Analysis of Drawdown Sensitivity in Shale Reservoirs using Coupled-Geomechanics Models
Productivity loss in some shale reservoirs is often attributed geomechanics-related factors, even when the exact mechanism(s) is undetermined. Overpressured reservoirs tend to experience a large increase in effective stress during depletion, which can contribute to a dramatic loss in fracture conductivity. Factors such as a low elastic modulus may also contribute to productivity loss through processes such as proppant embedment. The Haynesville Shale (Texas and Louisiana) is often cited as an example of geomechanics-related productivity loss since it is characterized by high overpressure (1.0 psi/ft in some areas) resulting in extremely high initial production rates followed by steep declines.
In this study, a simplified, single-fracture coupled model with different orientations relative to the principal stresses is used to investigate the impact of drawdown management on estimated ultimate recovery (EUR). The coupled-geomechanics reservoir simulation model was constructed using a comprehensive set of logs, rock mechanics tests and fracture conductivity measurements from a shale reservoir. In particular, various drawdown schedules (bottomhole pressure vs. time) and their effect on effective stress on the fractures were compared. Subsequently, fracture conductivity (Fc) relationships considering the maximum effective stress for each drawdown scenario were constructed and used in traditional (non-coupled) reservoir simulations to forecast gas EUR.
It was found that a managed drawdown schedule can improve EUR by up to 15%, mainly by reducing the effective stress on unpropped fracture regions. Importantly, the modelling results show that unpropped fracture "pinch points" near the wellbore may lead to very significant reduction in overall recovery. Model results showed that even proppant concentrations around 0.3 lb/ft2 maintained sufficient conductivity to not hamper well productivity in nanodarcy reservoirs. Traditional reservoir simulation approaches using compaction tables (pore pressure vs. Fc) lack the critical process of stress path dependency that governs the effective stress on the fractures and is necessary to adequately model this system.
Future experimental work should be directed at measuring unpropped fracture conductivity at high stresses and longer durations (creep) as unpropped fractures appear to be a main contributor to the observed geomechanical-related productivity loss.
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© Copyright 2015. Society of Petroleum Engineers
Presented at the SPE Annual Technical Conference and Exhibition, 28-30 September, 2015, Houston, Texas, USA