Well-Testing, In-Situ Fluid Sampling, and Stress Determination Methods for Unconventional Reservoirs

Abstract

Traditionally, production potential evaluation of newly drilled wells is determined with a drillstem test (DST) or a wireline formation tester to make decisions. Applying such techniques in unconventional formations is not always that easy owing to extremely tight formations that do not allow significant natural flow. Obtaining representative downhole fluid samples is also a major issue in unconventional wells. Unconventional gas condensate wells produce water for a long period after hydraulically fracturing the formation. Conventional fluid sampling acquires mostly water; surface sampling is not representative because the gas, condensate, and water flow rates change with slugs of fluid flowing to the surface. To obtain undisturbed fluid samples, a new wireline formation tester technique was developed for unconventional formations. This method uses straddle packers to isolate a section in the wellbore to pump into the formation and create a fracture. Because these tight formations have limited invasion during drilling, they clean up faster during the pumping out period, with the created higher surface area increasing the flow rate. Formation mechanical properties, such as fracture initiation, instantaneous shut-in pressure (ISIP), fracture closure and opening pressures, and fracture extension pressure will all be obtained for this new microfracturing and fluid sampling technique. Several well testing applications are discussed with corresponding analyses pertaining to unconventional gas and gas condensate wells in addition to some case studies for tight oil formations. The results provided valuable information to optimize production and evaluate reservoir potential. For example, a tight gas well was hydraulically fractured and did not produce after fracture stimulation because of water blockage; therefore, an injection/falloff test was recommended by injecting nitrogen into the formation and analyzing the falloff test. The well test analysis revealed that the early screenout caused an ineffective fracture length. The well was then refractured and has been producing since. One of the issues with unconventional wells, particularly in tight gas wells, is that the wellbore storage lasts much 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. A new extended diagnostic fracture injection test (DFIT) conducted with a downhole wireline operation that alleviated the problem of high wellbore storage and obtained pressurized pressure volume temperature (PVT) quality samples is also discussed. Because of controlled rates with an injection/falloff test in low-permeability wells, faster flow stabilization can be achieved, resulting in shorter wellbore-storage duration. This technique can obtain well and reservoir information in a shorter period of time than a conventional pressure-buildup test.