Abstract

A recently introduced wireline formation-testing tool incorporates new digital control technology to significantly improve formation pressure testing and fluid sampling. Closely spaced dual probes are simultaneously deployed for pressure testing and fluid sampling. Besides increasing tool reliability through redundancy, the two probes allow for advanced pressure-testing techniques. For example, formation anisotropy can be determined with an interference test between the two probes. Alternatively, a new pressure-pulsing technique can be used, which uses time delays between the probes' measured pressures to determine additional hydraulic diffusive properties of the formation. The drawdowns can be rate- or pressure-controlled from either probe, making it suitable for pressure testing in very low to high permeability formations (0.001 to 1000 md).

A primary focus of the new technology is improving the quality of reservoir fluid samples. An innovative pump-out design using a digital control feedback system enables the tester to continuously monitor the fluid sampled and to make precise adjustments to the rate and pressure at which the fluid is withdrawn from the formation and injected into a sample chamber. Because the tool design incorporates a powerful pumping-system motor and an efficient hydraulic system, invaded fluids surrounding the probes, such as mud filtrate, can be flushed 50% faster than other wireline test tools, thereby enabling quicker sampling of virgin formation fluids. Sample chambers can be filled against hydrostatic pressure to ensure pressure-volume-temperature (PVT) quality samples, and additional pressure can be applied to minimize phase changes that might otherwise occur because of temperature gradients in the borehole. The new system can also be used to perform a closed-chamber PVT test for the bubble point of the fluid sampled.

This paper details operational features of the new tester and introduces new pressure-testing and sampling interpretation techniques. Field-test results are used to demonstrate these features and techniques.

Introduction

Wireline formation testing tools have been available since the mid-1950's and have undergone generations of evolutionary changes. Early tools were conceived as simple sampling devices with a single probe that established formation fluid flow with a bullet perforation charge.1 Strain-gauge pressure transducers quickly followed, adding a new dimension to wireline formation evaluation.2 In the 1970's, dual chambers and pressure sensing were soon followed by the next generation of hydraulically powered tools with repeated pressure-testing capabilities and high-resolution quartz gauges.3,4 Multiple probes, pumps and fluid sensors were added in the late 1980's to early 1990's to purge the zone near the wellbore of filtrate while monitoring sample quality.5 Only recently have sample chamber-filling techniques been improved to reduce the shock experienced when the chamber is opened and filled.6

High-quality samples continue to be the driving force for each new advancement. Now microprocessor digital control technology combined with modern hydraulic servo systems add a new level of sampling control and improved quality. Even when a sample is carefully drawn into the tool, if it is not kept in a single phase in the chamber before it is retrieved to the surface, its quality can be corrupted. A complex sequence of events transpires during the pumping of the formation fluid, filling of a sample chamber and bringing the sample to the surface. The events must be precisely monitored and controlled to insure that a single-phase PVT-quality sample is delivered.

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