Linear Pressure Gradients - Myth or Reality?
- Mohamed Hashem (North Caspian Operating Company) | Hani Elshahawi (Shell International E&P Inc.) | Birol Dindoruk (Shell International E&P Inc.)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- August 2011
- Document Type
- Journal Paper
- 275 - 287
- 2011. Society of Petrophysics and Well Log Analysts
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- 421 since 2007
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For decades, formation-testing practitioners have adopted the practice of imposing linear trend lines on formation pressures in order to define reservoir fluid gradients and contacts. This practice was convenient for a long time due to the general unavailability of sufficiently precise pressure and density measurements. But the underlying assumption of constant density in the hydrocarbon column is often simplistic or even invalid. It is widely accepted now that reservoir and fluid complexities are the norm rather than the exception. We have missed these realities mainly because we did not look hard enough for them, because we did not have the enabling technologies to do so, but also because simplicity is usually more convenient than complexity.
Recent advances in measurements and workflows, including the development of higher accuracy and resolution pressure gauges and more reliable downhole fluid analysis tools, have resulted in significant improvements in the quality of formation pressure data. It is thus time to revisit the validity of conventional techniques such as linear extrapolation of pressures. In principle, the linear approximation of the reservoir pressure gradient is only valid if the reservoir fluid density varies over the fitted interval by an amount less than the accuracy of the measurement and of the fitting technique. Based on the examination of thousands of data sets, we contend that this basic assumption is frequently violated. Even more disturbing is the widely-adopted practice of extrapolating linear trends beyond the existing data set to establish fluid contacts without independent corroborating data.
In this paper, we will show nonlinear hydrocarbon gradients are the rule rather than the exception. Nonlinear gradients are prevalent, even in relatively thin hydrocarbon columns. Therefore, any attempt to force linearity or to extrapolate a pressure data set in a manner that is inconsistent with this understanding can lead to incorrect fluid contacts and inaccurate estimates of in-place hydrocarbon volumes. The location of fluid contacts (or perceived contacts) and their dynamic behavior have further implications in terms of well locations and proposed completion and depletion schemes. We will propose solutions and guidelines for the extraction of fluid gradient and contacts and establish criteria for the range of validity of conventional techniques using a case study as example.
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