This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 175116, “Effective Method for Monitoring Oil-Based-Mud Contamination in Wireline-Formation-Tester Sampling,” by W. Pineda, BP; F. Gozalpour, formerly BP; and Mehdi Hagshenas, BP, prepared for the 2015 SPE Annual Technical Conference and Exhibition, Houston, 28–30 September. The paper has not been peer reviewed.
The wireline formation tester (WFT) is a well-developed technology used to collect representative downhole fluid samples. Collecting low-contamination downhole samples in wells drilled with miscible oil-based mud (OBM) is challenging because the process must balance the need to acquire a sample with the desired level of contamination while using the minimum rig time.
The authors introduce a simple but effective method for monitoring WFT sampling when targeting the low levels of contamination needed for asphaltene-onset-pressure analysis.
Introduction
Flow-assurance problems caused by asphaltene deposition are common reasons for lost and deferred production in Gulf of Mexico Miocene reservoirs. High-quality samples are needed to measure the asphaltene-onset pressure and manage related flow-assurance problems. The asphaltene-onset pressure varies with the level of OBM contamination. The rise of asphaltene-related flow-assurance issues encourages operators to acquire high-quality downhole samples with the lowest possible OBM contamination.
To reduce OBM contamination, operators have to pump out fluid from the formation to the borehole before filling the samples at the desired OBM-contamination level. There is not a standard procedure to determine a sample’s contamination level during the pumpout. Engineers sometimes rely on empirical evidence and previous experience to determine contamination levels. This study introduces a practical method to estimate the contamination level from real-time fluid-analyzer data. A discussion of methods used to predict contamination is provided in the complete paper.
Conventional and Focused Probes
The most-common probes used for downhole sampling in the oil industry are conventional and focused probes. Conventional probes have a single inlet to take formation fluid; the inlet is surrounded by a sealing material to isolate formation fluids from the wellbore mud column. Focused probes are available from several service companies. These probes can use a hydraulic ring inlet (guard) surrounding the probe inlet (sample) to create a barrier between the probe and the borehole fluid. The guard and sample are connected to separate hydraulic systems that allow controlling the pump rates to maintain the pressure in the guard at or slightly below the pressure in the sample. By doing this, most of the fluid drawn into the sample will be uncontaminated fluid. After the cleanout pumping, fluid from the sample inlet will be used to fill the tanks.
