Log permeability for reservoir models is generally sourced from core measurements by calibrating to well-test permeability. Conventional approaches are challenged in carbonate reservoirs due to complex depositional and diagenetic alterations. The calibration to well test usually has the basis of pressure buildup (PBU) analysis from limited pilot holes. Horizontal drilling enhanced reservoir recoveries by horizontal drain holes instead of vertical producers. The certainty of a robust permeability model is scrutinized in history matching at the horizontal drain holes. Therefore, it is mandatory to reconcile the log permeability to drawdown permeability in horizontal drain holes by honoring measurement path. The measurement path brings in noticeable effects to permeability in heterogeneous carbonate reservoirs if it is pressure build up or pressure drawdown. The objective of paper is to demonstrate a solution to permeability estimation by sensitizing measurement type, scale, path, and environment in horizontal wells.
An analytical workflow is developed with field examples by integrating Multi-Probe production logging (PLT) with downhole gauge data while flowing. The prerequisites are to have gauge data for a sustained and stable flow period followed by a long shut in for pressure build up. Thereafter, Multi-Probe production logging was acquired for flowing passes and shut-in passes. In general, pressure transient behavior in horizontal well is mathematically represented by pressure diffusivity equation (Goode & Thambynayagam, 1987) with four possible flow periods. An automated process is programmed in python to detect transient flow regime from gauge data. This denotes the possible flow regime with a characteristic slope which represent the transient conditions during production logging. Multi-Probe PLT data is processed for inflow profile and zonal contributions from velocity and holdup profile conforming to reservoir flow units interpreted in vertical pilot well. Drawdown permeability is estimated from the solutions to pressure diffusivity equation based on estimated downhole fluid rates, identified flow regime and boundary conditions.
A discrete drawdown permeability from PLT flow profile is estimated as per transient flow regime in horizontal drain holes of a heterogeneous carbonate reservoir. Reconciliation of log permeability with drawdown permeability distinguished prominent flow units in the reservoirs. The results highlighted the critical pitfalls in static to dynamic reconciliation related to reservoir heterogeneity, measurement path, apparent skin variation across flow units and multi-phase effects. The workflow had overcome averaging nature of PBU permeability and the data scarcity in terms of PBU in vertical pilot wells.
The demonstrated solution involves an automated process to quickly detect flow regime and highlights the integration of prior gauge survey with production logging results. The merit of the solution is to detect baffles and to investigate performance of high permeability streaks across drain holes by reconciling log permeability with flow-calibrated drawdown permeability. The analytical workflow is pragmatic for reducing uncertainty in permeability distribution by capturing core scale heterogeneity and honoring transient production behavior.