FORMATION TESTING IN THE DYNAMIC DRILLING ENVIRONMENT

摘要

Formation Testers While Drilling (FTWD) tools have recently been introduced and offer an alternative to wireline testing. Several questions remain regarding the accuracy and repeatability of these measurements. First, the drilling environment is dynamic with hydrostatic pressure constantly changing. Some FTWD tools offer the capability to perform pressure tests with the mud pumps on or off. Invasion and the resultant supercharge effect can also be factors affecting FTWD measurements. The emphasis of this paper is to consider these factors through forward modeling methods as well as field examples. Several forward models are developed in this paper. The simplest assumes single phase invasion where the mudcake buildup is loosely coupled to the filtrate invasion. The second is a two-component fluid model where the mud filtrate invades and displaces the formation fluid. The third is a multiphase finite difference model that considers miscible and immiscible invasion that also couples mudcake growth to invasion. Invasion can result in FTWD or Wireline Formation Testers' (WFT) pressure measurements being elevated above formation pressure. This creates a supercharging effect as a result of mud filtrate invasion increasing the sandface pressure just behind the mudcake. A time lapse technique is used that enables the FTWD tool to estimate the mudcake properties and invasion rate. This information can be used in conjunction with the simulators to determine the supercharge magnitude. Pressures in the borehole are constantly changing due to several factors. It is usually desirable to keep the mud pumps on while testing to avoid hole deterioration and pipe sticking. Slower hydrostatic pressure changes can also occur due to drill pipe movement causing a swabbing effect. These pressure transients can be transmitted through the mudcake and into the formation. The magnitude of these pressure transients are simulated using the forward modeling methods developed in this paper. Field data from the new GeoTAP tool are presented to demonstrate the practicality of FTWD. The new tool requires the drill pipe to stop rotating/sliding for about 7 minutes per pressure test. During this time the pumps can be turned on or off. The field example demonstrates the robustness of the measurement. The field example included three repeat pressure tests taken at three different depth points where, in each case, the pressures recorded were within a few psi of each other. Final conclusions are drawn regarding FTWD technology and its future direction.