A Rapid Injection Flow-Back Test RIFT to Estimate In-Situ Stress and Pore Pressure in a Single Test

Abstract

Abstract Quantifying in-situ stress and pore pressure has significant applications in earth sciences and subsurface engineering, such as fault zone studies, underground CO2 sequestration, oil and gas reservoir development, injection into deep wells, and geothermal energy exploitation. The extended leak-off test (XLOT)/ pump-in and flow-back test (PIFB) have been the industry-standard for stress determination during drilling for some time, where drilling mud is injected to create a small fracture in an vertical open-hole below a newly cemented casing. This is followed by a shut-in phase and/or flow-back phase. However, there is still no consensus as regards to which methods should be used to pick closure stress from XLOT/PIFB data, and the influence of drilling mud and near wellbore hoop stress can further complicate the interpretation process. In addition, the risks of formation damage and wellbore integrity issues are also major concerns when executing XLOT/PIFB during high-cost drilling operations. Diagnostic Fracture Injection Tests (DFIT) is another standard method for estimating in-situ stress and other important reservoir/fracture parameters such as pore pressure and permeability. Such tests can be executed in either open-hole or cased-hole, and even carried out in horizontal wells. In very low permeability reservoirs, it may take several days for fracture closure and weeks to observe the after-closure flow regime. The required shut-in time for DFITs can be extremely long in unconventional reservoirs (weeks or months). In some circumstances, if the reservoir is naturally fractured and its effective permeability is strongly pressure-sensitive during the before-closure period, picking the closure stress in the DFIT data becomes ambiguous. In this study, we present a new approach to estimate in-situ stress and pore pressure using a Rapid Injection Flow-Back Test (RIFT) in low permeability formations, where the fracture closure process is facilitated via controlled flow-back that is followed by a shut-in period. This significantly shortens the time required to conduct a test that allows us to estimate the in-situ stress and the pore pressure. The time-convolution solution for RIFT is derived by preserving the physics of unsteady-state reservoir flow, elastic fracture mechanics, material balance, and progressive fracture closure. Our new approach not only provides an unambiguous way to estimate in-situ stress (even in naturally fractured formations), but also allows us to estimate pore pressure with data from a test that lasts only a few hours. It also provides to estimate effective fracture volume. Both numerical simulations and field cases are presented to demonstrate the advantages of RFIT, along with a discussion of cautions and the potential pitfalls when designing and executing RIFT.