Prediction and Scaleup of Waxy Oil Restart Behavior

Abstract

Abstract A major concern in the design of offshore pipelines is flow restart of waxy oil after a planned or emergency shut-down. As the offshore oil industry encounters increasingly challenging production environments, such as in deepwater or marginal fields, more accurate restart predictions are a requirement for reducing CAPEX and guaranteeing flow assurance. The present state-of-the-art does not generally provide reliable predictions of the pressure required to restart flow in a gelled pipeline; laboratory measurements tend to overestimate the restart pressure, so that empirical factors are used to scale the laboratory data to field conditions. Based on a series of experimental measurements carried out in a upgraded model pipeline, our analysis suggests that compressibility and the appearance of a solid-like fracture at the front of the propagating pressure wave have to be taken into account in order to better model the cold restart process. Introduction As the oil and gas reserves are found in the increasingly challenging production offshore environments, such as in deepwater or marginal fields, flow assurance emerge as one of the key aspects for the economical development of those assets. In case of waxy oils, a major concern is flow restart after a prolonged planned or emergency shutdown. In this situation, the waxy components of the crude oil crystallize and the whole oil undergoes a gelation phase transition. To start-up flow, an extra pressure is required. In order to design the transport facilities from the seafloor to the preliminary processing facility (on spars, TLPs, FPSOs, shore plants) where it is prepared for a subsequent shipment to a refinery, an accurate knowledge of the restart pressure is vital. This allows operators to reduce considerably the capital expenditures avoiding the risk of losing pipelines and delaying production for several months. The present state-of-the-art does not generally provide trustworthy predictions for the pressure required to restart flow in a gelled pipeline. The aim of this work was the development of a new pipeline restart model capable of generating reliable, rather than overly conservative, restart pressure predictions. Experimental For this study, two waxy oils were used. They belong to offshore fields one in West Africa, the other in North Africa. As industry best practice experimental measurements for pressure restart evaluation were carried out with a control stress rheometer and a model pipeline (1). Model Pipeline. A 16 m long, 6 mm internal diameter model pipeline was used. During this study, it was upgraded with a loop and an autoclave in order to be able to saturate the crude oils with gas components at pressure up to 2.0 MPa. The pipeline has four pressure transducers along the pipe and two syringe ISCO pumps control the pressure at the inlet and outlet respectively. The oils were feed to the rig at temperature above the cloud point. They were homogenized and, if necessary, saturated with the desired gas by flowing them in the loop. Then the pipeline test section was cooled at a fixed cooling rate (1.7°C/h). The oil was allowed to age for 66 h before the restart experiment was started1. The setup allowed considering different procedure for the restart, in particular the inlet pump worked in the following modes:At constant flow rate;At constant pressure loading rate ;At constant pressure loading rate in a stepwise sequence2;At constant inlet power (keeping constant the product flow rate times inlet pressure). In Table 1 are reported some of the testing conditions and procedures applied with the model pipeline. Model Pipeline. A 16 m long, 6 mm internal diameter model pipeline was used. During this study, it was upgraded with a loop and an autoclave in order to be able to saturate the crude oils with gas components at pressure up to 2.0 MPa. The pipeline has four pressure transducers along the pipe and two syringe ISCO pumps control the pressure at the inlet and outlet respectively. The oils were feed to the rig at temperature above the cloud point. They were homogenized and, if necessary, saturated with the desired gas by flowing them in the loop. Then the pipeline test section was cooled at a fixed cooling rate (1.7°C/h). The oil was allowed to age for 66 h before the restart experiment was started1. The setup allowed considering different procedure for the restart, in particular the inlet pump worked in the following modes:At constant flow rate;At constant pressure loading rate ;At constant pressure loading rate in a stepwise sequence2;At constant inlet power (keeping constant the product flow rate times inlet pressure). In Table 1 are reported some of the testing conditions and procedures applied with the model pipeline.