Technology Update: Wireline Sampling Technology Enables Fluid Sampling Without Contamination
Authors: JPT staff _ 
Published: February 2015

A focused sampling technique using a new wireline sampling tool was applied successfully in the Cairn Energy-operated Bhagyam field, Rajasthan, northwest India. Favorable results were achieved in this field, formation characteristics of which—highly viscous, waxy crude and oil-based mud (OBM)—had presented numerous challenges in obtaining good representative reservoir-fluid samples, even after long pumping times. Of the 18 samples collected from two wells, 83% were of pressure/volume/temperature (PVT) quality, and 33% of the samples showed zero mud-filtrate contamination.

Discovered by Cairn Energy in 2004, the Bhagyam field is one of 19 fields in the Barmer basin (Fig. 1). The high-permeability Fatehgarh sandstone is the basin's main hydrocarbon reservoir. Cairn Energy currently estimates oil reserves in this main reservoir (in three major fields) at 1.5 billion bbl, with reserves in Bhagyam field amounting to 25% of the total. The reservoir is saturated and exhibits compositional grading of oil. Hence, its crude-oil properties vary widely, from 15°API in the north to 52°API farther south. Although Bhagyam oils are not as dense as other heavy crude, with a 21–31°API oil-density range, they have high wax content that gives them a high viscosity and pour point.

Early in the exploration and development cycle, high-quality, representative reservoir-fluid samples provide essential information about the properties of hydrocarbon fluids, such as saturation pressure, density, viscosity, and gas/oil ratio (GOR). During the early development phase, an operator's determination of actual reservoir-fluid properties is critical for optimization of its well-completion and production-facility designs.

The formation characteristics and highly varied, highly viscous oils at Bhagyam presented challenges in acquiring representative, PVT-quality samples. The high wax content also made sampling difficult with both traditional formation testers and drillstem-test samplers, particularly because OBM drilling was used to avoid shale collapse in the reservoir section.

While drilling, contamination from miscible drilling-mud filtrate occurs when sampling oil in OBM wells, still presenting the biggest risk in obtaining good reservoir-fluid samples. During sample collection, OBM filtrate is collected with reservoir fluid, resulting in significant mud-filtrate contamination. This drastically reduces the sample quality and makes PVT laboratory analysis unreliable and often incorrect.

When attempting to sample with conventional openhole sampling techniques in the Bhagyam field, long pumping times did not achieve good, representative samples. Of the more than 20 samples acquired by use of traditional formation testers by Schlumberger and another service company, all were determined to be nonrepresentative. They were too contaminated to yield PVT properties during laboratory analysis. Mud-filtrate contamination can be assessed downhole by the live-fluid analyzer (LFA) in real time before fluid samples are collected. For example, at one sampling station in Well Bhagyam-4, LFA analysis quantified a 36% contamination—even after pumping for 105 minutes. Consequently, until recently, only simulated PVT data from mathematically corrected samples could be used for Bhagyam field development studies.