Multi-physic simulation of acidizing carbonate reservoir

In low-permeability and low-porosity reservoirs, oil is trapped in the rock and cannot be extracted by conventional recovery. In this case, well stimulation techniques such as fracturing can be used to increase the permeability of the rock. One way of well stimulation is matrix acidizing. In matrix acidizing, acid is injected into the rock and reacts with it, etching the wall of its pores, and creating channels called “wormholes”. Wormholes increase surface area and allow more fluid to drain to the well-bore, enhancing oil recovery. While acidizing increases oil production and recovery, it also increases the risk of rock collapse. It is therefore important to predict the contrasting effects of increased production rate and decreased rock strength due to wormhole formation. For this reason an integrated model adapted for computer simulation is developed to investigate the effect of wormhole formation on the strength of the chalk reservoirs. The wormhole can be modeled as fractures with different orientation and its deformation is coupled with the intact rock’s deformation. Numerical simulation is based on control volume methods for fluid flow in porous media using a Darcy model, and stress analysis is based on the finite element method using a constitutive model of chalk. For computational simulation of this multi-physics simulation an in-house CFD software, “Brilliant”, is used. In this work, the mechanical stability of wormholes and chalk reservoir under compaction loads will be investigated.

Stimulation of production and injection wells in carbonate reservoirs using an hydrochloric acid is a common practice to enhance flow towards the wellbore. There are two main types of carbonate acid treatment, fracture acidizing and matrix acidizing: -Aci d fracturing is obtained by injecting gelled fluids and acid into a carbonate formation at a pressure above the formation-fracturing pressure. -Matrix acidizing is performed when the stimulation fluid is pumped into the well at a pressure below the fract uring pressure. In this method acid penetrate into the porous rock. By dissolving the porous rock, conductive channels are created. These channels are known as wormholes. Wormholes increase the surface area between the porous media and the well, allowing fluid flux at lower energy consumption. This increases the drainage area of the porous rock and oil can flow easier. Wormholes are weakening the rock structure and the risk of breaking-down the formation increases. The changed stress level in the area wh ere the wormholes are formed influence the resulting achievement of the acid treatment. Simulation of the stimulation process and the stress analysis connected to it is a challenge in many respects. Wormhole structure and geometry can be quite complex an d modeling all details require extensive computing resources. The need for an approach that treats the subject efficient, but in line with the physics, is obvious. There are several approaches that can be imagined, but all are not equal with respect to time consumption and accuracy. A simulation system shall give an impression of the physical processes behavior and the interaction between them. The target of this study is to investigate the above subjects with emphasize on the industrial usefulness wi thin simulations. Equally important is the conformation between simulations and physical behavior with respect to fractures and wormhole formation and resulting stresses.