Abstract

Mud gas logging is a standard service on almost every drilling job. Its big advantage is the cost effectiveness and the large range of potential applications, from the classical wellsite safety measures to the identification of potentially productive zones in the subsurface. It provides significant input from the first geoscience interpretation, all the way through to the completion of the well.

Based on the example of an oil and gas field in the Norwegian Sea, this study presents the capabilities of mud gas data to assess a heavily compartmentalized field and the fluid contacts within. Historically, these datasets were used to describe and interpret wellbores individually. This study is one of the first approaches to describe a larger area based on multiple mud gas analysis datasets from various wells. In this particular case classical formation evaluation data is oftentimes not sufficient to identify fluid presence and boundaries in the subsurface. Hereby an advanced investigation of the mud gas data can greatly support the decision making process in operational issues and the development phase.

Of particular relevance for appropriate fluid interpretation from mud gas is a thorough understanding of various parameters such as the flows and the temperature on the surface and in the subsurface. Small differences can already lead to major mismatches in the interpretations. Additionally, this study also highlights the advantages of a fully advanced mud gas analysis system and the possibilities for post-processing that emerge from complementing the gas data by meta-data of the analysis extraction equipment.

Introduction

One of the earliest datasets available during a drilling job comes from mud gas analysis. This data is gathered by measuring the composition of the formation fluid, liberated by the drilling process, within the drilling mud after it has been circulated to the surface. A detailed analysis of gas concentrations and gas ratios are not limited to indicate the fluid type in the reservoir, but also reveal information such as an estimate of hydrocarbon saturation and porosity (Ritzmann et al. 2016).

Over the past decades, various mud gas extraction systems have been developed and the collection of methane (C1), ethane (C2), propane (C3), butane (C4) and pentane (C5) data in addition to total gas is already a standard service on almost every drilling job.

Due to technological advancements, the focus of development is now targeting advanced gas (AGAS) extraction and analysis systems, which do not only measure more compounds up to octane (C8) but also maintain constant extraction conditions at the surface. The major advantage of constant extraction conditions, coupled with a thorough understanding of the extraction process, is the comparability between various datasets in a quantitative way.

To give a short insight in the capabilities of mud gas logging beyond the description of a single wellbore, a field study is presented where an entire hydrocarbon field is assessed using only basic C1 to C5 mud gas data. Here, various gas ratios were compared on a regional scale for each single well to find similar compartments and create a map showing areas with a common geochemical signature.

MUD GAS ANALYSIS - DESCRIPTION

Information about the gas concentrations in the mud are obtained by setting up a gas extraction and a gas analysis system in the returning mud flow coming out of the wellbore and measuring its composition.

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