Downhole Analysis and Laboratory Analysis, Key Complementary Techniques for a Comprehensive and Effective Fluid Characterization

摘要

Downhole sampling and laboratory analysis are key complementary techniques offering a step change in fluid characterization. It is generally accepted that fluids in the reservoir are in chemical equilibrium. However this assertion, although convenient, is often invalid, for several reasons from source variations, due to in-reservoir reaction, degradation or precipitation. In a large majority of cases many of these phenomena are difficult to appreciate, because the techniques used for fluid sampling often lack the pin-point acquisition accuracy (geographical, depth and time) required to provide the information at a level of accuracy sufficient to detect subtle variations. Instead only a mixture of different oil is captured providing “averaged” oil composition and characteristics, whereas the reality in the reservoir may be markedly different. The vertical compositional gradient in oil column has been documented in many large oil column reservoirs. The geographical variation has been much less documented. In this paper we will demonstrate how variation at the scale of individual sand bodies in the reservoir, sometimes only tens of feet apart, can be very large (as observed in two sand bodies in the same well) and have a dramatic impact on oil property understanding and modeling. In turn understanding of these property variations and their impact on mobility, is a key factor for understanding fluid flow and for the choice of the correct secondary recovery mechanism for an improved recovery. The example of the Wara channelized sand, within the Greater Burgan complex, will illustrate the paper, where downhole fluid analysis is presented as a necessary complementary tool to improve the selection and construction of an accurate set of samples to ensure the data collection is complete and exhaustive before the well is completed.