A study of the potential of asphaltene precipitation due to gas injection in a UAE carbonate reservoir

Abstract

Asphaltene deposition is a potential major production constraint in several carbonate reservoirs in the UAE, Middle East. The negative effects of asphaltene include plugging of reservoir formation, wellbore, tubing and surface production facilities and causing shutdown of certain wells. In order to maintain the oilfield's production rate, one strategic program in the near term is to enhance oil recovery through gas injection. However, as gas injection may further exacerbate the asphaltene problem, it is essential to investigate, a priori, the effects of gas injection on asphaltene precipitation and deposition and this is the motivation for this study. In this study, a critical analysis of the asphaltene problems faced in the field as shown in Figure 1.1 has been made with hydrocarbon gas (HC) and carbon dioxide (CO2) gas as injectant under reservoir conditions and different ratios of gas injection. The results from experimental study have been interpreted. The effects of gas injection ratio and depressurization speed have also been carefully studied through experiments. Asphaltene precipitation growth history and nature of solids formed were investigated with the aid of high pressure microscope (HPM) as well. Meanwhile, the effects of stabilization time of the system at each step of depressurization on asphaltene onset pressure (AOP) have been studied in lab. From this study, a faster and more reliable method of AOP determination by using light scattering technique with near infrared range (NIR) light have been proposed. Formation damage has also been evaluated through core flooding experiments with oil blended with different ratios of gases. It has been found that asphaltene onset pressures (AOP) of the oil depends on gas injection ratio for HC and CO2 gas. With the increasing volumes of injected gases, asphaltene deposition envelope enlarges. AOP is also proved dependent on depressurization speed from experimental results. At the prevailing reservoir conditions, EOR through gas injection could precipitate more asphaltene out of the oil. In addition, it also appears that AOP is a turning point for asphaltene particle growth rate as well as its particle size and population. After the pressure drops below AOP, the precipitation rate is faster than that of pressure above AOP. Furthermore, the equal stepwise depressurization method developed in this study sees good agreement when compared with more accurate but slower depressurization method. It avoids overrunning the AOP and saves much time to operate as well. From the core flood experiment, it demonstrated that gas injection can cause formation damage due to asphaltene deposition. It is envisaged that the findings from this study will not only help develop a better understanding of asphaltene precipitation mechanisms and propose a more time-efficient and reliable method to determine AOP for petroleum industry but also produce a better gas injection strategy for the field under study so as to maintain the desired oil production rate.

Date of Award	Dec 2011
Original language	American English
Supervisor	Hemanta Sarma (Supervisor)