Reservoir Fluid Sampling of a Wide Spectrum of Fluid Types Under Different Conditions:
Authors: Nagarajan Narayana Rao 
Published: May 2016

Reservoir fluid PVT is the basis for efficient reservoir development plans, appropriate facilities, and plant design and operation and plays a vital role throughout the production life of the reservoir. Acquisition of representative high quality in-situ reservoir fluid samples is the first step in this process and requires special attention. The challenges of acquiring representative samples depend on a number of factors, including the type of the fluid being sampled, the initial reservoir conditions, and the formation properties.

The sampling issues related to various fluids are discussed, such as oils (i.e., heavy, black, volatile) and rich and lean gas condensates. Furthermore, the challenges involved in sampling fluids containing acid gas components and in cases where oil-based mud (OBM) is used during drilling are also outlined. In addition, the challenges of sampling reservoirs that are near-saturated and contain unconsolidated formation are highlighted. Finally, the challenges posed by unconventional plays are also described, such as shale reservoirs that tend to be in transient flow for long periods. The need for a rigorous decontamination technique for OBM contaminated samples is discussed. The following are the key elements for a successful sampling program:• A customized pre-job sampling plan• Meticulous on-site implementation with proper QA/QC• A contingency plan to acquire as best a sample as possible in case of unexpected operational difficulties• A rigorous method of decontamination to obtain in-situ composition in case of OBM contamination

In this paper, these elements and possible solutions of the formidable sampling challenges are discussed in detail and illustrated with field examples.

Introduction

Reservoir fluid PVT properties play a critical role in the entire life of the reservoir from exploration to abandonment and in optimizing the hydrocarbon potential from sub-surface to sales point. Representative reservoir fluid properties are required for reservoir evaluation and performance predictions. Reservoir fluid PVT is the key to estimating in-place volumes, evaluating a range of development scenarios, and designing appropriate production processes(1). In addition, the production facilities design and flow assurance requirements are dictated by the fluid properties over the entire range of operating conditions. The first major step in the process of obtaining reliable fluid PVT data is the acquisition of representative reservoir fluid samples through a customized sampling program. In addition to PVT samples, samples are needed for plant and process analysis, geochemical data for fluid source identification and reservoir continuity, and crude assay for refinery processes. The main objective of a successful sampling program involves several tasks as listed below.o Compilation of the list of samples required by various disciplines with the quality and quantity of samples neededo Selection of a sampling methodo Tool selection, tool string design, selection of specific sampling techniques and the associated procedureso Implementation with best practice quality control measuresThe key challenges in any successful fluid sampling program areo Possible two-phase flow in the reservoir resulting in the capture of non-representative sampleso Water and fines/sand production during samplingo Presence of solid forming compounds like waxes and asphaltenes and non-hydrocarbons such as CO2 and H2So Fluid contamination introduced by drilling and