Net Identification Techniques to Establish a Reliable Net to Gross Estimate
Authors: Turner Jane , Conroy Tim , van Deijl Ben 
Published: May 2016

Introduction

Net to gross (NtG) is a ratio that ultimately assists in defining the volume of hydrocarbons in place within a reservoir. NtG is a concept that has a concrete and immediate effect on reservoir modelling, investment decisions and development planning. As well as a technical contribution to the static model of the reservoir, NtG is often utilised in other subsurface applications such as rock typing, electrofacies and flow unit evaluation, petrology work and regional rerservoir studies. A key issue or query that commonly arises is how reliable is the NtG estimate and range. Even more importantly is the consideration of what can be done to decrease uncertainties associated with this parameter in order to create a robust NtG estimate. NtG should be evaluated using a collection of evidence from different data and techniques to create a well defined modelling parameter, with clearly communicated uncertainties. The associated e-poster presentation will discuss a variety of techniques used in establishing a reliable NtG estimate.

The discussion will reference a world class data set, extensive logs, kilometres of core and hundreds of routine and special core analysis results. Several net to gross methodologies which shall be discussed include;visual water permeability, high resolution computerised axial tomography (microCT scanning), core studies, laboratory results, mini permeametry results (finescale permeability profiling) and log analysis including NMR, image logs and conventional petrophysical analysis. Net to gross sensitivities will also be discussed along with comparison work between the various techniques employed. This is a deterministic approach for formation evaluation and is meant to compliment in depth discussions with reservoir engineering. These discussions should be centred on the dynamic behaviour of the rock, preferably from finescale sector models or similar. This approach will discuss the available dataset for the project, an introduction to NtG, a discussion on the validation of permeability cut-offs, different techniques used to estimate NtG range and a comparison of NtG techniques. A conclusion on the current work in quantifying the uncertainties related to NtG will also be presented.

The largest operator of oil and gas production in Australia has acquired several world class petrophysical datasets. Data includes; wireline logs, pressure data including sampling and mini-DST tests, and core, both conventional core and sidewall cores. Within the datset there are thousands of routine core analysis results at overburden conditions, hundreds of special core analysis results and core petrology information from a variety of sources. Net reservoir is defined as reservoir rock that has the capacity to flow fluids and/or provide energy in a production time scale. Net reservoir definitions vary between companies, organisations and countries. Defining NtG is paramount to its understanding, communication and quantification. Petrophysically, net has to be communicated in terms of permeability, the parameter which details arock's ability to flow gas. Permeability is the only parameter that comes close to describing the ability to flow gas or contribute energy. The more data available on permeability and net, the better the reservoir understanding will be and the easier it will be to explain reservoir processes.

Net reservoir can be assessed by considering results from a number of investigative techniques. Important considerations include; over the life of the field will pressure from low permeable rocks contribute energy to the system, does the special core analysis data from drainage and the imbibition cycles indicate that there is movable fluid down to the permeability cut-off. It is important to quantify the impact of using a different cut-off on NtG. Consider if other traditional net indicators such as volume of shale or porosity either have uncertainty considered too large or are not representative of the permeable reservoir accurately. Net sensitivity can also be investigated by looking at the Equivalent Hydrocarbon Column (the sum of the product of porosity by gas saturation by net reservoir column). Consider the fraction of the reservoir rock that exists between specified permeability cut-offs. Investigate how sensitive the NtG is to the assigned permeability cut-off and assess the volume impact on recovery.