Geosteering and reservoir mapping from deep resistivity and acoustic measurements have revolutionized the drilling industry. Looking around, ahead, and aside information allows for optimal well placement in complex reservoir structures, while drilling increasingly difficult well trajectories. Downhole fluid analysis (DFA) while drilling, aka fluid mapping while drilling (FMWD), can make geosteering even more powerful by incorporating actual InSitu fluid information into landing and geosteering decisions. This paper looks at successful FMWD based well placement made in a Malaysian field.
Field XYZ is a prolific oilfield with a strong gas cap and moderate water drive. The operator has planned a new greenfield campaign to boost declining production by drilling new horizontal oil producers from the existing well centers. The Late Oligocene - Miocene sandstones are well known homogenous units. However, salinity, resistivity contrast, and long transition in the swept zone become a major challenge. Active production, gas cap, aquifer movement, pressure depletion and complex porosity and permeability distribution cause uncertainties on the actual fluid contacts. Fluid typing from petrophysical measurements is inconclusive. Modelling for reservoir mapping resistivity-based measurements highlighted the challenges to land the wells in the thin oil pay zone with degrading rock quality at the bottom and uncertain OWC depth. The operator decided to drill a pilot section through the reservoir sequence to acquire critical subsurface data, identify actual fluid contacts and set the appropriate landing point. An FMWD service was integrated in the bottomhole assembly to provide a comprehensive reservoir fluid characterization.
Eight (8) acquisitions have been performed. Before acquiring DFA stations the initial program called to draw fluid gradients from pretests. This method turned to be misleading and inconclusive due to limited reservoir thickness, variable permeabilities and depletion. It was changed to a more extensive DFA acquisition with less reliance on pressure-based gradients. Differential sticking is a major historical risk due to active production. A new operational workflow was engineered to mitigate this risk. Conventional long pump-outs were fragmented into a set of repeat shorter ones at same depths within a stationary time limit set by drilling. Fluid characterization was thereby safely and efficiently performed by ensuring proper control of borehole stickiness with no stuck pipe event or excessive overpull, and without compromising the formation evaluation objectives or clean-up process. Acquired fluid information in the sand sequence provided new insights of the fluid distribution and contacts, yielding increased accuracy on the landing position of the production sections, and well producibility.
While still mostly perceived as an exploration/appraisal formation evaluation technique, FMWD is a powerful and disruptive geosteering solution. The combination of InSitu and while-drilling fluid, petrophysical and producibility data allows optimal positioning of wellbores, eventually yielding increased reservoir drainage.
