Advances in Multilayer Reservoir Testing and Analysis using Numerical Well Testing and Reservoir Simulation

Abstract

Abstract This paper presents multilayer testing and analysis techniques to obtain layer permeabilities and skin factors using pressure and flowrate transient data from sequential flow tests acquired with production logging tools. The analysis of these tests by application of numerical well test analysis is highlighted. An integrated workflow is shown and a new analysis technique is presented which incorporates numerical reservoir simulation and an automated history matching procedure. A gradient method has been successfully applied to modify simulation model input parameters to match synthetic and field examples of multilayer transient tests. The study also highlights the potential for the application of this method and for automated history matching techniques to pressure transient and flowrate transient analysis and multilayer test analysis and interpretation. Introduction Hydrocarbon reservoirs are commonly heterogeneous and they often comprise of units or formations where layers may have distinct values for thickness, permeability, porosity and skin factor. In well testing terminology, these reservoir formations are called multilayer systems. The effects of layering on conventional pressure-transient tests and their interpretation have been described by many authors1–3. In some situations, the layers may have such low permeabilities that they act as barriers to vertical flow and inhibit reservoir crossflow between more permeable units. This is known as a multilayered reservoir system without crossflow or commingled. For cases where communication can take place between layers and formation crossflow can occur, this is known as multilayered reservoir with crossflow. In cases that are more complex, a well may produce from a composite reservoir consisting of commingled zones that may contain crossflow layers. Characterisation of layered reservoirs4is important from a reservoir evaluation and management standpoint because of the influence of layering on primary and secondary oil recovery. Layered reservoirs pose special problems; for example differences in layer permeabilities can lead to unbalanced depletion and poor recovery. Waterflooding projects in such reservoirs can lead to poor sweep efficiency because unswept oil is usually bypassed in the lower permeability zones. For commingled reservoirs, contrasts in well performance and production could be due to large variations in permeability-thickness product or skin. The problem addressed here is the challenge of determining reservoir parameters in multilayered reservoirs, i.e.estimating layer permeabilities, skin factors and formation pressures from well test data. Attempts to identify reservoir parameters for individual layers have applied some of the following methods.Isolation testing and drillstem testing (DST) of individual layers by selective perforation, or by combinations of straddles and/or isolation packers;conventional buildup and variable-rate drawdown testing; andproduction logging. Isolation and selective methods can be problematic and they are expensive to perform. Conventional well tests (buildup and/or drawdown) and their analysis reveal the total system behaviour, and often give no indication that layering phenomena are present. Buildup tests are often affected by crossflow between layers and from wellbore storage effects; pressure data alone may not provide any information about crossflow or wellbore dynamics. However, there are some cases where conventional well tests do work1,3.