Downhole Fluid Analysis and Sampling: Some Results and Learnings from Shallow Offshore Niger Delta

Abstract

Abstract Downhole fluid analysis (DFA) and sampling entails deploying special tools and sensors to measure some physical and chemical properties of reservoir fluids for quality control in-situ before extracting samples of same fluid downhole. In principle, samples preceded by DFA should yield more representative fluid properties for reservoir characterization and other applications than obtainable with surface sampling. Typically, DFA tools are either optical or nuclear. The latter employs nuclear sources and detectors, while the former utilizes light sources and detectors to measure the optical properties of reservoir fluids. Combined with proprietary algorithms, the optical properties are exploited to determine relevant fluid properties such as fluid type, density, gas-oil ratio (GOR), composition, and condensate-gas ratio (CGR). Towards improving confidence in DFA, this paper presents the results and learnings from deploying optical-type DFA services in some oil and gas reservoirs in the shallow-water area of the Niger Delta. We compare results from DFA against outcome of laboratory studies (gas chromatography and phase behaviour) conducted on same fluid samples. Deviations between DFA and laboratory measurements of fluid compositions and other properties are about 2 - 308%. For the fluid samples examined in this study, the largest deviations between DFA and laboratory measurements apply to composition (~308%) and GOR (~74%). Relevant learnings and improvement areas for future DFA applications include (i) proper pre-job planning amid subsurface uncertainties, (ii) rigorous quality-control of results plus post-job engagements, and (iii) robust integration of results with other independent sources such as petrophysical logs, static-pressure profiling, equation-of-state models, and analogues. Based on this study and learnings from other applications, major improvement areas for DFA include (i) reliable determination of fluid contamination downhole before initiating sample extraction, (ii) robustness of compositional analysis in terms of accuracy and range of components covered, as well as (iii) accuracy of GOR/CGR measurements. Body text 2 paragraphs.