Maximizing the effective fracture half-length (Xf)eff is critical to gas well deliverability. A critical mechanism limiting the (Xf)eff is the water block that can occur because of capillary discontinuity at the matrix-fracture interface and multiphase flow in the fracture proppant pack. In this work, mechanisms were studied in order to understand the fundamental pore physics and to propose a workflow to evaluate chemistries to mitigate these damaging mechanisms, increase gas relative permeability and thereafter (Xf)eff.
To investigate capillary discontinuity at the fracture-matrix interface and fluid entrapment in the propped and unpropped, induced fractures, reservoir rock and fluid were first characterized with state-of-art methods. The requirements for extending (Xf)eff were then established via the relationship of gas relative permeability and the ratio of pressure drop and capillary pressure. A chemical fluid system was tailored, and its performance was tested to 1) mitigate capillary blockage at the matrix-fracture interface and 2) remove aqueous phase trapping in the proppant pack at Xf of 10 ft to 100 ft range from the wellbore. The system was then implemented and evaluated in two new Eagle Ford gas wells.
The Eagle Ford gas window reservoir rocks were characterized as strongly water-wet with an in-situ contact angle of 35°, indicating high tendency of water retention in the matrix-fracture network. Pore size distribution was obtained from 3D FIB-SEM imaging and it showed a bimodal distribution with 10-40 nm pores and several hundred nm- to micron-sized nano-fractures. The pores smaller than 500 nm (which contribute ~46% of pore volume) have the propensity to be blocked by water according to the estimated threshold capillary pressure.
The customized chemical system showed 1) in the capillary discontinuity blockage mitigation test with core-flood, the tailored system improved gas relative permeability by 15% at in-situ temperature and pressure; and 2) the system removed 50% of residual water in the proppant pack and the gas effective conductivity was increased by 60%. The combination of capillary blockage mitigation and multi-phase flow trapping removal efficiently extends the (Xf)eff and improves gas production.
In this work, we present a novel and comprehensive method to evaluate the tendency of water block in shale gas formation based on fundamental rock and fluid characterization. A solution is also developed to resolve the blockage that extends the (Xf)eff and improves gas well deliverability.