An Analytical Approach to Consistency Checks of Experimental PVT Data

Abstract

Abstract Pressure-Volume-Temperature (PVT) relationships have long been studied as a basis of understanding the phase behavior of fluid systems. In order to understand the behavior of these fluid systems, smaller quantities usually referred to as samples are obtained and studied under varying PVT conditions in order to define the character of the fluids. PVT experimental measurements provide key data for reservoir engineering and production applications; however the importance of having valid samples may be overlooked during preparation of the laboratory study and interpretation of the results. PVT data is an important input into the evaluation of oil and gas resources, reservoir simulation and surface equipment design. Oil and Gas companies are required to disclose estimates of their oil and gas resources on an annual basis. Accurate reporting of resources is largely dependent on an accurate fluid description, PVT derived properties like the formation oil and gas volume factor can have a significant effect on the final resource estimate. PVT data is also employed in reservoir simulation models and their effect span from the model initialization through history matching to production performance forecasting. The amount of oil and gas initially present is largely impacted by the PVT properties input; the recovery mechanisms which play an important role in the history matching and forecasting phases are also influenced by the PVT properties. A complete PVT analysis of a subsurface sample can cost anywhere around $20,000, thus in a large Niger Delta field with say 50 plus reservoirs, over a million dollars in evaluation cost can be incurred. There is a case reported in an Oil Technology publication of April 2012 where oil recovery was underestimated by as much as 40% when separator gas was used to represent both reservoir gas and average produced gas. Test lines for subsea well testing cost as much as US $60 million and with the accompanying logistics challenges, most operators have moved to install flow meters as an alternative with added optimization benefits. These meters can only operate at their full potential if they are precisely calibrated and benefit from high quality volumetric sampling that reflects the changing fluid and process conditions of the reservoir. Accurate measurement of PVT properties is important to avoiding some of the pitfalls mentioned above. This paper presents some of the analytic approaches employed in quality checking PVT data covering the application of mass balance compositional consistency checks, material balance checks on Differential Liberation and Constant volume Depletion experiments for oil and gas condensates respectively. In some instances, parameters are estimated from different sources and compared. These methods if properly applied can lead to huge savings owing to accurate facility sizing and accurate data which aids optimization efforts including subsea installations with flow meters in place. Accurate PVT data also ensures proper reporting of in-place volumes which otherwise can lead to heavy penalties as well as reputational issues if wrongly assessed and reported.