Yang Zhihui

Imbibition characteristic of shale gas formation and water-block removal capability

Large scale hydraulic fracturing is one key technology for shale gas development. Thousands of liquids are pumped into shale formation. However, fluids flow-back rate is surprisingly low for same cases, which is always lower than 30%. The production is not influenced too much by the low flow-back rate and some of these wells even have higher productivity. This phenomenon cannot be interpreted by conventional theory, in which gas will be blocked seriously by the water. It indicates that shale formation has water-block removal capability. Experiments are proposed around this fracturing imbibition issue including water imbibition and solution conductivity online test, nuclear magnetic resonance tests with water imbibition and pulse decay permeability tests with water imbibition. The interaction between water and shale and the dynamic effect are analyzed to make clear the relationship between water imbibition and water block removal. Fracturing fluids are imbibed into the formation matrix and complicated physical and chemical effects happen between fluids and shale. There are huge differences between different shales including the imbibition capacity, diffusion capacity and ion exchange capacity. The imbibition capacity, diffusion capacity and fracture induced capacity are the main factors affecting water block removal capability. Pressure enhancement, Liquid diffusion into deeper matrix and micro-fracture induced by water-rock reaction are treated as the main mechanisms of water block removal through the study. A water block removal potential valuation methodology is proposed taking account the initial water saturation, imbibition capacity, diffusion capacity, fracture induced capacity and clay chemistry effect. The water block removal potential can be evaluated by the water imbibition, nuclear magnetic resonance and plus-decay permeability tests. A grading evaluation of typical shale gas formation is proposed based on this method. The work is the pioneer study analyzing the water block removal mechanism and controlling facts, and it is valuable to make clear the shale gas productivity factors and put forward guidance to shale gas reservoir stimulation.