Summary

A comprehensive system for the separation and handling of sand from produced fluids was designed and installed on a facility in the Gulf of Mexico. This system involves two multicone desander systems, one on the water outlet and one on the oil outlet of a low-pressure separator, to provide separation of sand from produced fluids. The separated solids are collected, dewatered, and transported to a unique, simple-solids handling system designed for complete fluid containment and safe handling. The focus of this paper is to present the design of the separation and handling system and to discuss the various challenges encountered during and after commissioning. The topics covered include desander operation (pressure drop and separation), slurry discharge from the desander, dewatering of slurry, recycling of fluids, and transport of the collected solids from platform to shore.

Introduction

All oil and gas wells produce solids, in varying concentration, with the produced fluids. These solids, such as sand, clay, or silt, may originate from the reservoir, or they may be corrosion byproducts from downhole tubing or other process equipment. These solids are normally smaller than 250 µm in diameter and vary in concentration for different fields, with a typical range of 5 to 250 ppm. This type of solid is designated as a "natural" solid (although corrosion products may not be natural), as opposed to an "artificial" solid, such as fracture sand, which may be larger. The system described is designed to handle "natural" solids.

Even at relatively low flow rates and velocities, solids can cause considerable problems in a production system.1 Severe erosion of pipe work and valves can be experienced in high-pressure production. As solids accumulate in tanks and gravity separators, the conditions for bacterial growth and associated production of hydrogen sulfide become ideal. This in turn can be a source of further corrosion. Accumulation of solids in process vessels can also affect retention time and thereby reduce separation performance. Typical solids-handling scenarios require periodic solids removal by sand jetting or shutting the system down and digging them out by hand. Either method is only a partial solution at best, and both can significantly impact the quality and quantity of oil production.

Solid-liquid hydrocyclones, termed "desanders" for this paper, are a superior alternative to conventional equipment, such as plate interceptors or filters. Desanders are a solution that can remove solids online, in a controlled fashion, without system disruption or shutdown.1 Their weight and size is usually 10 to 25% of standard equipment, and the capital cost is proportionally lower. Desanders require little, if any, operator interaction, and owing to the fact that they have no moving parts, maintenance is minimal. Benefits and operation of solid-liquid hydrocyclones are detailed in the Appendix. Owing to a combination of new drilling technologies, tighter environmental regulations, and more compact production facilities with higher uptime requirement, the need for a sound solids-control strategy is greater than ever. Desanders are a proven technology providing a robust solution to these challenges.

Sand-Production Difficulties

The South Pass 78 field is located in the Gulf of Mexico, and it consists of 41 production wells routed from three satellite platforms.

Once the fluids reach the platform, production is routed through high-pressure (HP), intermediate-pressure (IP), and low-pressure (LP) manifolds. The HP and IP two-phase separators accomplish bulk gas removal, while the liquids flow to a cone-bottomed LP bulk separator. Flash gas is removed in the LP bulk separator, and the produced liquids are separated into oil and water streams. The oil stream flows to a chem-electric treater for removal of residual water, and then to the clean-oil surge vessel for temporary storage. Sales oil passes through the lease automated custody transfer (LACT) system and leaves the platform via pipeline. Bulk water from the LP bulk separator, along with separated water from the chem-electric treater, is first treated by a corrugated plate interceptor (CPI). The CPI skims off any residual oil from the water stream. The water receives final treatment by the mechanically induced gas flotation cell before overboard discharge.

As the South Pass 78 platform's production life matured, problems resulting from solids production began to be encountered in both the oil and water treatment systems. These problems manifested themselves as equipment plugging, emulsion stabilization, and erosion.

The LP bulk separator was designed as a cone-bottomed tank. With the water outlet at the apex of the cone, most of the solids produced would discharge to the water-treatment system. The large solids would settle in the cone-bottomed tank and pass through to the CPI. The plates in the CPI experienced constant plugging, which required frequent operator intervention and resulted in equipment downtime. Some solids would pass on to the flotation, which is the final and most critical piece of water-treating equipment. The cells were filling at a rapid rate, resulting in severe efficiency loss caused by reduced residence time.

Small solid particles would pass through the LP bulk separator in the oil phase. When these fine solids reported to oil treating, they would create emulsion pads, which inhibit the oil/water separation ability. Furthermore, the solid particles are able to pass to the LACT system and erode the metering elements. This reduced sales-metering accuracy and operational life of this costly system.

To remove the sand and solid particles from production streams, a desander was installed on each outlet of the LP bulk separator to improve the efficiency of the platform-treatment systems. In addition to the desanders, a sand-removal system was required owing to restriction of overboard sand discharge by the National Pollutant Discharge Elimination System (NPDES).

Design of Solids-Handling Systems

Solids handling and disposal in the oil and gas industry can be broken down into five areas: separate, collect, clean, dewater, and haulage.2

  1. Separate: The first step in solids handling is separation of the solids from the well or process fluid stream. Separation may be accomplished by solids-separation-oriented equipment, such as desanders, filters, or sand jets. Serendipitously, the process equipment itself may also perform this duty as a byproduct to fluid-phase separation, producing settled tank bottoms or vessel drains.

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