Reservoir Fluid Evaluation from Real Time Pressure Gradient Analysis: Discussions on Principles, Workflow, and Applications

Abstract

Modern formation pressure testing while drilling (FPWD) tools provide accurate formation pressure measurements, even under very challenging drilling and formation testing conditions. Pressure data from logging while drilling (LWD) tools are primarily used for drilling safety related pore pressure profiling, mud weight optimization and ECD controls. Its full potential in formation and reservoir evaluation has yet to be further explored. Pressure gradient analysis is a particularly costeffective solution for preliminary reservoir fluid evaluations while drilling or shortly after the well is drilled. Integrated with other LWD and surface logging data, pressure gradient analysis is able to provide valuable insight for planning subsequent fluid sampling, well completion and reservoir development programs which require accurate assessments of fluid types, fluid contacts, reservoir compartmentalization, and connectivity. Based on data and experience from recent worldwide FPWD and gradient jobs, we present a recommended best practice workflow for pressure gradient analysis using real time pressure testing data. The main procedures of the workflow consist of pre-job preparation, real time wellsite execution, quality control, statistics and error analysis, proper graphic presentation, data integration and interpretation. Special effort is made to identify fluid types, contacts, vertical fluid variation and reservoir barriers from pressure gradient analysis. It is found that the integration of pressure data with openhole logs and surface mud gas logging is particularly effective in reducing uncertainties in the interpretation. Possible factors responsible for departure from linear pressure gradient trend lines are discussed together with the limitations of gradient analysis. Quick reservoir evaluation in terms of fluid types, oil water contact (OWC), oil gas contact (OGC) and flow barriers can be obtained independently with the use of pressure gradient analysis from FPWD data. More reliable results are achieved by the integration of pressure measurement data with the openhole log interpretation, surface logging and offset well information collected in the pre-well planning stage as well as while the well is being drilled. Introduction For several decades, the pressure gradient plot has been used for identifying reservoir fluid types, contacts and reservoir connectivity (Chen, 2003; Steward et al., 1979, 1982). The information obtained from pressure gradient analysis has contributed to improved reservoir characterization and optimized completion and production strategies. Vast majorities of previous gradient work have been carried out with data from wireline formation pressure testing data and production logging tools (PLT). In recent years, increased usage of LWD formation pressure testing has broadened the spectrum of available raw data for pressure gradient analysis (Meister et al., 2003, 2004; Buysch et al., 2005).

Reservoir Characterization from Gas Ratio Analysis Using New High Efficiency Gas Extraction System

“…In spite of both approaches being capable of independently obtaining fluid properties, the correlation between fluid types and contacts between gas ratios and pressure gradient analysis can complement each other and result in elevated confidences in the interpretation and better understanding of reservoir compartmentalization and vertical communications (Zhou et al 2008). In spite of both approaches being capable of independently obtaining fluid properties, the correlation between fluid types and contacts between gas ratios and pressure gradient analysis can complement each other and result in elevated confidences in the interpretation and better understanding of reservoir compartmentalization and vertical communications (Zhou et al 2008).…”