Improved Pressure Transient Interpretation for Unconventional Reservoirs Utilizing the Transient Drainage Volume

Abstract In unconventional reservoirs, rate transient analysis is routinely used to interpret production for reservoir and well characteristics, and for the prediction of economic reserves. With the inclusion of pressure data, we have developed a novel analysis based upon sequential Pseudo Steady State (PSS) flow and a modified flowing material balance plot, from which a richer set of information can be inferred: the transient drainage volume, the instantaneous recovery ratio, the transient well productivity, and the instantaneous shut-in pressure. Unconventional reservoir production analysis challenges many of the assumptions of routine pressure transient analysis (PTA) and rate transient analysis (RTA). For PTA analysis, the near well reservoir region is often over-pressurized due to hydraulic fracturing operations and the initial reservoir pressure is not known. For RTA analysis, the bottomhole flowing pressure may take months to stabilize and reservoir scale boundary dominated flow is never attained. We introduce a novel methodology and a modified flowing material balance plot based upon a transient extension to boundary dominated flow (TBDF). It is applicable if both pressure and rate information is available, and resolves the limitations listed above. A model of transient boundary dominated flow (TBDF) is used to relate the transient bottomhole flowing pressure, flow rate, and cumulative production from the beginning of the analysis period (the observations) to a transient drainage volume, the average pressure at the beginning of the transient and the well productivity (an interpretation). When applied to early time data the model can determine the initial near well reservoir pressure from which we obtain the bottomhole flowing pressure drop, and which allows the use of routine PTA analysis. During production, the model infers an instantaneous shut-in pressure, and the transient productivity and drainage volume of the well. A modified flowing material balance plot based on the pressure normalized rate will predict the economic ultimate recovery (EUR) and can be used to assess the impact of gas lift operations, for instance, on the EUR. The methodology is validated using numerical flow simulation models and demonstrated on field data sets, including data drawn from the SPE data repository (https://www.spe.org/en/industry/data-repository/). Details are provided on the methods found most effective in filtering and smoothing the field data, to aide in the technology transfer of the novel analysis methodology. The proposed methodology introduces a novel transient boundary dominated flow analysis which is used to determine the initial average reservoir pressure in the near well region, the transient drainage volume of a well during production, the average reservoir shut-in pressure in that region, and the transient well productivity. A modified flowing material balance plot can be used to determine the EUR and to assess the impact of gas lift operations on the EUR.