Major obstacles in production from hydraulically re-fractured shale formations: Reservoir pressure depletion and pore blockage by the fracturing fluid

Gas production from ultra-low permeable shale resources declines drastically at early times upon which organically rich undrained zones are still left at high gas content. In order to enhance nearly flat gas production rates, re-fracturing virgin zones using the newly emerged technologies has been widely implemented in the past few years. Therefore, numerical optimizing tools for re-fracturing must capture several production steps including the irreversible reservoir conditions; the depleted reservoir pressure and the associated fracturing fluid pore blockage close to hydraulic fractures. These have not been considered in the production models in the literature. In this paper, we introduce a multi-step production model including the geomechanics effects, followed by a detailed sensitivity analysis for the most influential parameters in gas production. A synthetic shale gas reservoir model is created with logarithmically spaced, locally refined grids (LS-LR-DK) inside the stimulated reservoir volume to accurately capture the physics of flow in shale gas reservoirs. This model consists of the following sequential steps: 1) production from the first set of hydraulic fractures and well shut-in at the re-fracturing time; 2) activation of the second set of hydraulic fractures induced by re-fracturing, and fracturing fluid (water) injection into all fractures to simulate the fracturing fluid invasion into the matrix; 3) fracturing fluid soaking period; and 4) production from the renovated fracture network (the new and old hydraulic fractures). This model provides a mechanistic approach to include and simulate the following obstacles in gas production enhancement using re-fracturing: 1) reservoir pressure depletion in the initially stimulated reservoir volume as the depleted reservoir pressure cannot strongly repel the invaded fracturing fluid out of the matrix; 2) deep fracturing fluid invasion due to the pressure depletion and the alteration of single phase flow to two phase flow. The results showed that the pressure depletion and the resultant water retainment in the pore space reduced the production enhancement by 5% compared to the base case without these effects. This modification in gas production can influence the risk assessments for further investment on re-fracturing a field yet producing at low rates, and may revise the number of considered fields for re-fracturing.