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.
This paper presents a novel workflow for seismic net pay estimation with uncertainty. It is demonstrated on the Cassra/Iris Field. The theory for the stochastic wavelet derivation (which estimates the seismic noise level along with the wavelet, time-to-depth mapping, and their uncertainties), the stochastic sparse spike inversion, and the net pay estimation (using secant areas) along with its uncertainty; will be outlined. This includes benchmarking of this methodology on a synthetic model. A critical part of this process is the calibration of the secant areas. This is done in a two step process. First, a preliminary calibration is done with the stochastic reflection response modeling using rock physics relationships derived from the well logs. Second, a refinement is made to the calibration to account for the encountered net pay at the wells. Finally, a variogram structure is estimated from the extracted secant area map, then used to build in the lateral correlation to the ensemble of net pay maps while matching the well results to within the nugget of the variogram. These net pay maps are then integrated, over the area of full saturation gas, to give the GIIP distribution (Gaussian distributions for the porosity, gas expansion factor, and gas saturation for the sand end member are assumed and incorporated in the estimate of GIIP). The method is demonstrated on the Iris (UP5 turbidite) interval. The net pay is corrected for reduction in the amplitudes over part of the area due to shallow gas. The sensitivity of the GIIP to the independent stochastic variables is estimated (determining the value of information) so that business decisions can be made that maximize the value of the field.
No abstract
Shallow localized gas pockets cause challenging problems in seismic imaging because of sags and wipe-out zones they produce on imaged reflectors deep in the section. In addition, the presence of shallow gas generates strong surface-related and interbed multiples, making velocity updating very difficult. When localized gas pockets are very shallow, we have limited information to build a near-surface velocity model using ray-based reflection tomography alone. Diving-wave refraction tomography successfully builds a starting model for the very shallow part. Usual ray-based reflection tomography using single-parameter hyperbolic moveout might need many iterations to update the deeper part of the velocity model. In addition, the method generates a low-velocity anomaly in the deeper part of the model. We present an innovative method for building the final velocity model by combining refraction, reflection, and wave-equationbased tomography. Wave-equation-based tomography effectively generates a detailed update of a shallow velocity field, resolving the gas-sag problem. When applied as the last step, following the refraction and reflection tomography, it resolves the gas-sag problem but fails to remove the low-velocity anomaly generated by the reflection tomography in the deeper part of the model. To improve the methodology, we update the shallow velocity field using refraction tomography followed by wave-equation tomography before updating the deeper part of the model. This step avoids generating the low-velocity anomaly. Refraction and wave-equation-based tomography followed by reflection tomography generates a simpler velocity model, giving better focusing in the deeper part of the image. We illustrate how the methodology successfully improves resolution of gas anomalies and significantly reduces gas sag from an imaged section in the Greater Cassia area, Trinidad.
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