A theoretical and experimental study on asphaltene deposition in well tubing

Abstract

Asphaltene deposition is a major unresolved flow assurance problem in the petroleum industry, which may occur anywhere in the production system from reservoir through flowing pipes to the separator, whenever the condition is fallen into the asphaltene precipitation and deposition envelope. It is one of most expensive technical problems. Asphaltene deposition could cause formation damage, production loss due to the blockage of both subsurface and surface flow conduits. Understanding of the mechanisms of asphaltene deposition and the effect of composition, pressure, temperature, and flow conditions is essential. However, there are no universal mechanisms of the asphaltene problems. The potential processes related to asphaltene deposition consist of precipitation (formation of asphaltene solids out of liquid phase), aggregation (formation of larger asphaltene particles), and deposition (adsorption and adhesion onto the surface). It is believed that aggregation and deposition compete with each other. The main deposition mechanisms include gravity settling, shear dispersing, and molecular diffusion. Only with a better understanding of the mechanisms and an early diagnosis of the problems, it is possible to develop more efficient and economic mitigation methods, including both remedial and preventative approaches. With the purpose of better understanding of the mechanisms of asphaltene deposition, a series of asphaltene deposition experiments is conducted by using a simple couette device with synthesized oil obtained by dissolving "asphaltene" solids from well tubing deposits into toluene with controlled proportion. Shearing is provided to simulate the condition in flowing pipe. The effects of shear rate, deposition time, asphaltene content, water content, Toluene to Heptane ratio, and titration time on asphaltene deposition are investigated. Scanning electron microscopy (SEM) images are taken to analyze the asphaltene aggregates and confirm the findings from the experiments. Prediction is critical in the aspect of controlling and mitigating of asphaltene problems. SARA preliminary screening that is considered as a first hand predetermination of the stability of crude oil is discussed based on SARA data from oil field. The prediction of the asphaltene precipitation depth in well tubing is conducted by combining the production profile and asphaltene precipitation envelope. In order to predict the thickness of asphaltene deposition layer in well tubing, a simplified force balance model is proposed based on particle sliding mechanism. The drive force of asphaltene particle aggregation and deposition is regarded to be the van der Waals force. Based on this model, the effects of flow rate, fluid properties (viscosity and density), tubing size, and particle sizes on deposition thickness are analyzed.

Date of Award	2011
Original language	American English
Supervisor	Tao Zhu (Supervisor)