New Reservoir Dynamic Connectivity Measurement for Efficient Well Placement Strategy Analysis Under Depletion

Abstract

Abstract A dynamic measurement of reservoir connectivity based on a parameter called resistivity index (RI) is evaluated. The RI parameter takes into account the pressure effects from wells but does not solve pressure equations. This technique is compared with other static connectivity measurements and with streamlines (another dynamic-connectivity technique) for gas reservoirs producing by depletion. Reference flow simulations were used to validate these comparisons. Two real field models were used in this study. In both cases, the RI analysis reproduced the reference flow simulation results significantly better than the other techniques while requiring orders of magnitude less time than the finite difference flow simulator. The results of an uncertainty study based on the RI supported the approval of the infill-drilling program for the first field model. This technique has been implemented as a special option for a commercial geostatistical software application. Introduction Many reservoirs produce by depletion for a significant amount of time or at least as an initial strategy, particularly in gas fields. Therefore, many decisions are made during the appraisal stage or in an infill-drilling program when depletion is the main production mechanism. It has been reported that conventional layer-cake models have a tendency to overestimate recovery by neglecting compartmentalization or lack of reservoir connectivity. To be more realistic, reservoir models must be larger and more complex, what results in longer flow simulations. Hence, the analysis of different well placement strategies such as geometry, location, orientation and spacing for optimizing production is time-consuming. The analyses are even more demanding when they include any uncertainty sensitivity. Furthermore, most well decisions are done at appraisal or initial production stage when the uncertainty is largest. Reliable fast tools are essential for allowing detailed analyses within a practical timeframe. Some fast reservoir performance estimator (FRPE) tools currently available include cut-off based connectivity calculations1 (also known as connected cell bodies, CCB) and streamline based calculations2,3. While the former neglects any pressure effects or distance from producing wells, the latter was mainly designed for steady-state systems. Some streamline implementations handle non steady state systems, but the main advantage of speedup compared with a finite-difference flow simulator is reduced. A brief review of these estimators and the RI is presented. The volumetric calculation based on the RI and the main elements of the software implementation are also shown. Two real field models were used in this study. The first evaluates the infill-drilling potential of an onshore US tight-gas reservoir, assuming vertical well completions. The second evaluates deviated well trajectory strategies for a North Sea offshore tight-gas reservoir. General description about these two fields and the results from full flow simulation (FFS), which were used as reference reservoir performance are presented. After a comparison between the different FRPE and validation of the RI technique, its application for estimating production performance under uncertainty for the two field cases is presented. Finally, some conclusions are discussed. Fast Reservoir Performance Estimators FRPE are methods to estimate the cumulative production or drainage volume through simplifications of the reservoir model and/or of the fluid flow equations. FRPE are mainly indicators of the connectivity in the reservoir.