Analysis of Drawdown Sensitivity in Shale Reservoirs Using Coupled-Geomechanics Models

Abstract

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 geomechanicsrelated 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 (F c ) 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/ft 2 maintained sufficient conductivity to not hamper well productivity in nanodarcy reservoirs. Traditional reservoir simulation approaches using compaction tables (pore pressure vs. F c ) 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.

Pressure Drawdown Management Strategies for Multifractured Horizontal Wells in Shale Gas Reservoirs: A Review

“…Even after the stress is restored, the permeability can only recover to 10–15% of the initial state. 33 …”

“…Even after the stress is restored, the permeability can only recover to 10−15% of the initial state. 33 Wilson et al 33 proposed a single-fracture numerical model with different maximum horizontal in situ stress distribution orientations to study the influence of pressure drop strategy on the ultimate recoverable reserve (EUR). The research results show that by alleviating the impact of effective stress on the natural fracture compaction, the reasonable pressure drop scheme can increase the EUR by 15%.…”

“…When the effective stress increases, the loosening, spalling, and migration of particles occur at the rough interface of the fracture and have a great impact on physical parameters such as porosity and permeability. Even after the stress is restored, the permeability can only recover to 10–15% of the initial state …”

Numerical Investigation of Fracture Compressibility and Uncertainty on Water-Loss and Production Performance in Tight Oil Reservoirs

“…The Sw within NF is relatively high (17%) for the base case, which limits the flow of oil in NF. However, the oil production gap between three models is not large because tight reservoirs do not require very high fracture conductivity, and proppant that is evenly distributed within a fracture should be more important for well productivity [64]. From Figure 11, it can be found that the oil phase appears in NF after one day of flowback, when stress-dependent porosity is absent.…”

Bottom-hole pressure drawdown management of fractured horizontal wells in shale gas reservoirs using a semi-analytical model

“…Formulating a reasonable pressure-controlled production plan is the key to obtaining ideal productivity of over-pressured fractured shale gas reservoirs. The common research methods for the dynamic performance analysis of shale gas wells with pressure drawdown management mainly includes: field test 11 , physical simulation [12][13][14] , and numerical simulation 11,15,16 . Restricted to the strong regionality and high research costs, field tests are mostly used in large-scale field tests to provide engineering practice knowledge for theoretical scientific research and physical simulation work.…”