifier becomes less effective at watercuts (WC) greater than 60% and the peak viscosity can be as high as 10,000 cp. The restricting factors are the maximum-allowable-operating-pressure (MAOP) of 46 bara for the pipeline startup. A two-step ramp-up schedule was recommended to keep the peak pressure to an acceptable level. A limited number of well VSDs (Variable Speed Drive for ESP wellhead control) are required based on the slower ramp-up scheme and production profile.

Some guidelines are given in this paper for managing emulsion crude oil system issues.

Introduction

The N-field/M-field project consists of the development of two additional satellite fields that feed an FSO on the West African coast. The N-field is located approximately 5 km northeast of the existing MND Gathering Station. The majority of the production comes from the M-field located approximately 33 km northwest of the MND Gathering Station. The additional production from N-field/M-field will be tied into the existing facilities and infrastructure in West Africa. Recoverable reserves for the two field areas are estimated at approximately 100 million barrels of crude. The development plan calls for 5 producing wells in N-field and 25 wells in M-field. Electric submersible pumps (ESP's) will be used as the primary method of artificial lift. 

Under certain conditions, oil and water may form an emulsion that will not separate at the surface without special treatment. An emulsion is a colloidal dispersion of liquid droplets in another liquid phase (oil-in-water or water-in-oil emulsion, depending on which is the continuous phase). As watercut increases, emulsification of oil results in a significantly increased viscosity that severely hampers wellbore and pipeline delivery capabilities.  Demulsifiers are products used to break or prevent the formation of emulsions. Most of the demulsifier products are hydrophilic surfactants that have the ability to revert the water-in-oil emulsion into two separate phases.

The following key technical issues were considered in the study:

• Maximize standardization of design and equipment to reduce project costs.

• Include turndown capability needed in early and late life. The production profiles are based on reservoir simulation. The peak rate in the pipeline sizing should reflect a 90% service factor.

• Optimize the trunkline and well flowline and determine the correct trade-off between ESP horsepower and pipeline diameter.

• Determine surge volume requirements for the M-field trunkline slug catcher (co-mingled with N-field at the end of the M-field trunkline).

• Test VSD requirements for cold startup and shutdown scenarios. Transient modeling is required to determine an optimal start-up procedure that will aid in defining VSD requirements.