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SPE 161626

Enhanced Assessment of the Distribution of Organic Matter in Unconventional Plays and Tarmat in Reservoirs Using a New Laser Pyrolysis Method on Core D. Dessort, TOTAL; F. Gelin, TOTAL; D. Duclerc, TOTAL; R. Le-Van-Loi, TOTAL

Copyright 2012, Society of Petroleum Engineers This paper was prepared for presentation at the Abu Dhabi International Petroleum Exhibition & Conference held in Abu Dhabi, UAE, 11–14 November 2012. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract

Estimation of the amount, the distribution and the quality of the sedimentary organic matter (kerogen) in unconventional plays or tarmat in reservoirs are often key to a proper assessment of the viability technique and/or commercial of these plays. Conventional core- or log-based methods for the evaluation of organic carbon content in various types of formation (e.g. tarmat in reservoir, oil shale and gas shales) are not always satisfactory due to limited resolution and/or non-reliable data.

The acquisition of quantitative and high resolution (centimetric or sub-centimetric) logs of organic carbon on core can be performed, at the labs or at the coring site, using a continuous power laser (LIPS: Laser Induced Pyrolysis System). The technology was first developed to identify the presence of tarmat in carbonate reservoirs where the results impacted the assessment of the reservoir quality, the GOIIP, the presence of permeability barriers and the response of electric logs.

More recently it has been successfully applied on various unconventional studies. For instance it was possible to have a high resolution, accurate and quantitative measurement of total organic carbon in oil shale or gas shale plays. It then allowed (i) to estimate the yield of petroleum which can be produced from oil shale Pyrolysis (ii) to extrapolate, model and map the quantity of the remaining petroleum potential of oil shale deeper in the basin.

This technology will greatly benefit the mapping of these unconventional plays by providing, in particular, a very accurate tool to calibrate conventional well logs with respect to the distribution of the organic matter.

Introduction:

Laser Pyrolysis on core

The LIPS (Laser Induced Pyrolysis System) is a new instrument being developed (fig.1). It performs the automatic acquisition of high resolution logs (for instance, centimetric) of organic carbon on cores (e.g. tarmat, source rock, oil shale, shale gas).

The LIPS includes:  One class IV laser (20 MegaW/m2) providing a continuous infra-red laser beam through an optical fiber.  One or more detectors (mass spectrometer, PID, FID…).  3D movable and programmable head which guides the laser beam and collects the material expelled from the core

during the laser shoot.  An acquisition and data processing system.

The cleaning of the core surface is achieved by a preliminary laser impact at low power. The “L1” signal is produced during

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this impact.

A second, high intensity laser shoot impacts exactly the same place, pulverizes and pyrolyses the rock and the organic matter. Nanoparticles and gases produced by the impact are continuously detected. The detector is not sensitive to the CO and CO2 released during the thermal decomposition of carbonates. During this impact the “L2” signal is acquired. The intensity of the signal increases with the quantity of organic material impacted by the laser. This signal is then calibrated with the Rock Eval, by Fisher Assays or thin sections on limited number of samples.

Because the small size of the laser impact the LIPS preserves the core integrity. The typical duration of an acquisition cycle is 60 seconds. The data acquisition can be performed on hundredth meters of core.

The technology was first developed to identify the presence of tarmat in carbonate reservoirs where the results impacted the assessment of the reservoir quality, the GOIIP, the presence of permeability barriers and the response of electric logs. More recently it has been successfully applied on various unconventional studies. This technology will greatly benefit the mapping of these unconventional plays by providing, in particular, a very accurate tool to calibrate conventional well logs with respect to the distribution of the organic matter.

Application to tarmat in reservoir

Tarmats are a dark brown to black, thick, semisolid to viscous mixture of heavy hydrocarbons enriched in asphaltenes that occurs naturally in reservoirs. One key concern regarding tar mats is their impact on reservoir connectivity and petrophysical properties, their effect on electrical logs and on reserve calculation and water flooding. To gain full control of these issues, many questions need to be answered:

o What are the processes behind their formation?

o Where are the tarmats located and how many are there?

o What is the effect of bitumen occurrence on reserve calculation, on petrophysical properties and on electric logs ?

o What is their horizontal continuity and can we predict and model tar mats distribution in a reservoir?

o Do they constitute strong permeability barriers or only partial ones?

Conventional methods are not always satisfactory in tar mat detection because:

o The log based methods and the seismic processing do not provide enough resolution and in addition they need to be calibrated.

o The core based methods rely on analysis of selected core samples using Rock-Eval, Iatroscan and thin sections. The results depend heavily on the sampling interval, i.e. the wider the sample spacing, the higher the chance of missing small scale tar mats.

o The difference of color does not necessary indicate the presence of tarmat in carbonate reservoirs.

The fig. 2 shows selected results obtained on a carbonate reservoir in the Middle East. When the porosity is measured the % of bitumen in rock porosity can be calculated. The study shows that the color of the rock is not always a reliable criterion to characterize (and quantify) the tarmat. The difference in visual appearance can be the results of difference of mineralogy, rock type, etc. In particular dolomite can show several different colors because it can include calcite, sulfide minerals, fluorite, barite….

Application to unconventional resources It is generally agreed that worldwide conventional petroleum supply will reach its productive limit, peak, and begin a long term decline. This is one of the reasons why unconventional resources such as oil shale and shale gas and other should be developed:

 Oil Shales are shallow organic-rich fine-grained sedimentary rock (shale or carbonate), containing significant amounts of immature to marginally mature kerogen yielding oil in commercial amount upon pyrolysis.

 Shale Gas are fine-grained sedimentary rocks (shale to siltstone) containing a minimum of 0.5 wt % TOC. Gas shales may be thermally marginally mature to mature and contain biogenic to thermogenic methane. Gas is generated and stored in situ as both adsorbed (on organic matter) and free gas (in fractures and pores).

Each shale play is unique and heterogeneous. The assessment of unconventional shale resources can use Organic Geochemistry. The accurate measurement of total organic carbon (TOC) as well as the remaining petroleum potential is important for numerous reasons:

 For calculating the yield of petroleum which can be produced from oil shale pyrolysis,  to estimate the quantity of gas adsorbed on organic matter in gas shales (the gas adsorbed onto organic material is

released as the formation pressure is drawn down by the well).

 To extrapolate, model and map the remaining petroleum potential of oil shale deeper in the basin. Conventional methods for TOC and petroleum potential measurement are not always satisfactory because:

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 The log based methods and the seismic processing based methods do not provide enough resolution and in addition they need calibration with conventional methods.

 The core based methods rely on analysis of selected core samples using Rock-Eval. The results depend heavily on the sampling interval, i.e. the wider the sample spacing, the higher the chance of missing small scale organic-rich layers.

The fig.3 shows selected results obtained on 100 meters of an organic-rich oil shale formation. Owing to the great number of data points uncertainties and statistics can be calculated as well as the averaged petroleum potential in every interval or facies. In addition the high resolution allows displaying Milankowitch’s climatic cycles.

Conclusion The automated Laser Pyrolysis can produce high resolution (centimetric) and quantitative logs of TOC or petroleum potential over hundredth meters of core.

Fig.1: Laser Induced Pyrolysis System (LIPS)

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Fig.2: Application of high resolution TOC to tarmat in reservoir

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Lithology

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Facies

Climatic precession +

Earth obliquity

LIPS Measurement

Eccentricity & Stages of Glaciation

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2-Frequencies Model

Fig.3: Application of high resolution TOC to oil shale.

TOC