Wireline Sampling Technology Enables Fluid Sampling Without Contamination

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.

Fig. 1—Bhagyam field, located in the Barmer basin.

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.

The QuickSilver Probe, a wireline formation tester and technique developed by Schlumberger, differs from its sampling predecessor to address this mud-filtrate-contamination problem. The technique separates drilling-mud-filtrate contamination efficiently from the formation fluid in the early stage of the sampling process, allowing cleaner samples and faster collection compared to traditional probe-type wireline formation testers.

Probe design was driven by the phenomenon that filtrate flows faster to the sampling point than does formation fluid because of its fluid-viscosity ratio and the influence of the reservoir’s vertical and horizontal permeabilities. The solution splits the fluid flow into two paths, with the central sampling area isolated from a surrounding guard area around the perimeter.

These central and perimeter areas in the downhole tool are essentially concentric probes, which are connected to independent pumps and separate, discrete sample and guard flowlines, with each having full downhole-fluid-analysis capabilities. During sampling, the pumps create higher intake velocities at the surrounding guard area, creating an easier path for the contaminated fluid and directing it to flow toward the perimeter, while the formation fluid is drawn into the central sampling area. In essence, a cone is created, with mud filtrate moving along the wellbore wall and taken in at the guard probe. Formation fluids flow directly into the sample probe, resulting in a less contaminated, more representative reservoir sample.

The approach was used in two Bhagyam wells. In Well 1, the probe drew in fluid that registered 0% OBM contamination on the LFA detector in only 27 minutes of pumping time. Independent laboratory analysis confirmed no contamination. In Well 2, after 52 minutes of pumping, the focused-sampling-technique/LFA combination sampled fluid that averaged 2.2% contamination. However, subsequent laboratory analysis determined a 0% contamination level. Of the 18 samples collected from the two wells, 15 were of PVT quality, and 6 samples showed zero contamination, as was later confirmed in the laboratory (Fig. 2). Time required for sampling was less than half the time required with conventional methods.

Fig. 2—Plot of 15 samples taken with the QuickSilver Probe; 15 were of PVT quality, and 6 showed zero contamination.

Information provided by Sanjay Kumar and Stuart Wheaton, Cairn Energy, and Dibyatanu Kundu and Sameer Joshi, Schlumberger.