Quantification of the effects of bed lamination and wellbore deviation on the interpretation of formation-tester measurements

摘要

Formation-tester measurements (FTM) acquired in thinly bedded formations and in highly deviated wells often show a large pressure drop during the drawdown period of a pressure-transient test. The measured large pressure drop can be an indication of a low permeability layer at the probe location. We invoke the effects of bed boundaries to explain additional pressure drop in the FTMs. An accurate analysis of the FTMs requires simulating mud-filtrate invasion prior to the pressure-transient test Specifically, the interplay between gravity, capillary, and viscous forces leads to a highly non-symmetric fluid distribution around the wellbore perimeter in deviated wells. In this condition, it becomes crucial to have depth control and perform fluid sampling at a suitable probe location with respect to the perimeter of the wellbore. A three-dimensional (3D) compositional fluid-flow simulator is employed to simulate mud-filtrate invasion and subsequently FTMs in deviated wells. The simulator is specifically designed for problems where the greatest variations occur in the vicinity of the wellbore. We test the accuracy of the algorithm with a series of drawdown-buildup tests in highly deviated wells; a comparison of results obtained with the available analytical solutions and the numerical simulator shows a very good agreement. Subsequently, using the developed simulator, we study invasion, pressure transient tests after invasion, and fluid sampling after invasion. Results show that invasion in a high-permeability formation causes non-symmetric distribution of fluid around the wellbore of a deviated well. Simulation results confirm that a probe-type FT records significant pressure drops when it is placed at the vertical vicinity of the well; this effect leads to a lower measured permeability compared to the actual permeability. It was also found that pressure drop during drawdown is affected by invasion; pressure drop in invaded formations is greater than in un-invaded formations. We also quantify cleanup time for sampling with a probe-type FT deployed in deviated wells. Results show that the cleanup is achieved faster when the probe is located at the high side of the well.