Problems With Application of Material Balance Time to Transient Flow Data in Diagnostic Plots
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2017, Unconventional Resources Technology Conference (URTeC). Although material balance time was originally proposed by Blasingame and co-workers to model wells in boundary-dominated flow, later investigators proposed that logarithmic rate vs. material balance time lead to the same conclusions as logarithmic rate vs. time plots during transient flow for smoothly changing rates and are superior for identifying and analyzing data in boundary-dominated flow. However, application of this idea to field data often shows different curve shapes for the two methods of plotting during transient flow. The purpose of this study was to identify the cause of this discrepancy and to suggest an approach to flow regime identification in which we might have more confidence. Our hypothesis was that the cumulative production used in calculating material balance time will not be consistent with the rates during transient flow for multi fracture horizontal wells (MFHW), completed in ultra-low permeability reservoirs, when (1) fracture damage was present (more common than not) and (2) pressure drawdown and production varied during the early months of a well's production history. To test our hypothesis, we analyzed simulated production data in several ways: (1) production at constant BHP, no fracture damage; (2) variable BHP, no fracture damage, BHP values known as functions of time; (3) variable BHP, no fracture damage, BHP values unknown; (4) constant BHP with fracture damage; (5) variable BHP with fracture damage and known BHP; and (6) variable BHP with fracture damage and unknown BHP. We found that our hypothesis was correct, and that, during transient linear and bilinear flow, the slopes of logarithmic plots of rate vs. time and rate vs. material balance time can be quite different. Linear and bilinear flow can be identified with a slope of (-1/2) and (-1/4) on logarithmic rate-time plot for smoothly decreasing rates. However, they can be delayed or disguised significantly on logarithmic rate-MBT plots with large fracture skin and varying BHP. We conclude that a simple logarithmic rate-time plot leads to a more definitive identification and analysis of the transient flow regime in MHFWs than the logarithmic rate-material balance time plot. On the rate-time plot, transient linear and bilinear flow can be identified with a (-1/2) and a (-1/4) slope which may be delayed or distorted on the rate-material balance time plot. For boundary-dominated flow identification and analysis, the material-balance-time plot remains the preferred method.
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Proceedings of the 5th Unconventional Resources Technology Conference
