Characterization of Sampling-While-Drilling Operations

Abstract

SummaryThe while-drilling environment poses several potential challenges to acquiring representative formation-fluid samples; in particular, sampling times are expected to be restricted (perhaps severely) and dynamic filtration conditions are expected to predominate. With the aim of formulating effective strategies for sampling-while-drilling (SWD) operations, it is natural to seek answers to at least the following questions: When should samples be acquired—immediately after drilling the formation when invasion is shallowest, or after some time when the mudcake is more mature? What is the effect of circulation rate on sample quality? What levels of sample quality are achievable under while-drilling conditions?The first part of the paper describes the SWD tool used to investigate the questions posed. The tool is composed of the following components: a probe module; a pump-out module, which contains within it fluid-property sensors, including a resistivity cell and a 10-channel optical spectrometer; sample-capture modules; and a power-generation module consisting of a dedicated mud turbine and alternator that provide the required pumping power to the sampling tool.The second part of the paper is devoted to the description and analysis of a series of tests performed to characterize SWD operations. In particular, the following aspects were studied for their effect on sampling efficiency and sample contamination: time after drilling, mud-circulation rate, ceasing and resuming pumping operations as a means of mitigating sticking risk during protracted pumping operations, and the integrity of samples captured under drilling conditions. All tests were performed in water-bearing formations drilled with a water-based mud. A wireline sampling (WLS) tool was run after the while-drilling tests were completed in order to compare the performance of the two sampling tools.The principal conclusions derived from the tests conducted are that present-day sampling technology can be made to survive and operate effectively under drilling conditions, and that there appear to be substantial advantages to acquiring samples soon after drilling a formation.