Shi-Yi Zheng

Uncertainty in well test and core permeability analysis: a case study in fluvial channel reservoirs, northern North Sea, Norway

Reservoir permeability is one of the important parameters derived from well test analysis. Small-scale permeability measurements in wells are commonly made using core plugs or, more recently, probe permeameter measurements. Upscaling of these measurements for comparisons with the permeability derived from drill stem tests (DSTs) can be completed by statistical averaging methods. DST permeability is commonly compared with one of the core plug averages: arithmetic, geometric, or harmonic. Questions that commonly arise are which average does the DST-derived permeability represent and over what region is this average valid? Another important question is how should the data sets be reconciled where there are discrepancies? In practice, the permeability derived from well tests is commonly assumed to be equivalent to the arithmetic (in a layered reservoir) or geometric (in a randomly distributed permeability field) average of the plug measures. These averages are known to be members of a more general power-average solution. This pragmatic approach (which may include an assumption on the near-well geology) is commonly flawed, owing to several reasons that are expanded in this article. The assessment of in situ reservoir permeability requires an understanding of both core (plug and probe) and well test measurements in terms of their volume scale of investigation, measurement mechanism, interpretation, and integration. This article presents a comparison of core and well test measurements in a North Sea case study. We undertook evaluation of three DSTs and associated core plug and probe data sets from Jurassic fluvial channel sandstones in a single field. The well test permeabilities were generally found to differ from the core estimates, (Begin page 1930) and no consistent explanation could be found for the group of wells. However, the probe permeameter data were able to further constrain the core estimates. This study highlights the uncertainty in effective in situ reservoir permeability, resulting from the interpretation of small (core) and reservoir (well test) scale permeability data. The techniques used are traditional upscaling combined with the Lorenz plot to identify the dominant flowing interval. Fluvial sandstones are very heterogeneous, and this exercise is instructive in understanding the heterogeneity for the guidance of reservoir models in such a system.

The geochoke well test response in a catalogue of systematic geotype curves

SPE 93992 The Geochoke Well Test Response in a Catalogue of Systematic Geotype Curves P.W.M. Corbett SPE Y. Ellabad 1 J.I.K. Egert 2 and S. Zheng SPE Heriot-Watt U. Copyright 2005 Society of Petroleum Engineers This paper was prepared for presentation at the SPE Europec/EAGE Annual Conference held in Madrid Spain 13-16 June 2005. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper as presented have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the

The integration of geology and well testing for improved fluvial reservoir characterisation

Fluvial reservoirs are important hydrocarbon reservoirs world-wide. It is the fluvial depositional characteristics that give rise to the complex reservoir architectures/geometries, spatial distributions/patterns, internal heterogeneities/petrophysical properties as well as the connectivity between flow anits/channel sands that combine to give great uncertainty in characterising the effective reservoir properties, characterising the effective reservoir properties. Well testing, which measures the dynamic response of the reservoir, is potentially a very important tool for investigating these properties in fluvial reservoir systems. Through the integration of geoscience and engineering, the uncertainty resulting from reservoir description and well test analysis in such heterogeneous systems can be substantially reduced. This paper reviews the latest techniques developed from the integration of geology and well testing for fluvial reservoir characterisation. Starting from the classification of fluvial systems, deterministic geological models, based on the two end members of fluvial systems (Meandering and Braided), have been mapped out. A well test interpretation model for meandering channel reservoirs, the Pseudo-channel model has been distilled, which removes the assumption of the uniform formation thickness. Numerical solutions, termed Geotype curves have been derived. A new measure for the reservoir heterogeneity leading to an Effective flow interval has been developed, which shows a significant improvement from the conventional core analysis method. Geoskin as a geological phenomena has been identified and the technique quantifying its value with respect to geology has been developed. A wealth of geological data sets, derived from ancient (outcrops) and modern rivers, have been integrated. Fluvial Flow System Diagnostic Plots (FFSDP) have been developed, with which ten well tests of Tertiary channel sand reservoirs from the Gulf of Thailand have been evaluated. Based on these studies, a Two-stage diagnostic procedure for well test analysis has been developed. A confident interpretation requires that the tested system should be clearly mapped out prior to the transient data analysis in ensuring the correct selection of the interpretation model. Then the meaningful reservoir parameters can be inverted. The final match to the tested data should be made by the Numerical solution from the defined reservoir system, rather than force an analytical model, which are mostly idealised, for the match. An improved understanding on the scale and limits of the disciplines involved in reservoir characterisation is crucial for the integrated studies. This study gives insights into the integration and scaling of measurements as well as the need for improved geological, petrophysical and dynamic descriptions.