Abstract

A series of three fixed platforms off the California coast produce fluids sourced from sandstone and carbonate formations that individually produce brines with substantial amounts of calcium and bicarbonate, respectively. These fluids are sent to a central facility onshore for processing, requiring that the combined fluids be treated for calcium carbonate scale. During acid stimulation treatments of wells, the fluid flowback from the wells initially has very high calcium levels that greatly increase risk of scale formation for a short period of time. To better understand the dynamic scale risk created during stimulation flowback, a monitoring program using a novel in-line deposition monitoring probe was implemented. The probe was placed in-line at the onshore processing facility to track changes in the production fluids prior to and during the acid stimulation program. The resulting data was correlated to several events that occurred within the process including a process shut-in, deposition of heavy oil residue that fouled the probe during a period of poor separation, and scale deposition upon reduced chemical treatment.

Real-time data obtained during flowback from two acid stimulation jobs is presented, demonstrating how it was possible to optimize the scale inhibitor treatment program such that no scale deposition occurred during these treatments. The value of the real-time monitoring data versus other methods of assessing the risk of scale formation within these fluids is highlighted and the ability to optimize inhibitor treatment rates in real-time demonstrated.

The development and implementation of on-line real-time monitoring for deposition control of an active and changing scaling environment shows the clear value that this technology brings to scale control within process facilities.

Introduction

Three offshore platforms (P1, P2 and P3) are located in the Santa Barbara channel (California). The produced fluids (Table 1) are transported to an onshore facility for processing via a sub-sea pipeline after combination of the fluids at the P2 platform. As the produced waters have a positive scaling index (SI) for calcium carbonate scale, the fluids are treated at each platform using a phosphonate scale inhibitor. An emulsion breaker is added at the onshore facility prior to processing through the free water knock-out (FWKO) vessels and the heater treater units. Processed water is then passed through the media filter unit to remove residual oil and solids. Hydrochloric acid is added to the water to reduce the calcium carbonate scaling potential. This fluid passes on to a heat exchange unit used to transfer heat from the processed fluids to the incoming emulsion crude before treatment at a bio-processing station and final discharge off-shore.

Potential scale deposition is minimized from the platform to the on-shore treatment facility by treatment offshore. However, as the fluids (crude emulsion) arrive on-shore, process changes could induce scaling. The first point of change is at the heat exchanger unit where the crude emulsion temperature is initially raised and could induce scale deposition, as calcium carbonate is less soluble with increasing temperature. As fluids travel through the remaining portion of the system, further heating in the heater treater units and processing time both encourage scale deposition.

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