In-Situ Fluid-Composition Analyses Improve Reservoir Management
Authors: Carpenter Chris 
Published: August 2017

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 181526, “New Wireline, In-Situ, Downhole Fluid Compositional Analyses To Enhance Reservoir Characterization and Management,” by Gabor Hursan and S. Mark Ma, Saudi Aramco, and Wael Soleiman, Sami Eyuboglu, Neeraj Sethi, and Nacer Guergueb, Halliburton, prepared for the 2016 SPE Annual Technical Conference and Exhibition, Dubai, 26–28 September. The paper has not been peer reviewed.

This study focuses on recent experience in Saudi Arabia with crude-oil compositional analyses during pumpout with a wireline formation tester (WFT). It summarizes experience with the in-situ measurement of methane, ethane, propane, saturates, aromatics, and gas/oil ratio (GOR) on the basis of multivariate optical computing (MOC) conducted at more than 200 pumpout stations in a total of 37 wells drilled with a variety of inclinations, bit sizes, and drilling fluids in several oil and gas fields.

Introduction

In reservoir-fluid characterization performed in the laboratory conventionally, samples of representative formation fluids are analyzed to determine bulk fluid properties, fluid-phase behavior, and chemical properties. Exploration and evaluation wells are often drilled exclusively for fluid-analysis purposes for which the only way to analyze or capture formation fluids is a downhole pumpout WFT (PWFT). Capturing high-quality reservoir samples is one of the most important objectives in any PWFT job.

The keys to ensure fluid cleanup during pumpout are (1) a set of downhole sensors measuring the pumped fluids and (2) the unambiguous contrast between the drilling-mud filtrate and reservoir fluids as measured by at least one of these sensors. Avoiding such fluid ambiguity is a challenge in many situations. Optical sensors respond with high sensitivity to chemical compositions of fluids. Implemented downhole, these tools can provide a powerful means of differentiating between the oil-based-mud (OBM) filtrate and reservoir oils.

The most valuable benefit of these measurements is the prompt availability of in-situ fluid-composition data without the additional steps of acquiring, transporting, and analyzing physical fluid samples in the laboratory. This process requires durable, robust, and accurate optical sensors that operate reliably and consistently in the hostile downhole environment. One recently developed optical-sensor system is based on an optical device known as an integrated computational element (ICE) that performs the mathematical operation of MOC. For each fluid component, an ICE “core” is engineered such that only one particular fluid component or property is accentuated in the detector response and everything else is muted. This detector response is then used to infer the abundance of the fluid component of interest. These optical elements are typically very broadband and may have a response that extends from 400 to 5000 nm. The high bandwidth of these optical elements combined with their intrinsic high signal/noise ratio enables laboratory-grade optical analysis downhole.