Deposition of "Asphaltenes" during CO2 Injection and Implications for EOS Description and Reservoir Performance.
Authors: Broad Thomas J. , Muhamad Rais Tajjul Ariffin , Hussain Ahmad , Bani Basheer Ahmad 
Published: February 2016

It is essential that precipitation of "asphaltenes" is recognised early in the planning stage of any EOR project so that appropriate testing can be performed to evaluate whether there will be a negative impact on reservoir performance.

This paper presents detailed evaluations of extended compositional and PVT data which were obtained during extensive EOR tests using a light Middle East crude and CO2. The compositional measurements clearly confirm that "asphaltene" precipitation occurs during gas injection and that the properties of the heavy end fraction are significantly reduced.

Before the C36+ data could be successfully interpreted, it had to be "calibrated" with data from a true boiling point distillation; this is explained. Detailed evaluations of the C36+ concentrations in the normalised C11+ fraction during swelling, equilibrium contact and forward multiple contact tests are presented.

The changes in the C36+ concentrations result in changes in the physical properties of the plus fraction. It is essential that these changes are reflected in the fluid characterisation and tuning of the EOS. Also included are detailed predictions from various EOS models with and without taking into account the alteration in the physical properties of the C+ fraction.

Because minimal tuning is performed, predictions from the EOS models should be reasonably precise. The paper includes preliminary predictions of EOR behaviour using different injection gas compositions.

The paper concludes with recommendations for modifications to analytical programs for investigating gas injection processes which would lead to a better understanding of the fluid behaviour. Also included are suggestions for future research to better characterise the deposited "asphaltenes" which will hopefully result in more accurate predictions as to whether these precipitated asphaltenes will negatively impact reservoir performance.

Introduction

The Dukhan field onshore Qatar consists of several stacked anticline reservoirs and was discovered in the late 1930s. Production started in the early fifties and various EOR processes have been implemented since the mid seventies. Base oil properties vary between the different reservoirs, for example, API gravities range from around 35 to 40°API. Solution gases generally contain less than 1 mole percent H2S. Asphaltene contents are significantly less than 1 percent.

Various gas injection processes are now being considered. Comprehensive EOR studies have been executed on several fluids from both the Arab C and Arab D reservoirs to determine the fluid behaviour as CO2 contacts the reservoir oil. These included:PVT experimentsTrue Boiling Point DistillationSwelling testEquilibrium contact testForward multi contact testSlim tube tests to determine MMP

During the course of these experiments, fluid compositions were measured by flashing portions of the pressurised fluids to atmospheric pressure and analysing the subsequent phases through C36+ by gas chromatographic techniques. Interpretation of these reported compositions is not straight forward and compositions need to be calibrated for TBP data.

However, the reported compositions do provide insights into the fluid behaviour and guidance on tuning the EOS.

Asphaltenes or High Molecular Weight Hydrocarbons (HMWH)

Asphaltenes are generally considered to be complex aromatic structures with molecular weights of the order of 100001.

As with many light reservoir oils in the Middle East, asphaltene contents are not sufficient to explain the changes in the observed C+ properties during gas injection processes2. The implication is that other High Molecular Weight Hydrocarbons (for example, paraffins and resins), are also precipitated. Throughout this text, the term "asphaltenes" has been used to describe all precipitated HMWH (including aromatics, paraffins, resins, etc).