Novel Method for Estimating SHmax in Naturally Fractured Formations Using DFIT Data: Case Study on Unconventional Shale Gas

摘要

Maximum horizontal stress (SHmax or SH) and associated horizontal stress anisotropy are some of the essential inputs for drilling and completion optimization such as 3D geomodel and geomechanical model building, drilling mud design and well trajectory analysis, solid production prediction, sweet spot identification in tight/shale plays (i.e. low stress anisotropy as well as high brittleness), and multi-stage hydraulic fracturing design optimization with stress shadowing analysis (e.g. well spacing, fracturing stage spacing, injection rate, fracturing fluid, proppant, etc.). Minifrac/Diagnostic Fluid Injection Test (DFIT) prior to hydraulic fracturing can provide essential reservoir information, such as closure pressure (e.g. minimum horizontal stress, Shmin or Sh), formation pressure, and effective permeability from the G-function analysis. Nolte and Smith (1981) identified Pressure Dependent Leak-off (PDL) behavior in the G-function analysis, and defined PDL net pressure to estimate SHmax associated with small fissure opening along the main hydraulic fracture during the Minifrac/ DFIT. The definition might be suitable for estimating SHmax for non-fractured formations such as conventional ductile sandstones, which could create small fissure openings (e.g. cm size) along the main hydraulic fracture. However, in conventional and unconventional fractured formations such as fractured carbonates, tight sandstones, and shales, the methodology was not applicable because of limitation in the assumptions, which excluded interaction with existing natural fractures (i.e. natural fracture opening during the Minifrac/DFIT). A novel methodology has been developed to estimate SHmax in conventional and unconventional fractured formations from Minifrac/DFIT field measurements. The SHmax Estimation from DFIT method (SHED) is considered more direct compared to the existing methods, since the common industry-accepted methods, such as the image log method from breakouts and the poroelastic horizontal strain method, require Shmin inputs from Minifrac/DFIT tests, additional logs such as image logs and sonic logs, and additional image and geomechanical analyses to define SHmax. Whereas, SHED only requires Minifrac/DFIT test data, typical G-function analysis, and natural fracture patterns from existing geological maps and other inputs, such as seismic surveys and Discrete Fracture Network (DFN) analysis. SHED has been validated and applied in several unconventional shale gas fields utilizing available Minifrac/DFIT field measurements.