Advanced Compositional Gradient Analysis

Cañas, Jesus A.¹
 Pop, Julian¹
 Dubost, Francois¹
 Pomerantz, Drew¹
 Mullins, Oliver¹

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Formation testers supply a wealth of information to observe and predict the state of fluids in petroleum reservoirs,through detailed pressure and fluid analysis measurements. With the correct understanding of fluid characteristics in the reservoir, reserve calculations and adequate development plans can be prepared. Additionally, flow barriers may then be revealed, as across such barriers, fluid may appear different. In order to test for the existence of such barriers, pressure or fluid differences, in their context must be assessed. Formation tester data must therefore be treated by means of a systematic analysis, so that different sources of information lead to an integrated, preferably, consistent conclusion. Downhole fluid analysis, geochemical analysis in conjunction with pressure gradient analysis is simultaneously analyzed to reflect and make deductions concerning the correct state of fluids in complex fluid columns.

In this paper, a novel methodology is presented to account for non-linearity in pressure gradients due to varying fluid density. State-of-the-art analytical chemistry methods support this analysis. We examine three field cases in which compositional gradients were observed with both downhole fluid analysis measurements and pressure gradients. An equation of state model is used to describe the fluid column according to a simple fluid equilibrium model. A non-linear pressure gradient regression fit is also examined. The choice and appropriateness of the pressure gradient model that accounts for the observed fluid density changes observed. The result of the modelled fluid and pressure analisis are compared to actual
downhole measurements of the pressure profile and insitu fluid logs.