RESERVOIR CHARACTERIZATION TARGETING DRAINS
From the start the seismic data are key data to get a structural picture of the field and its reservoirs than the well data, even if, these later ones are important already at this stage. As soon as geoscientists are focussing on a particular reservoir, sometimes on one of its reservoir blocs, well data will be at the front stage to get a comprehensive understanding.
Borehole Image Logs, available in logging oil industry since 1980s are very valuable data able to bring further intelligence to characterise reservoirs. Once data is processed, QC-ed, the interpretable output images generated, and the basic study carried out, it is important to focus on the potential/actual draining features. This is based on a well-known fact observed in reservoirs and expressed under different notions by geoscientists, that fluids have preferential paths.
My experience, hands-on during this last 19 years in petroleum geosciences, was mostly on this second stage of reservoir characterisation. Going deeper in the characterisation of reservoir mainly fractured ones using mostly well data and more particularly borehole images.
Through my experience, focussing on reservoirs has a dual aspect related to fractured reservoirs and to carbonate formations and in some cases siliciclastic.
This implies getting an overview of what is commune to fractured reservoirs, the fracturing process and the types of other formations involved with emphasis to get the most important learning for our main interest.
In this regard, structural topics such as, specific tectonic environments, faults, minor faults, fractures, fault- and fracture-zones, analogies with field data, methodologies to identify and define them, correlation between sets, wells, reservoirs-blocs and reservoirs are essential to be able to go further in the process of characterization.
The interest of that is not only theory basis, but because it is well know that fluids use fractures and sometimes faults as path ways, so an important part of the studies will focus on that, that’s the structural and tectonic aspects of the geological process of fracturing within its context. These structural elements, mainly faults and fractures, can also be impermeable barriers or impeachment for fluid flows.
Moreover during this last decade, geosciences in petroleum acquired that the present stress in a field is involved in impeding or facilitating fluid circulation depending on circumstances related to direction of the in-situ stress vs. fracture orientation.
The matrix, mainly its sedimentologic ones, such as cavities or karsts in carbonates, infers in porosity and more importantly permeability in synergy with tectonic features.
Identifying the main features, from the matrix and the overprinted alteration or tectonic, infers to define the associations of them involved in fluid flows, impeding or facilitating, building up baffle or drains.
Drains can be identified, I call “pipe-layer” because they shape as layer or by the intersection of two or more defining at this time an axis at their intersection named “pipe-channel”.
Based on studied fractured reservoirs, from different parts of oil productions areas, notions of draining features are further characterised in two major types I call them: Pipe-Layers and Pipe-Channels.
At start of the analysis process when defined, they are named as potential/actual based mainly on geological features, further other data are needed (such as production tests, flow meter for example) to differentiate between potential and actual.
The challenge is, once these draining features are defined and characterised at well scale, to correlate and model them at reservoir scale. That’s to say a matrix, intersected by the fracture population, and the drains transecting all that. To model that in order to target them by adequate drilling.
To get further insights go to this link to download the article presented in Sonatrach JST11, Oran (Algeria) in 2018.
And published in 2018: “Borehole-Image-Based Characterization of Reservoirs Targeting Draining Features (Fluid pathways)” Article p. 26-30, in SPWLA TODAY Issue5 Vol 1 2018