Pressure Transient Analysis of Shale Gas Wells With Non-Uniform Fractures

Abstract

Abstract Uniform fractures with equal half-length and fracture spacing are commonly assumed when conducting pressure transient analysis and production performance estimation for shale gas wells. However, production loggings and microseismic in many shale gas wells illustrate that only about one third to one half of hydraulic fractures produces. Meanwhile, the productive fractures usually contribute quite differently to the production rate. Therefore, modeling and analyzing of non-uniform fractures properties like fracture type, half-length and fracture spacing are of great importance to understand hydraulic fractures' effect on dynamic flow regimes, pressure and production performance. In this paper, one model with non-uniform fracture properties is used combing shale gas properties like desorption, pressure dependent permeability and fracture properties like un-even distance, un-even half-length. Type-curves of interpretation models with un-uniform facture and the ones with uniform fractures are compared. Rate normalized pressure (RNP) method is used to analyze non-uniform fractures' effect on shale gas wells' flow regimes and drainage volume. Parametric analyses for five reservoir parameters and six well & fractures parameters are conducted. The reservoir parameters include reservoir permeability, porosity, pressure dependent permeability (PDP), desorption and Outer boundary area. The well and fracture parameters include clustered fracturing and evenly spaced fracturing with fixed total fracture half-length (TFH), fracture half-length, cluster spacing, cluster number per stage, stage number with fixed TFH, stage number with fixed fracture half-length. Results show that a series of flow regimes including three linear flows and three pseudo-steady state flows can be diagnosed for shale gas wells with non-uniform fractures. Three pseudo-steady state flows correspond to flow within stage volume, flow within stage plus space between stages volume, and flow within SRV separately. However, the flow regimes reflecting stage volume is missed for shale gas wells with uniform fractures. Permeability determines when the three pseudo-steady state flows happens, but doesn't change the related three volumes. Desorption doesn't change the shape of the pressure and pressure derivative curve, and it only changes the apparent drainage volume. PDP impose severe effect on shale gas well's pressure drawdown and apparent drainage volumes. Increasing the modulus results in decrease of the apparent volume being drained especially when the modulus is larger than 0.01MPa-1. Evenly spaced fracturing has lower pressure derivative and pressure drawdown than non-uniform fracturing, which means it can yield more gas production for the same amount of injected propant and fluids. Fewer stages fractures with longer half-length have lower pressure derivative and pressure drawdown, and will reach SRV boundary earlier than more stages fractures.