Characterization of Sampling While Drilling Operations

Abstract

Abstract Although formation pressure-while-drilling (FPWD) tools have been introduced relatively recently the service is well accepted not only for its application to drilling optimization but also for reservoir characterization, especially in developed reservoirs. The next step in the progression of reservoir-related services to the drilling environment is clearly the acquisition of formation fluid samples while drilling. The while drilling environment, however, poses several potential challenges (and opportunities).For example: when should one sample, immediately after drilling the formation when the invasion is shallowest, or, after some time when the mud cake is more mature?; what is the effect of circulation rate on mud cake integrity?; and so on. The first part of the paper describes a sampling-while-drilling (SWD) tool which is comprised of the following components: a probe module; a pumpout module, which has contained within it fluid property sensors, including a resistivity cell and a ten-channel optical spectrometer; a multi-sample module; and, a power generation module, consisting of a dedicated mud turbine and alternator which provides 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 which were performed to characterize sampling while drilling operations. In particular, the following aspects were studied for their effect on sampling efficiency and sample contamination: time after bit; mud circulation rate; ceasing and resuming pumping operations as a means of mitigating sticking risk during protracted pumping operations; and, captured sample integrity under drilling conditions. All formations tested contained water as the formation fluid and the well was drilled with a water-based mud. Samples were acquired at multiple times during a single station and at multiple stations during the sampling operation in order to evaluate the effect of the test parameters on sample contamination and clean-up. To aid in the laboratory contamination analysis the mud was doped with bromine and iodine tracers. In addition, a water soluble blue dye was mixed with the mud to allow real-time monitoring of clean-up during pumping by means of the optical spectrometer. A wireline sampling tool was run after the while drilling tests were completed in order to compare the performance of the two sampling tools. In addition to the usual LWD logs, gamma ray, array resistivity, density-neutron and spectral gamma ray, a shallow array resistivity tool was run to quantify the extent of invasion at various times during the drilling/sampling operation. The principal conclusions derived from the series of 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 advantages to acquiring samples soon after drilling a formation.