Many acidizing treatments in carbonate reservoirs do not produce expected increase of productivity, especially in the case of open-hole horizontal wells. This is due to the poor modelling of near well-bore mechanisms. This paper describes a new 2D numerical simulator which features a better description of the physics and which is validated on experimental data.
The numerical simulator presents the coupled mechanisms of flow and dissolution at the origin of the wormhole instability. The macroscopic dissolution equation is derived from a volume averaging of the advection/diffusion and reaction equations written at the pore scale. The flow description is based on Darcy-Brinkman equation which accounts for Darcy's flow in the matrix and Stokes flow in the wormhole.
Two series of experimental results are used for the validation: acidizing experiments with carbonate cores previously published by IFP and other investigators and new model experiments. These experiments consist in injecting under-saturated salt water into a porous medium made of salt. The dissolution instability, channel development and wormhole propagation are recorded by a video camera.
The results predicted by the numerical model reproduce most of the observations in salt and carbonate:
the effect of the flow rate on the dissolution regime, i.e. compact, wormholing or uniform behaviour;
transitions from wormholing to ramified which slow down the wormhole propagation rate;
the effect of the injection conditions on the wormhole propagation rate, the optimum flow rate and the maximum distance of wormhole penetration. Optimum conditions for wormhole propagation are deduced.
The model is built to provide relations on wormhole propagation rate depending on the reservoir properties and the injection conditions. These relations will then be integrated in a near well-bore simulator coupled to a reservoir simulator for a better design of the acidizing treatments in carbonates.
Corresponding Author : Mrs Brigitte Bazin, IFP.
The dissolution by acid injection in carbonate formations results in the formation of highly conductive channels commonly called wormholes. The efficiency of the matrix acidizing treatments depends strongly on the structure of the dissolution figure. It is schematically characterised by one of the following types: compact, wormholing or uniform1. Compact patterns develop at low flow rate whereas uniform dissolution patterns are almost observed at high flow rate. In between, wormholing figures are likely. Previous studies have shown that the development of the wormholing pattern is the most efficient in terms of propagation distance reported to the volume of acid injected. The main difficulty in these treatments is to design the treatment parameters, acid concentration and acid injection rate, to develop a dissolution figure in the wormholing regime.