Tga/Dsc Investigations Of Saskatchewan Heavy Oils And Cores
Authors: Verkoczy B. , Jha K.N. N. 
Published: March 2010

Thermal gravimetric analysis (TGA) and pressurized differential scanning calorimetry (PDSC) investigations were performed on Saskatchewan heavy oils collected from wells under primary, steamflood and fireflood production, and on cores, extracted oil and mineral matter. Thermal fingerprinting of oils demonstrated that the recovery processes cause characteristic changes on the produced oils. Kinetic and thermochemical data were estimated for thermolysis, low temperature oxidation (LTO), high-temperature oxidation {HTO), cracking, coking and combustion reactions in cores and separated oils and were found to be dependent on the composition of the oil and rock. The results showed that the oxidation and combustion reaction rates were non-linearly dependent on the heating rates indicating a need for simulating reservoir conditions in the laboratory in order to generate meaningful data. Although tile formation of coke was independent of heating rates, the presence of mineral matrix increased its yield by 37 to 315 per cent.

The TGA/PDSC affords a quick and inexpensive technique to generate reliable data on kinetics, enthalpies and fuel (coke) laydown in cores and separated oil under various conditions.

Introduction

Heavy oil reservoirs of Saskatchewan are characterized by thin pay (&lt; 10 m), bottom water, high permeability, heterogeneity, high oil saturation and high viscosity oil (500 – 20 000 mPa. at 20 °C)(1). The primary and secondary processes recover less than nine per cent of initial oil-in-place(l). Thermal methods, especially in situ combustion, appear to be the most promising enhanced oil recovery technique for these reservoirs.

An understanding of the reservoir geology and oil chemistry is essential for screening, designing and application of the in situ combustion process. The changes in the composition of the reservoir oil during thermal stress will determine the fuel (coke) laydown, the amount of oxygen required, the efficiency of oxygen utilization, and the extent of oxidation, cracking, coking and combustion reactions. Since design and numerical simulation of the in situ combustion process requires kinetic and thermochemical data in addition to reservoir description, reservoir fluid properties and well data, TGA and PDSC techniques have been used to generate Arrhenius parameters and enthalpy values for core, extracted oil and mineral matter and to determine mineralogical changes in the core material(2–5).

The results of TGA, differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies on two Lloydminster heavy oil cores, extracted oil and mineral matter have been reported previously(4–5).

This paper presents TGA thermal fingerprinting results of eight primary and tertiary wellhead oil samples and demonstrates the effects of the recovery processes on produced oil. The TGA, PDSC and XRD results obtained on four heavy oil cores, extracted oils and mineral matter were used to estimate activation energies, pre-exponential factors, rate constants, enthalpies for thermolysis, oxidation, cracking, coking and combustion reactions, and the propensity of coke formation.

Experimental

Sample Preparation and Analysis

Eight primary and tertiary wellhead samples were collected from four heavy oil reservoirs in the Lloydminster area of Saskatchewan currently undergoing steam injection or in situ combustion.