Abstract

Pyrobitumen is a black solid insoluble carbon-rich deposit derived from thermal degradation of hydrocarbons. This organic material has been commonly found in carbonate rocks worldwide. In a recent SPE paper, we have shown that pyrobitumen can cause fines migration, oil-wetting and acid sensitivity problems. More importantly, the presence of pyrobitumen severely occludes porosity and reduces permeability. There are no known chemical treatment processes to remove pyrobitumen in-situ near the wellbore.

The objective of this study is to design a chemical treatment process, which will enhance well productivity of pyrobitumen-containing formations by removing the organic material in-situ. Since pyrobitumen is insoluble in any organic solvents, several strong oxidants are evaluated at elevated temperatures. These beaker experiments show that sodium hypochlorite is the best oxidant The kinetics of the oxidation process is carefully measured in these beaker tests. Subsequent coreflood experiments are performed to study the effectiveness of pyrobitumen removal using the sodium hypochlorite treatment at various temperatures, with different types of pyrobitumen, and in the presence of residual oil. Detailed petrographic analyses of the pre- and post-flooded core samples are conducted to find out the extent and location of pyrobitumen removal from the pore structure of cores. Effluent samples from the coreflood tests are analysed to understand the oxidation process of pyrobitumen in the core. In addition, concerns of scale precipitation, corrosion byproducts, and chlorinated hydrocarbon production from the sodium hypochlorite treatment are also addressed.

The coreflood results show that significant improvement of core permeability by thirty to forty fold can be achieved by removing pyrobitumen from core samples using the newly developed chemical process. Visual and microscopic examination of the core samples before and after the treatment shows that the pyrobituzen material is removed by the oxidant. These laboratory results demonstrate that similar degrees of well productivity improvement is attainable by using this process.

Introduction

The term "pyrobitumen" was first introduced by Abraham (1945) who based his bitumen classification system on the analysis of chemical composition and physical properties. Some of the properties of pyrobitumen reported by Jacob (1989) were:

  • colour, jet black, opaque in transmitted light;

  • hardness, <2.5;

  • density, 1.0-1.2 g/cc;

  • reflectance % in oil, 0.01-0.7;

  • fluorescence, 0.1-2.0; and

  • solubility in CS2, insoluble.

A most recent publication by Shaw et al. (1995) provided a comprehensive study on the effect of pyrobitumen on hydrocarbon recovery. These authors showed that pyrobitumen can cause fines migration, oil-wetting and acid sensitivity problems. More importantly, the presence of pyrobitumen severely occludes porosity and reduces permeability. The objective of this study is to enhance well productivity of carbonate formations by chemically removing pyrobitumen in-situ because until now there are no known chemical treatment processes. As a result of this study, US and Canadian patents were filed and granted for the treatment process.

Experimental Procedures
Sample Selection

The Rainbow Keg River and Bigstone Leduc formations' cores were chosen because samples from these reservoirs yielded sufficient extracted pyrobitumen to be used for preliminary kinetics measurements. They also contain different pyrobitumen types and represent different reservoir conditions. The Rainbow Keg River pyrobitumen is a lower thermal grade pore- and vug-filling fine grained mosaic epi-impsonite. The Bigstone Leduc pyrobitumen is a higher thermal grade pore- and vug-lining coarse grained mosaic epi- to meso-impsonite. Five 1.5 inch diameter core plugs were chosen for this study, four from the Rainbow Keg River formation and one from the Bigstone Leduc formation.

Pyrobitumen Determination

The weight of pyrobitumen in the carbonate samples is determined using the whole rock ashing method. This method involves the complete oxidation of pyrobitumen at temperatures ranging from 450–600 C.

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