A New Method to Estimate Formation Pore Pressure in Fractured Areas of Shale Gas Reservoir

Abstract

The matrix of shale gas reservoir has the characteristics of low porosity and ultra-low permeability, but also develop natural and hydraulic fractures. Many field operations observed that the formation pore pressure after hydraulic fracturing is higher than the original formation pore pressure. Drilling in the fractured areas of shale gas reservoir, including natural fractures and artificial fractures, always faces greater overflow risk. The pore pressure test after fracturing also shows that the test value is higher than the formation initial pore pressure. However the conventional formation pore pressure predicting methods are difficult to obtain the accurate pore pressure in the fractured areas of shale gas reservoir.In this paper, the causes of formation pore pressure changing and fracturing pressurization effect in the fractured zones of shale gas reservoir are analyzed. Based on the assumption of closed and constant volume material balance and the gas equation of state, the internal mechanism of the increase in pore pressure caused by fracturing operation in low-permeability shale gas formation is theoretically analyzed. A new method to predict pore pressure changing of shale gas reservoir during drilling and hydrofracturing operations is proposed. The pore connectivity of shale gas reservoir is poor, and the permeability is tens of nano-darcy. The permeability is usually between 0.01-0.20 millidarcy when natural fractures are developed. After fracturing, the permeability can be increased by 2-3 times. Based on the principle of effective stress, a pore pressure calculation model with the influence of permeability change is established by using logging data. Three influence terms are considered in the model: the first one is the overburden pressure term, the second one is the skeleton stress term, and the last one is the influence term of permeability change rate on local formation pore pressure. The proposed method shows simple principle, clear mechanism and easy to use in the field. Field applications show that this method could quickly and accurately predict dynamic formation pore pressure changing of shale gas reservoir. It offsets the shortcomings of traditional formation pore pressure calculation methods.