Formation-Tester Pulse Testing in Tight Formations
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 155037, ’Formation-Tester Pulse Testing in Tight Formations (Shales and Heavy Oil): Where Wellbore-Storage Effects Favor the Determination of Reservoir Pressure,’ by Hamid Hadibeik, SPE, The University of Texas at Austin; Mark Proett, SPE, Dingding Chen, SPE, and Sami Eyuboglu, SPE, Halliburton; and Carlos Torres- Verdin and Rooholah A. Pour, The University of Texas at Austin, prepared for the 2012 SPE Americas Unconventional Resources Conference, Pittsburgh, Pennsylvania, 5-7 June. The paper has not been peer reviewed.

Tight-formation testing, when mobilities are lower than 0.01 md/cp, poses significant challenges because conventional pressure-transient buildup testing becomes impractical because of the long buildup-stabilization time. An automated-pulse-test method is introduced for testing tight formations. It reduces testing time significantly and makes determination of formation pressure and permeability possible. Based on pressure data during the shut-in period, the next drawdown or injection test is designed such that the flow rate is a fraction of the initial pulse rate, followed by another shut-in test. This procedure continues until the difference in pressure at the beginning and at the end of the shut-in period is reduced to within a specified limit of pressure change; then, an extended transient is recorded to a stabilized shut-in pressure.

Introduction

Reservoir properties, such as formation pressure and permeability, can be measured with formation testers. Fluid sampling in laminated reservoirs under the influence of dynamic mud-filtrate invasion was tested by use of various probe-type formation testers. A complication in testing tight formations is that the measured pressure is supercharged and is greater than the reservoir pressure. The measured shut-in pressure usually is assumed to be the formation pressure. In a permeable formation, mudcake can form quickly and normally is very effective in slowing down invasion and maintaining the wellbore-sandface pressure near the formation pressure. However, this assumption is unrealistic, especially in low-mobility formations for which there could be no sealing mudcake to isolate the reservoir from hydrostatic pressure. In tight formations, the invasion rate is slowed by the formation, and mudcake may form slowly or not at all. As a result of the lack of sealing mud-cake, the pressure measured in these cases is substantially greater than the formation pressure. A pulse-test technique was used in combination with the spherical-flow model to obtain the true formation pressure. The early transient- and spherical-flow period can be distinguished from the late-transient-flow period by calculating the stream-line evolution of flow near the source.

To measure the reservoir pressure in tight-formation testing, current technologies require several hours of wait time to reach the stabilized pressure, but often it is difficult to reach stabilization or to determine when it occurs. The automated-harmonic-testing method introduced here reduces the testing time substantially when a packer- or probe-type formation tester is used, and it automatically converges to the shut-in pressure.