Beyond Linear Analysis in an Unconventional Oil Reservoir

摘要

Current linear analysis is based upon the tri-linear model, which assumes a continuous and homogeneous porous medium within each flow region. This recent study, however, reveals that a discontinuous stimulated reservoir volume (SRV) is extensively associated with horizontal wells with fewer frac stages and/or within areas with high-stress anisotropic ratio. To characterize such isolated SRV scenarios, three parameters-fracture half-length, SRV permeability, and the half-width for each isolated SRV-must be inferred from the analysis, which creates a difficult challenge for the existing rate transient analysis (RTA) procedure. This paper presents a new RTA methodology and associated diagnostic plots to infer all three parameters simultaneously using the information beyond the linear flow period. The analyzed results illuminate optimizing the number of frac stages and well spacing. The newly developed "beyond linear analysis" procedure has been used successfully to analyze more than 100 Bakken wells with various completion types and frac stages. The resultant RTA-derived statistics have been used for the construction of type curves, well history matching, completion assessments, and optimal well spacing. Almost all the wells with fewer than 20 frac stages show an isolated SRV around each hydraulic fracture; these are good candidates for refrac. Although a high number of frac stages (e.g. >30 stages) will boost the initial production rate (IPR) and more quickly deplete the SRV, it is our conclusion that the estimated ultimate recovery (EUR) is not significantly enhanced beyond a certain number of frac stages. Moreover, both the incremental SRV and flow capacity associated with each additional stage steadily decrease. This suggests that there is an optimum number of frac stages and wells that maximize both the IPR and the EUR of a drilling spacing unit (DSU). This new RTA methodology allows, for the first time, detailed SRV and fracture description. The precise RTA model not only greatly relaxes the dependency on compaction and PVT properties suppression during the history match, but also clarifies the "stress shadow effect" from adjacent stages and the relationship between the shear fracture displacement/dilation and inter- stage intervals.