Well-Testing Challenges in Unconventional and Tight-Gas-Formation Reservoirs

Abstract

Summary: Well–testing techniques provide valuable reservoir information, which is used to evaluate production potential and helps make development and completion decisions. Traditionally, newly drilled wells are tested using a drillstem test (DST) or wireline formation test (WFT). These methods are typically designed to flow the well under controlled conditions and then shut in the flow to achieve a pressure buildup (PBU). In this paper, we present eight case studies to highlight well–testing challenges in tight–gas–formation wells in unconventional reservoirs. These challenges are accompanied by methods and solutions to adequately address them. During the first two cases, the duration of the PBU was not long enough to overcome high wellbore storage and reach radial flow. This was because of surface shut–in. A simulation is provided for Case 2 to highlight the importance of downhole shut–in. A hydraulically fractured, horizontal, tight–gas–formation well is presented in Case 3, where a 46–day–long PBU was conducted. However, owing to low permeability and high wellbore storage (WBS), this still resulted in a nonunique solution. The use of a wireline–conveyed formation tester to mitigate WBS effects is presented in Cases 4, 6, and 7. Case 5 is an example of a surface diagnostic fracture injection test (DFIT), with surface shut–in. In addition to the conventional PBU analysis, microfracturing and subsequent fluid sampling from tight formations have also been discussed. Finally, an injection/falloff (IFO) case is presented in a hydraulically fractured formation to discuss the challenges and solutions. Because of controlled rates with an IFO test in low–permeability wells, faster flow stabilization can be achieved, resulting in shorter WBS durations. This technique can obtain well and reservoir information faster than a conventional PBU test. One issue with unconventional wells, particularly tight–gas–formation wells, is that the WBS lasts significantly longer than in standard wells. A downhole shut–in tool or any test that minimizes the WBS effects can greatly reduce the duration of such tests and help improve the quality of the analysis results. A discussion on formation compressibility has been provided to highlight the shortcomings of the traditional Hall's correlation for application in unconventional formations. This paper discusses several well–testing applications with corresponding analyses pertaining to unconventional wells. The results provided valuable information to help optimize production and evaluate reservoir potential.