Well-Testing Challenges in Unconventional and Tight Gas Reservoirs

Abstract

Traditionally, newly drilled wells are tested using a drillstem test (DST) or wireline formation test (WFT) to evaluate production potential and help make development and completion decisions. These methods are typically designed to flow the well under controlled conditions and then shut in the flow to achieve a pressure buildup (PBU). Depending on how it is conducted, testing could involve a typical DST, closedchamber DST, slug test, or surge test. Later, during the life of a well, other tests, such as standard drawdown-buildup, reservoir limit, flow-after-flow, interference, and pulse tests are conducted to evaluate productivity, remaining reserves, amount of damage, depletion, and production allocation. Even though injection/falloff (IFO) is also an established testing type, it is not conducted as often as a standard pressure drawdown (PDD) and PBU test within the petroleum industry because of possible formation damage resulting from a noncompatible injection fluid. Such tests are usually conducted to determine injection efficiency for water injection wells, communication between an injection and a production well during a secondary oil recovery project, such as waterflooding, in mini- or microfracturing to determine formation and rock mechanics properties, and in nonproducing wells where a standard PDD and PBU test would not work. Because of controlled rates with an IFO test in low-permeability wells, faster flow stabilization can be achieved, resulting in shorter wellbore-storage durations. This technique can obtain well and reservoir information faster than a conventional PBU test. One issue with unconventional wells, particularly tight gas wells, is that the wellbore storage lasts significantly longer than in standard wells. A downhole shut-in tool or any test that minimizes the wellbore storage effects can greatly reduce the duration of such tests and help improve the quality of the analysis results. An extended diagnostic fracture injection test (DFIT) conducted with a downhole wireline operation that minimizes high wellbore storage issues is also discussed. 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.