Modelling flowback as a transient two-phase depletion process

•We propose a two-phase model for analysing transient flowback rate/pressure data.•The model accounts for rapidly changing fluid saturations in hydraulic fractures.•Field data application estimates hydraulic fracture half-length and pore-volume.•It can estimate the pore-volume of active secondary fractures in tight reservoirs.•It can forecast post-flowback hydrocarbon production and load recovery.

The existing rate transient models for fractured horizontal wells assume single-phase fluid flow. This assumption is violated in early times, when hydraulic fractures (HF) are filled with fracturing water and hydrocarbon. This calls for a model that captures the transient 2-phase (gas/oil + water) flow in HF, and can be used for history matching rate/pressure data measured during flowback operations. This paper extends the existing linear dual-porosity model (DPM) and develops a flowback analysis model (FAM) which accounts for transient 2-phase flow in HF.FAM assumes that water-saturation in HF drops with time, which causes a non-linear increase in relative-permeability. Hence, it adopts an explicit dynamic-relative-permeability (DRP) function of time for the hydrocarbon-phase to account for water-saturation drop in HF. The resulting DRP function is obtained by integrating cumulative flowback hydrocarbon + water (from 15 tight-oil, tight-gas and shale-gas multifractured horizontal wells) data with drainage relative-permeability curves from existing literature. DRP is then incorporated into the DPM to obtain FAM flow equations. These flow equations capture the fluid physics from flowback till “full” hydrocarbon production.The FAM equations are solved using Laplace transforms. Type-curves are generated by numerically inverting the resulting Laplace-space solutions to time-space using the Gaver-Stefhest algorithm. FAM converges to DPM at the limit of constant water-saturation. Flow-regimes observed comprise linear, bilinear, and boundary dominated. This paper proposes an interpretation workflow for analysing flowback data. The application of this workflow on 2-phase rate + pressure flowback data from a shale gas well provides a means to (1) estimate effective HF half-length and volume of interconnected secondary fractures, (2) evaluate flowback performance and (3) forecast hydrocarbon recovery. The results encourage the industry to start careful and accurate data measurement immediately when wells are put on flowback.