Identifying and Assessing Productivity in CO2 Bearing Gas Zones Using Wireline Logging and Testing Data

Abstract

One of the common challenges in the Malay-ThoChuBasin of Vietnam is that hydrocarbon accumulations consist of multiple layersand compartments (tens to hundreds) with variable reservoir quality, hydrocarbon types and CO2 contents. Understanding the CO2 distribution before committing to a productiontest is essential. Neither mud logging nor Logging While Drilling (LWD) nor traditionalwireline measurements are able to quantify CO2 content. An efficient method toquantify CO2 content is to usenear infrared equipment mounted along the Wireline Formation Tester (WFT) flowlinedownhole. This approach can then be validated against laboratory measurements. Permeability is a critical parameterwhen dealing with heterogeneous reservoirs management and well performance. Thecontinuous permeability measurement from Nuclear Magnetic Resonance (NMR) loggingcan be optimised if calibration information is available, particularly from intervalpressure transient tests (IPTT) obtained from "Mini-DST" packer formation testingof selected flow units, and even from pumpouts performed for downhole fluidanalysis and sampling. The calibrated continuous permeability curve from theNMR log and hydrocarbon compositions from the WFT sensor are then used to design conventional drillstem testing, and can also be used to model deliverability of reservoirs nottested by a conventional DST. Three case studies are presented. Thefirst is a well that encountered multiple reservoirs with variable CO2 content. Downhole measurements using the WFT sensor were acquired from every reservoir>2m thick with PVT samples obtained from four reservoirs for calibration. Subsequent comparison and delumping of the downhole compositions produced areliable estimate of CO2 content and basic PVT parameters in all significantreservoir sands. The second case study demonstratesapplication of an IPTT obtained from a dual-packer "miniDST" on a thin, highCO2 hydrocarbon-bearing sand to calibrate the NMR continuous permeability curve. This was then used to model the deliverability of a thick, low CO2 reservoirwithout the need for a full production DST. The third case studyshows anintegration of downhole compositional measurements, NMR permeabilitycalibration on wireline packer IPTT and comparison to an actual production DSTfrom one of the reservoir intervals.