Sampling While Drilling: An Emerging Technology
Authors: Villareal Steven , Pop Julian , Bernard Francois , Baxter Martin , Hakam Ahmad , Firth Andy , Megat Amanda , Fey Scott , Haq Shahid , Vahi Nitin 
Published: October 2012

Even though it is very early days in the realisation of sampling-while-drilling (SWD) as a service the picture regarding what might be achievable in practice is beginning to emerge. Previous experience, accumulated primarily in sampling water bearing formations drilled with water-based muds, has demonstrated that the sensors used to infer sample contamination and derive formation fluid properties are capable of withstanding the rigours of drilling without losing precision, that the information provided by these instruments can be used in real-time to control the sampling process and capture wireline-quality samples, even when employing mud-pulse telemetry, and that, once captured, sample integrity can be maintained until the samples are transferred at surface. Based on this experience operational guidelines have been formulated, in particular, regarding the most opportune time to sample during the drilling process.

The first part of the paper describes SWD in a high-angle appraisal well drilled to assess the continuity and quality of several target sands and to establish the degree of continuity of these sands with the main field. The well was drilled using an oil-based mud. Samples would be acquired after the well had reached total-depth so that the most appropriate sampling points could be identified by means of open-hole logs. Multiple water, oil and gas bearing formations were identified. Sampling duties were split between the SWD tool and a drill-pipe-conveyed wireline sampling (WLS) tool in an attempt to rationalise the well's sampling programme. To enable a comparison of the relative performance of the two tools, two oil sampling stations were chosen where multiple samples would be acquired under similar operating conditions by both tools. Additionally, water samples were collected at two stations by the SWD tool and scanning was performed at one (wet) gas station to confirm the formation fluid type. Laboratory analysis of the oil samples at the common stations showed that the contaminations and fluid properties of the samples acquired by the two sampling tools were very similar.

The second part describes results obtained in an appraisal well and sidetrack in a different field. Both pilot and sidetrack were high-angle wells drilled using oil-based muds. The purpose of these wells was to identify and evaluate the commercial potential of unproduced hydrocarbon bearing zones. Both wells were drilled to total-depth prior to conducting sampling operations and no wireline operations were planned in either well. Five oil samples were acquired in the pilot well in two zones and four oil samples and two water samples were recovered in the sidetrack.

The results obtained during the sampling-while-drilling operations described suggest that it is possible to consistently acquire quality formation fluid samples during drilling operations, even under less than optimal sampling conditions and strict time-on-station constraints. The quality of the samples recovered is sufficient to perform reliable PVT analyses.