Spotlight: How to... use CT scanning in well bore analysis

Materials World magazine
,
24 Sep 2018

Ben Morgan of Nikon Metrology explains how the company’s computed tomography scans of rock core samples have been used to improve oil and gas flow predictions.

Drilling is a key element of the oil and gas industry, but is inherently an expensive process. No matter where the well, off-or-onshore, minimising the risk associated with drilling operations is key to a successful project. Exploration companies look to minimise risk in drilling operations to make sure they do not miss the target yield. Therefore, the inspection of core sample is important to the operation.

Scanning for solid migration

Ian Patey, COREX Manager Formation Damage and Stimulation Group, explains the company’s methods for using micro-CT to aid drilling. ‘Operational fluids are used to assist the drilling and completion of boreholes by providing hydrostatic pressure and preventing formation fluids from entering the well bore, as well as to cool and clean the drill bit and evacuate drill cuttings. We advise on the best operational fluids to use for a particular job to avoid formation damage.

‘The fluids inevitably interact with the wellbore, changing the structure and porosity of the rock which can in turn restrict or increase the flow of gas or oil from the reservoir. In the former case, hydrocarbon recovery is compromised.

‘One of our jobs, therefore, is to take representative core samples from various depths, particularly at critical intervals in the strata, to identify any alteration in the rock. We look, in particular, at the addition or removal of solids that could indicate accumulation or strippage from the framework and hence negative or positive changes to flowrate.’

Leigh Wright, formation damage geologist and project co-ordinator at COREX’s Aberdeen laboratory, said, ‘Being able to simulate wellbore operations and reservoir conditions accurately, whether to pre-screen a proposed operation or troubleshoot an existing project, requires data from rock samples taken using an array of different equipment for observation, measurement and chemical analysis.

‘However, micro-CT scanning is the key technology, as it is central to providing a true picture to our clients. We have found that simple before-and-after permeability measurements on a sample can be a misleading metric to predicting oil or gas flow impairment.

‘Much more accurate is being able to look at the deposition or removal of solids, such as clay fines to see how they have migrated, which shows the physical alteration. A major advantage of CT is
that it can separate materials based on their density characteristics. Three-dimensional change modelling from scan data using visualisation and analysis software can help identify how the rock
is actually altering over time.’

Non-destructive testing

Micro-CT scanning relays risk to operators. Rock samples flown down from Aberdeen to Nikon Metrology in Tring, UK, for analysis are typically 25–38mm in diameter, and 25–50mm long. On arrival, the applications engineers have only two days to perform subcontract inspections.

These microfocus sources produced in Tring reveal very fine detail, and high-flux, rotating targets enable fast scanning times. The non-destructive technique preserves the item under examination, which can often be expensive, enabling material analysis, assembly evaluation and metrology CT. Accurate simultaneous measurement of internal and external dimensions is possible, plus comparison with CAD models and geometric dimensioning and tolerancing.

High-resolution micro-CT scanning is non-destructive and can reveal bedding features, fractures, cements and heterogeneities that are smaller than one micron. The images and data sets produced can show grain size and shape, pore size and network, cements and mineralogy and can potentially visualise fluids and saturation.

The resolution of a typical medical CT scanner would be sufficient to show coarse features but not individual grains, pores, micro-fractures, voids and fine features. Conventional testing techniques, such as scanning electron microscopy and thin section, are commonly used to understand what has happened within core samples and they provide valuable data. However, they are destructive testing procedures and unable to examine the entire volume of a core sample to show distribution of damage quickly or cost-efficiently, so there is potential to miss the mechanisms involved.

Predicting success    

Plans to advance its rock core analytical services using volumetrics to quantify the alteration to the sample over time.

The idea is to segregate the damage mechanisms by simulating them in 3D using the virtual scan model, with the flexibility to anticipate how the rock structure will alter under the influence of a single change, a second change, or both. The technique is being developed to include the ability to overlay combinations of up to seven variables, providing a powerful analytical and predictive tool.