Pressure Measurement and Pressure Gradient Analysis: How Reliable for Determining Fluid Density and Compositional Gradients?

Abstract

Pressure-depth plots have been used for over thirty years to evaluate fluid density, fluid contacts, and pressure compartmentalization in formation tester pressure surveys. However in the Niger Delta region and other offshore deepwater environments, many reservoirs are multilayered and highly variable in terms of connectivity, permeability and fluid properties. Such complexity and reservoir heterogeneity means conventional pressure-depth plot and pressure gradient analysis of wireline pressure data is not easy, and identification of in-situ fluid type can be difficult. There is also mounting evidence for the presence of compositional gradients in the hydrocarbon columns of some reservoirs - this raises questions about the conventional approach to pressure gradient analysis and uncertainties in inferring fluid properties and contacts from pressure gradients. In this contribution using several field examples, we discuss and review formation pressure measurement techniques and data quality, and compare conventional and advanced methods of pressure gradient analysis and fluid contact determination. The interpretation techniques compared include traditional pressure-depth graphical methods, the excess-pressure method and statistical tests. Depth dependent fluid property variation from fluid gradients, PVT properties and EOS-models are compared and discussed. Guidelines are presented on how to interpret wireline pressure measurements in multilayered siliclastic reservoirs, perform connectivity and compositional gradient assessment. We describe how to improve on these interpretations by performing more advanced formation testing procedures; some of which are based upon new and emerging technology. Introduction Formation pressure profiles have long been used as important tools for determination of reservoir pressure, evaluation of fluid type from in-situ densities, identification of fluid contacts, and differential depletion, and inter-reservoir connectivity. Pressure profiles and fluid density gradients provide valuable information for reservoir evaluation and management, and input to aid decisions about well completion strategies and field production schedules. Modern wireline formation testers are equipped with quartz pressure gauges of high accuracy, resolution and repeatability. Theoretically, this higher resolution is sufficient for detection and evaluation of small fluid density changes and pressure variations within individual wells - this capability potentially leads to some new applications and interpretation techniques for formation pressure data. These new applications could include improved integration of pressure measurements and pressure gradient analysis with reservoir fluid and thermodynamic models. However such an approach requires good data quality control and data analysis, since pressure measurement problems, supercharging, wettability effects, and depth measurement errors can make it difficult to acquire representative pressure data and hence limit fluid gradient accuracy and reliability.

Wireline Formation Testers Unlock Potential in a Deepwater Gas Condensate Reservoir

“…This raised some initial questions about the pressures and hydraulic connectivity between Sand 1 and Sand 2. The pressure gradient established in Sand 1 was further analyzed using the excess pressure technique (ref) to assess if there were any subtle fluid density variations within Sand 1 and test the initial assumption of a single and linear formation fluid gradient (Jackson et al, 2007). The excess pressure analysis of the pressures measured across Sand 1 provided some evidence for a subtle variation of fluid density variation with depth (see Fig.…”

Complexity of Wettability Analysis in Heterogeneous Carbonate Rocks, A Case Study

“…Al., 1984;Elshahawi et al, 1999;Elshahawi et. Al., 2000;Pham et al, 2005;Carnegie, 2006;Chenggang et al, 2006 andJakson et al, 2007) have commented on estimating accurate formation pressure gradient from the wireline formation pressure testers. One of the main conclusions from their works is that oil can be produced with a low water cut in a reservoir with measured water gradient.…”

Application of a New Method to Estimate Free Fluid Level and Recognition of Compositional Grading Using Wireline Formation Testing Data

“…The uncertainty in the location of the fluid contacts has a significant impact on the reserves estimate and future decision on production wells. [1][2][3][4] There are different Modern Wireline Formation Testing tools like Schlumberger's MDT/XPT which could measure reservoir formation pressure with high accuracy. In a reservoir with considerable mobility, reservoir formation pressure is directly proportional to in-situ fluid density.…”