Formation damage testing is commonly used to gather information and aid in risk-reduction when making operational decisions. The nature of laboratory testing means that it is a higher risk to rely on permeability and pressure measurements alone, so various techniques (including scanning electron microscopy and thin section) are used to gather additional information and aid interpretation. The current techniques provide excellent high-resolution data but are limited in terms of capturing the change throughout an entire core sample.
The paper presents a new approach which utilises micro-CT scanning to produce high-resolution data of entire core samples. The images of core produced are superior to those from the commonly-used medical scanners, and give insight into core properties as well as areas such as drilling mud constituents infiltration, mud-cake structure and thickness, and alterations in the pore network. Through a technique that we have called "difference mapping", data sets captured before and after laboratory testing are compared to reveal the distribution of changes within samples. Difference maps can be used to provide additional interpretation of tests results as well as combining with current techniques to target their sampling locations. The combination of laboratory data with tools that allow visualisation of both the distribution and nature of damaging mechanisms makes laboratory data more valuable and therefore decreases risk in operational decision-making.
The technique is illustrated by a case study from Centrica's South Morecambe gas field. Here a series of experiments were carried out to aid in the selection of drilling mud for a cased & perforated well. Whilst permeability was relevant, it was most important to have a fluid that did not contribute deep damaging mechanisms or produce high fluid losses. Laboratory test data showed very significant reductions in permeability, which would normally be a concern if there was not an understanding of the nature of damage. Micro-CT scanning, in combination with geological analysis, showed that the damaging mechanisms were concentrated within the drilling mud-cake, attachment of the drilling mud-cake to the core sample, and drilling mud constituents within the first few pores of the core sample. Only scattered change, caused by some drilling mud filtrate retention and clay fines mobilisation, was seen deeper in the majority of samples; in a cased & perforated scenario the vast majority of damage would therefore be expected to be bypassed. This illustrated the value of the combination of micro-CT scanning and geological techniques to allow greater insight and more meaningful conclusions.