Introducing the GEM

A Complete Geological Model.

In P&A you need to know what you need, not what you have got.
A GEM is not a recycling of information culled from existing well documents. Instead it is a complete reinterpretation of the whole well in order to maximize the value of all available information, including new learnings from later wells and wider offset experience in the basin or area. There are no raw logs requiring the user to ‘read’ and interpret data, but a set of new and consistent interpretations made in three key Geological Models.
 

A Stratigraphical Model.

The Stratigraphical Model is constructed from a review of rock types, formations and cyclo- or sequence-stratigraphic elements encountered in the well and in its key offsets. The purpose of the model is to achieve a common stratigraphy at the highest possible resolution across the wells as a group. This means that if - as is common for production wells - the tophole lacks the key log suites needed to build a high resolution PPFG, synthetic logs based on tool runs in offset wells can be prepared at the highest possible resolution, using the new stratigraphic model in both wells as a 'container' for our estimation algorithm.

 

For this reason, we will identify all recognizable lithostratigraphic elements, from Group to Member level throughout the well (but especially in the tophole), even when little attempt was made to discriminate stratigraphy by the original drill team. We will also delimit cyclo- or sequence-stratigraphic elements where necessary (e.g., in the absence of any clear or established lithostratigraphy) and, in frontier regions such as the African Atlantic Margin, we will construct wholly new stratigraphies based on our observation of local to regional correlations. Our final stratigraphical model, whatever its key components, always uses the latest published nomenclature and is calibrated to the latest Gradstein timescale.

 

A Lithological Model.

The Lithological Model is constructed from a review of cuttings, mudlog, LWD and wireline data, including the review of offset well data and/or our synthetic log estimations in sections without electric logs and/or cuttings returns. Where necessary, we will also use petrophysical algorithms and AI to determine lithology. The purpose of the lithological model is to assist in the identification of permeability but also to provide fallback elements of the PPFG rock property model at a sufficiently high resolution should well log data be found to be inadequate for this purpose.

 

A Pore & Fracture Pressure Model.

The Pore Pressure and Fracture Gradient (PPFG) model is a tailored product made for each well, and it does not use regional assumptions or depth-based interpolation from a model developed in a single example well some distance away (take our P&A Challenge to find out why). The model is an log-based Effective Stress estimation which uses standard rock and fluid physics linked to indications of changing fluid type, pressure and lithology as expressed in the wellbore itself. Each well is modelled independently, and proxy indicators of pressure and strength (e.g measured or derived sonic, resistivity and density data; fluid and lithology type interpretations) are used to determine the PPFG gradient.The model in each well is always a 'Best Technical Estimate' (i.e., a P50), which achieves the best possible history match to relevant observational data. Take our P&A Challenge to find out why conservative pre-drill models, intended only to assure mud weight windows, lead to lost value when used for P&A.

The log-based model is wholly independent of observational data (RFT, LOT and FIT and drilling performance/events). These can then be used for validation (Take our P&A Challenge to find out why PPFG models should be based on logs and not observations). LOT/FIT/RFT data from offset wells is reassessed and correlated to the subject well by repositioning with respect to the newly-modelled stratigraphy rather than a simple TVDSS correlation, in order to provide a properly correlative history match. Drilling performance and events are listed in an on-depth narrative immediately adjacent to the PPFG model. When the PPFG model matches the RFT/LOT/FIT and wellbore observations, confidence in the gradient character expressed between those observations is very high. In this way, we are able to express changes in rock strength which may impact abandonment planning and moreover be confident that the PPFG model is a genuine best estimate. Take our P&A Challenge to find out why this makes a difference to your engineering P&A plan.

 

A Complete Engineering Model.

If you want the best barrier location, you'll need the best perspective on your wellbore.
Because the GEM is drawn to scale (usually 1:5,000) the wellbore and completions schematic is drawn precisely against the geological models. The geology, cement bond and former drilling environment around a preferred barrier location can be assessed at a glance, and candidate barrier locations can be immediately ranked. In this way, the GEM drives forward assurance and replaces unscaled 'excel drafted' well schematics as a tool for barrier selection.
 

A Drilling Incident and Performance model.

All drilling and engineering reports are comprehensively reviewed in preparation of the GEM, with a complete narrative of drilling performance and downhole incidents given in a dedicated track. The information is captured and presented on-depth and every event can be seen in its geological and engineering context at a glance. The drilling incident model acts primarily as an observational validation of the log-based PPFG Model, but in-casing, slot recovery and cement data is also captured to enhance the definition of the engineering models.

 

A Mud System and Cementing model.

The drilled mud type (including weight) is presented on the GEM as a continuous column scaled against both the geology and the completions. This supports the interpretation of drilling events and performance in the geological models, but it also supports the interpretation of the cement job. As a minimum, the cement job will be reviewed and the actual or estimated TOC described on the GEM with the cemented part of the annulus shown with a shaded fill. Fully-risked cement job interpretations can be optionally incorporated when commissioned alongside the GEM (for example, as part of our RACE service).

 

A Comprehensive Well Tubulars model.

In every GEM, all casing, tubing and completions within the well are newly-drawn to scale using our unique components library, to which new components can added as required. The grade, connection type and other key data for the tubulars are summarized in GEM header. Particularly with older wells and those which have been slot recovered, there may be multiple generations of well schematic and all are reviewed and cross-checked before a final scheme is proposed.

 

A Cement and Formation Bond model.

The GEM can incorporate any bond tool interpretation, made by ourselves or other parties. The interpretations (in the form of a bond quality flag) are always shown at scale against the detailed engineering and geological models, assisting the 'at a glance' estimation of potential barrier locations.

A risk-based approach to cement bond logs, where we use our geological and engineering models to calculate a Chance of Success (CoS) of a successful annular bond at any point in the well, can also be substituted for the simple bond flag. Click the button below to explore our Risked Annular Cement Evaluation (RACE) service.

 

A Complete Well P&A Scope.

In P&A, you need to know exactly where you are going.
The goal of the GEM is not to simply derive and present the geological and engineering models, but to interpret them for the end user and achieve a P&A Zonation of the well.
Flow Zones are areas of the well where the geological models identify permeability together with hydrocarbons or brine overpressure.
Permeable Zones are ares of the well where the geological models identify permeablity together with normally pressured brine.
Primary Isolation Windows are areas of the well where impermeable rocks have sufficient strength to retain hydrocarbons or brine from a given Flow Zone.
Combined Isolation Windows are areas of the well where impermeable rocks have sufficient strength to retain hydrocarbons and/or brine from multiple deeper Flow Zones. Combined Isolation Windows must be identified to achieve the cost efficiency of a combined isolation barrier.
By comparing the Isolation Windows to the well schematic in the scaled GEM, P&A engineers can immediately identify and rank the barrier location opportunities, derive a risk-based work scope and plan contingency barrier locations.
GEM example

In this example GEM, Flow Zones and their Primary Isolation Windows are identified (far right) by interpretation of the permeability, rock strength and fluid models (far left). Flow Zones share a colour coding with their Isolation Windows.

Interpretation of the fluid and rock strength models (far left) allows the identification of a narrow Combined Isolation Window (shown in bright green, far right). In this section of the well, there is the possibility to safely set a single barrier which will isolate multiple Flow Zones from surface.

This barrier opportunity can be compared against the completion of the well via the mud system flag (centre left), a CBL interpretation (centre right) and the completion and cementation schematic (centre).