Characterisation, Origin and Repartition of Tar Mat in The Bul Hanine, Arab D Reservoir, Qatar
Authors: Jedaan Nizar Mohamad R. , Fraisse Christian J. , Pluchery Eric , De Groen Vincent Laurens Nico , Dessort Daniel , Al Abdulmalik Abdulmalik 
Published: February 2016

Both core description and the log detection have evidenced the presence of bitumen inside the Bul Hanine Field (figure 1), which can be particularly abundant in some wells. This tar mat severely impacts reservoir production behaviour because it acts as a permeability reducer and a barrier to flow. Properly understanding its distribution and its propagation throughout the reservoir is then essential for the prediction of reservoir performance under various development plans, for instance when water flooding the field.

The objectives of this study were to:Characterize the tar mat and understand its formation mechanism.Evaluate its occurrence in wells: type, thickness and distribution, in the various rock types.Propagate this distribution in a 3D reservoir model for the entire field.

Fulfilling these objectives has allowed more accurate volumetric estimations, taking the tar mat into account in the dynamic reservoir modelling as well as in planning further development of the field.

Tar mat occurrence was investigated across more than 5400 ft of cores from 26 wells, 90 well logs and a large number of cuttings samples. Two tar mats were identified in the reservoir. The upper tarmat was formed in the crestal area at early stage of the oil charging (early phase segregation?). The second major one was formed at deeper depth.

The tar mat in the Jurassic reservoirs is composed of asphaltenes. Tar mat formation is explained as follows:A charging of oil, expelled from the Source Rock, followed byGravity segregation of Asphaltene Precursor Entities (APE) within the oil column on top of permeability barriers and paleo-OWC,The precipitation of asphaltenes triggered by a secondary light oil charge.

The methods applied in this study include geochemical characterisation of the bitumen of the Bul Hanine Field, a quantification of the tar content in cores using simple techniques (optical observation, Rock-Eval, Iatroscan, image analysis), and extending this quantification through wireline data in non-cored wells and then, subsequently across the field. In the reservoir model, through the relationship between reservoir quality (rock-type) and bitumen content, the distribution of tar mats can be inferred and traced across the entire field.

Introduction

Tar mat occurs in the Bul Hanine Field, particularly in Jurassic reservoir1. Bitumen occurrence can be a problem due to its effect on oil in place calculation (since bitumen is not movable, it should then be removed from the volumetric calculation) and its impact on reservoir quality. Tar mat impacts on the development plans of an oil field when it behaves as a permeability barrier. Injecting water under the Tar mat might result in inadequate pressure support because of poor communication across the Tar mat2.

For these reasons, it is important to know where Tarmat occurs in the field (both laterally and vertically), and what controlled its distribution. This information, supplemented by a good knowledge of compartmentalization of the field, could then be used to plan the location and design of peripheral field injectors and ensure optimum sweep efficiencies.

Objectives of This Paper

A series of investigations was carried out with the aim of data collection in order to:Identify the presence of solid bitumen in rock samples,Suggest assumption on its origin to help in predicting its occurrence in the field,Give a quantitative estimation,Describe and model its distribution in the reservoir,

The detailed study of the tarmat in Bul Hanine Field was carried out using standard techniques used in Organic Geochemistry.